[Advances in Food Research] Advances in Food Research Volume 8 Volume 8 || The Blanching Process

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  • THE BLANCHING PROCESS

    BY FRANK A. LEE

    New Y'ork State Agricultural Experiment Station, Cornell Uniuersity, Geneua, New York

    I. Introduction . . . . . . . . . . . . . . 1. Mineral Substances . . . . . . . . . . . 2. Sugars and Proteins . . . . . . . . . . . 3. Carotene . . . . . . . . . . . . . 4. Thiamine . . . . . . . . . . . . . 5. Riboflavin . . . . . . . . . . . . . 6. Niacin . . . . . . . . . . . . . . 7. Ascorbic Acid (Vitamin C ) . . . . . . . . . 8. Chlorophyll . . . . . . . . . . . . . 9. Sulfur Compounds . . . . . . . . . . .

    10. Enzymes . . . . . . . . . . . . . 111. Special Techniques for Blanching . . . . . . . . .

    1. Steam Pressure . . . . . . . . . . . . 2. Electronic Blanching . . . . . . . . . . .

    a. Weight Changes . . . . . . . . . . . 3. Special Treatments . . . . . . . . . . .

    Unblanched Material . . . . . . . . . . . V. Summary . . . . . . . . . . . . . .

    References . . . . . q . . . . . . .

    11. Changes in Nutrients and Other Substances During Blanching .

    IV. Recent Studies on Fundamental Changes During Frozen Storage of

    Page 63 65 65 68 72 74 75 75 76 81 83 84 88 86 87 87 91

    95 104 106

    I. INTRODUCTION

    The preservation of vegetables for relatively long term storage is a very important industry. Indeed, the canning of peas in the United States alone amounted to 31,222,000 cases for the year 1955. As far as the frozen product is concerned, 231.2 million pounds were packed in that year (Western Canner and Packer, 1956).

    One of the important processes in the preparation of vegetables for freezing, canning, or dehydration, is blanching, or as it is sometimes called, scalding.

    The process of blanching involves the treatment by means of some form of heat, usually either steam or boiling water. The time and tem- perature used depend upon the final processing to be employed, as well

    63

  • 64 FRANK A. LEE

    as on the nature of the material to be packed. Other means to achieve this end have been studied.

    Blanching as a pretreatment of vegetables for canning has as its object (1 ) the removal of tissue gases; (2) the shrinking of the material so that adequate fills can be had in the can; and ( 3 ) the heating of the material prior to filling so that a vacuum will be obtained after heat processing and boiling. The first of these is necessary to reduce internal stresses in the can, which in turn, tend to avoid such undesirable results as buckling. The second of these makes it possible to meet the legal requirements concerning fill. While it is true that the process used for the removal of the tissue gases will at the same time inactivate any enzymes present, it might well be noted that if some were left intact, the cooking process, so necessary in canning to effect sterilization, would complete the inactivation of the enzymes. It is for this reason that enzyme inactivation, per se, in blanching as a part of the canning process is not so important,

    Blanching is necessary as a part of the preparation for freezing preservation ( I ) to inactivate the enzymes in the tissues and ( 2 ) to shrink the material so as to conserve space in packing. The inactivation of the enzymes is very important in this process because no final cook or sterilization is used previous to freezing, and freezing storage, at least at the temperatures commonly employed, does not prevent unde- sirable deterioration in flavor, odor, and color on the part of the enzymes in the tissues. Enzyme inactivation in material to be dehydrated is important, because again, no further cooking previous to storage, is involved.

    In the early days of the preservation of vegetables by freezing, it was found that mere storage at -18OC. ( O O F . ) did not prevent the develop- ment of off-flavor, off-odor, and off-color in the stored products. The early work of Joslyn and Cruess (1929) in this connection is summarized as follows.

    These workers showed that steam or water blanch at 100OC. (212OF.j for 2 5 minutes, followed by chilling in cold water or a blast of cold air and packing in brine in the case of certain vegetables resulted in vegetables which were considered to be equal to the fresh even after several months of storage at -18 to -9.5OC. (0-15OF.).

    In 1930, Barker recorded work on the preservation of vegetables by freezing in which samples of peas were stored at each of the folIowing temperatures: -so, -50, -100, and -18C. (27OF., 23OF., 14OF., and O O F . ) for four weeks, and then thawed and cooked. At each tem- perature autolytic changes occurred affecting the color and flavor. The autolysis was markedly retarded at the lower temperatures but even at

  • THE BLANCHING PROCESS 65

    -18OC. (OOF.) slight changes occurred, and an objectionable flavor appeared although the color was normal,

    A means of preventing these autolytic changes was found in blanch- ing, i.e., partial cooking, before freezing. Peas cooked for about 8 minutes, and then frozen in water a t -18OC. (0F.) were stored suc- cessfully for 4 months, and there seemed no reason why this period could not be extended. After thawing and further cooking, the color was still excellent, and there was no trace of the distasteful flavor noted in peas frozen without preliminary blanching. The success of the blanching was presumed to be due to the destruction of the catalytic systems by the heat.

    While blanching times and perhaps methods of storing have changed somewhat, it is quite obvious that these workers twenty-eight years ago expressed views that were fundamentally sound. The necessity of the blanching step in the preparation of vegetables for freezing, for canning, and for dehydration has been demonstrated on many occasions since then. This step is now universally employed under commercial condi- tions, and it is recommended to home processors by the books and bulletins on this subject.

    Adam et al. (1942) concluded that blanching has an appreciable effect on the pressures developed in the cans and on the total weight of contents. They considered it necessary, but stated the times should be as short as practicable.

    II. CHANGES I N NUTRIENTS AND OTHER SUBSTANCES DURING BLANCHING

    Many studies were conducted to determine the effects of steam and water blanching on the vegetables so processed. Much of this work was done to determine whether steam or water blanching is the more efficient when one considers the loss of nutrients as a result of blanching. In certain of these studies, time and temperature of the blanch were an important part of the work.

    1. MINERAL SUBSTANCES Horner ( 1936-1937a ) observed that during blanching, considerable

    loss of potassium and phosphates occurred in all vegetables. Shrinkage of the vegetables, accompanied by a reduction in weight took place also.

    In the following blanch times, peas, 3 minutes at 100OC. (212OF.), beans, 3 minutes a t 82OC. ( 180F.), carrots, 7 minutes a t 100OC. (212OF.), and potatoes, 5 minutes a t 100OC. (212OF.), the percentage losses respectively were: peas, 39% K,O (potassium oxide), 20% P,05

  • 66 FRANK A. LEE

    (phosphorous pentoxide); beans, 40% K,O; carrots, 16% K,O, 15% P205; potatoes, 9% L O , 9% P,O,.

    Calcium was found to be generally absorbed by the vegetables dur- ing blanching, the extent of the increase depending upon the nature of the vegetables, the hardness of the blanching water, and the time of blanching.

    TABLE I Gain in Calcium Content during Blanching a

    Calcium oxide content ( W ) Gain of calcium oxide

    vegetables in vegetables Vegetable Water

    Raw Blanched

    Peas 0.0100 0,0199 0.0302 52 Beans 0.0087 0.104 0.124 19 Carrots 0.087 0.0432 0.0521 21 Potatoes 0.0087 0.0118 0.0157 33

    (1 Horner ( 19361937b).

    Lee and Whitcombe (1945) studied the effects of the blanching of vegetables in various types of potable water. These authors, in confirma- tion of Horners findings, found that vegetables prepared for freezing preservation by blanching in hard water showed significant increases in the calcium content. They found also, that the blanching of vegetables in water containing one part per million of iron, resulted in no significant changes in the iron content of the peas and snap beans used in the experiments.

    Kramer and Smith (1947) made a study of the effect of duration, temperature, and type of blanching on the mineral composition of peas, green beans, lima beans, and spinach for preservation by canning. Steam blanching was found to cause no significant change in the composition of any except spinach, where moderate losses were noted in the ash and phosphorus contents and slight gains noted in the calcium contents. For water blanch in general, the effect of time was more important than temperature, The mineral components showed changes as follows. The severest water blanch caused a reduction of 54% in the ash content of spinach. The calcium content of green beans was not affected by the water blanch, that of lima beans slightly increased, and that of peas and spinach increased by as much as 79 and 54%, respec- tively. The phosphorus content of spinach as reduced by as much as 40%. but rarely more than 10% for the other vegetables.

    Data showing the effect of blanching on the mineral composition of

  • TABLE I1 Changes in Proximate and Mineral Composition of Canned Peas as a Result of Blanching, Fill-In Weight Basisa

    Fancy grade, No. 4 sieve s u e Blanching Mineral composition

    ( "F. ) ( min. ) Moisture Protein Fat Fiber Ash Carbohydrate Calcium Phosphorus (mg./100 g.)

    Type of blanch Temp. Time

    None - - 82.19 5.04 0.36 1.86 0.56 9.99 19 63 Water 180 3 81.99 5.03 0.40 2.04 0.60 9.89 25 74 Water 180 6 82.65 4.82 0.37 2.09 0.54 9.53 30 61 Water 180 9 83.24 4.75 0.34 2.11 0.51 9.05 33 61 Water 190 3 82.22 4.94 0.40 2.11 0.56 9.77 29 65 Water 190 6 82.65 4.63 0.38 2.14 0.55 9.65 31 63

    Water 200 3 81.93 4.99 0.41 1.99 0.55 10.13 30 67 m td Water 200 6 83.04 4.76 0.38 1.92 0.56 9.34 32 65 F * 1: Water 200 9 83.76 4.46 0.35 1.93 0.55 8.95 34 62 Steam 210 1 81.14 5.42 0.38 2.06 0.61 10.39 22 70 n E 2: Steam 210 2 80.54 5.75 0.42 2.24 0.56 10.49 22 78 Steam 210 3 80.74 5.57 0.40 2.42 0.57 10.30 23 79 d

    90.85 2.81 0.56 0.96 1.74 2.98 119 70 x None - - Water 170F 1 91.60 2.69 0.56 1.03 1.48 2.64 110 59 Water 170 4 92.63 2.47 0.56 1.04 1.21 2.09 137 49 Water 170 7 92.54 2.57 0.53 1.03 1.13 2.20 150 49 Water 185 1 92.37 2.63 0.51 0.86 1.31 2.32 141 50 Water 185 4 92.61 2.64 0.48 0.92 1.07 2.28 155 45 Water 185 7 93.00 2.61 0.42 0.92 0.98 2.07 144 49 Water 200 1 91.63 2.77 0.60 1.05 1.38 2.57 137 58 Water 200 4 92.61 2.66 0.52 1.10 0.92 2.19 153 42 Water 200 7 92.79 2.70 0.48 0.96 0.80 2.27 183 43 Steam 210 0.75 91.77 2.59 0.57 0.91 1.52 2.64 134 53 Steam 210 1.5 91.66 2.77 0.56 0.97 1.54 2.47 128 60 Steam 210 3 91.77 2.85 0.62 0.88 1.49 2.39 120 62

    Proximate composition (9)

    Water 190 9 84.28 4.57 0.36 1.97 0.58 8.24 32 65 2

    Changes in Proximate and Mineral Composition of Spinach as a result of Blanching, Fill-In Weight Basis.

    M v) v)

    m 4

    aKramer and Smith (1947).

  • 68 FRANK A. LEE

    peas are presented in Table 11, along with data on other substances. Steam blanching reduced the moisture content by 1 to 2%. The corres- ponding small increases in all the components therefore indicate that there was little or no change in the mineral composition of peas as a result of steam blanching.

    Regardless of temperature, the short water blanching period of 3 minutes caused a slight loss in moisture content. As the blanching time was increased, however, the moisture content increased with time and temperature until the 9-minute blanch at 93OC. ( 2 0 0 O F . ) resulted in a 1.57% increase in moisture for the fancy peas and the 12-minute blanch at 93OC. (200OF.) resulted in a 2.15% increase in the moisture of the standard peas. The increase in moisture content was compensated for by a decrease in ash and in other materials. Although considerable losses were recorded for the ash content, especially of the standard peas, the total ash content of about 0.5% did not influence materially the general proximate composition.

    The calcium content increased rapidly with time and temperature of the water blanch, from 19 mg. for the unblanched to 34 mg. for 100 grams for the fancy peas blanched for 9 minutes at 93OC. (200F.), and from 33 mg. for the unblanched to 45 mg. per 100 grams for the standard peas bIanched for 12 minutes at 93OC. (200OF.). This is in agreement with Horner, and with Lee and Whitcornbe.

    The phosphorus content was not materially affected by any of the blanching treatments, whereas Horner reported considerable loss in phosphorus content of peas as a result of blanching.

    The loss of total ash components of peas was not due to losses in phosphorus, and certainly not to calcium, which was actually taken up from the blanching water, but to losses of other more soluble minerals, particularly potassium, which is present in comparatively large quantities.

    Loss in total ash as a result of water blanch of spinach was also significant; the maximum reduction for the 7-minute blanch at 93OC. (200OF.) varied from 1.74% to 0.80% or more than a 50% loss. As with peas, the calcium content increased with increasing time and tempera- ture of the water blanch until the 7-minute blanch at 93OC. (200OF.) resulted in a calcium content of 183 mg. per 100 grams compared to 119 mg. per 100 grams for the unblanched sample. The phosphorus con- tent declined from 70 mg. for the unblanched sampIe to 43 mg. per 100 grams for the most severe water blanch (Table 11).

    2. SUGARS AND PROTEINS Magoon and Culpepper (1924) found that considerable losses in

    sugars and other soluble substances resulted when peas, snap beans,

  • TABLE 111 Analyses of Stringless Green Pod String Beans, before and after Treatmenta

    Average components ( 8 )

    Treatment of material Alcohol Sugars Polysac- pento- Nitrogen

    Soluble Insoluble Reducing Total as starch protein if any Moisture Solids charides as

    reducing

    No treatment 89.40 10.60 4.46 6.14 2.34 0 2.34 2.60 1.15 1.72 8 Treated 4 min. with

    Treated 8 min. with

    Scalded 4 min. in

    E k

    1.73 Q Scalded 4 min. in 8

    8

    89.60 10.40 4.38 6.02 2.39 0 2.39 2.58 - 1.71 live steam

    live steam

    boiling water 90.05 9.95 4.15 5.130 2.01 0 2.01 2.52 - 1.65

    boiling water, then chilled 30 sec. in m cold water 90.83 9.17 3.73 5.44 1.78 0 1.78 2.33 - 1.63

    89.52 10.48 4.38 6.09 2.28 0 2.28 2.63 -

    0

    Scalded 8 min. in boiling: water 90.43 9.57 3.75 5.82 1.97 0 1.97 2.67 - 1.64 -

    Scalded 8 min. in boiling water, then chilled 30 sec. in cold water 90.67 9.24 3.61 5.5:; 1.80 0 1.80 2.35 - 1.48

    a Magoon and Culpepper ( 1924).

  • 70 FRANK A. LEE

    and spinach were scalded in boiling water and chilled in cold water thereafter; for 2 minutes and 4 minutes in the case of spinach and peas, and 4 minutes and 8 minutes in the case of snap beans, in preparation for canning.

    These authors concluded that the chilling step following scalding resulted in loss of nutrients. They further concluded in the case of spin- ach, peas, and snap beans that blanching in steam was preferable to that in boiling water because of the conservation of soluble nutrients. Table I11 gives data which these authors obtained in work on snap beans.

    Horner (19361937b) studied the loss of soluble solids in the blanch- ing of vegetables. This included the effect of blanching on the sugar and soluble nitrogen contents of peas, beans, and carrots. It was found that the amount of loss increases with the time of blanching in water, and is greater for small units (peas) than for larger ones. Steam blanch- ing reduced considerably the amount of sugar lost. These losses are illus- trated in Tables IV and V. Table VI shows the loss in weight of vege- tables on blanching.

    TABLE IV Loss of Sugars during Blanching 0

    ~ ~ _ ___~ ~

    Vegetable Type of blanch Total Sugars ( X ) Loss (g./100 g.) Peas

    Beans

    None 3 min. water 6 min. water 3 min. steam None 3 min. water 6 min. water 3 min. steam

    6.35 5.43 4.91 5.58 3.0 2.85 2.85 2.89

    - 22.6 34.6 17.5

    7.5 9.3 3.3

    -

    a Horner ( 19361937a ).

    TABLE V Loss of Protein during Blanching 0

    Vegetable Type of blanch N x 6.25 ( X ) Loss ( g. / 100 g. ) Peas None

    3 min. water at 212F. 6 min. water at 212F. 3 min. steam at 212F.

    3 min. water at 190F. 6 min. water at 190F. 3 min. steam at 212F.

    Beans None

    5.75 4.94 4.52 5.37 1.75 1.79 1.73 1.85

    - 22.1 31.5 12.2

    0.0 5.0 0.0

    -

    a Horner ( 1936-1937a ).

  • THE BLANCHING PROCESS 71

    TABLE VI Loss in Weight of Vegetables on Blanching@

    Vol. Weight of vegetable Loss Blanch in Mois- Temp. Time ture (OF.) (min.) water Before After wt

    of Vegetable

    ml. g. g. % %

    Peas Water 212 3 600 150 136 9.2 83.4 Water 212 6 600 150 127 15.4 84.2 Steam 212 3 - 150 141 6.0 83.0

    Beans Water 190 3 600 200 193 3.5 91.4 Water 190 6 600 200 191 4.5 91.4 Steam 212 3 - 200 194 3.0 91.5

    a Homer ( 193S1937a).

    Kramer and Smith (1947) studied the effect of duration, tempera- ture, and type of blanching on the proximate composition of peas, green beans, lima beans, and spinach prior to canning. Steam blanching caused no significant change in the composition of any except spinach, in which case moderate losses were noted in carbohydrates. For the water blanch in general, the effect of time was more important than temperature. Carbohydrate losses were most serious in spinach; they reached about 30% of the total found in the unblanched sample, as compared to a little over 10% for peas and lima beans, and only about 5% for green beans. Protein losses rarely exceeded lo!% for peas, lima beans, fancy green beans, and spinach, and reached only 5% for the more mature green beans.

    Regardless of the temperature, the short water blanching period of 3 minutes caused a slight loss in moisture content. However, the moisture content increased with time and temperature until the 9-minute blanch at 93OC. (200OF.) resulted in a 1.57% increase in moisture for the fancy peas and the 12-minute blanch at 93OC. (200OF.) resulted in a 2.15% increase in moisture for the standard peas, These increases in moisture content were compensated for by corresponding decreases in protein, ash, and carbohydrate contents. Almost one-third of the loss in solids was accounted for by the proteins, and about two-thirds by the carbohydrates.

    Of all the vegetables studied, spinach was the only commodity show- ing increases in moisture content and losses in carbohydrates as n result of steam blanching. Thus for the 3-minute steam blanch, the cnrbo- hydrate content was reduced to 2.39% as compared to 2.98% for the unblanched sample. Moderate losses were recorded for protein. Only slight losses in protein were noted for the water blanch, but consider-

  • 72 FRANK A. LEE

    able losses occurred in carbohydrates, the greatest being from 2.98% for the raw sample to 2.07% for the sample blanched 7 minutes at 85OC. ( 185OF. )

    3. CAROTENE A considerable amount of work was done on the effect of blanching

    on vitamin content of vegetables, both for freezing preservation and for canning. The fact that blanching does not affect noticeably the carotene content of vegetables was pointed out by several authors between 1937 and 1940 (Stimson et al., 1939; and Zimmerman et al., 1940).

    According to Guerrant et al. (1947) carotene retention by peas and spinach, after various conditions of blanching, was usually good. Zscheile and co-workers (1943) showed that greater retention of carotene re- sulted when vegetables to be frozen were blanched previous to storage than when they were stored unblanched. In this study, varying but comparable months of storage were employed at -2OOC. (-4OF.).

    TABLE VII Loss of Carotene during Blanching a

    Type of vegetable and variety

    Storage time Total Description at -20C. carotene

    (months) ( mcg. /g. )

    Spinach 3?4 Fresh 0 57.0 Giant Nobel 3?: Blanched 6 55.7

    3?1 Blanched 11 40.0 3% Unblanched 6 34.1 3?i Unblanched 11 20.8

    Asparagus Medium size 0 7.12

    Med. unblanched 8 2.88 Med. unblanched 13 1.65

    Alderman T.T. Fresh 0 5.06 Alderman T.T. Blanched 12 4.20 Alderman T.T. Blanched 23 4.07 Alderman T.T. Unblanched 12 2.30 Alderman T.T. Unblanched 23 1.91

    Martha Washington Med. blanched 8 5.57

    Peas

    =Adapted from: Zscheile et al. (1943).

    The method of alcohol-insoluble solids as a reference base to be used for blanching vegetables was recommended by F. A. Lee in 1945, as a result of work on the carotene content of carrots.

    Table VIII illustrates the great decrease of total solids which occurs during the blanching of carrots in water, the larger apparent increases

  • TABLE VIII Carotene Content of Processed Carrots a

    Treatment Total solids ( a )

    Alcohol- insoluble

    solids wet basis

    (%6)

    Carotene,

    kl Carotene, Carotene, Increase alcohol- Increase wet basis dry basis ( a ) insoluble ( 9 ) m F

    solids

    Raw Blanched 1 min. in boiling water Blanched 3 min. in boiling water Blanched 5 min. in boiling water Blanched 10 min. in boiling water Blanched 20 min. in boiling water Blanched 30 min. in boiling water Autoclaved at 10 lb. for 12 minutes

    12.74 11.24 9.87 9.58 8.71 7.84 6.39

    13.52

    3.46 88 3.36 91 3.46 90 3.39 93 3.43 90 3.35 90 3.24 91 3.83 104

    690 810 910 970

    1030 1145 1430 770

    - 17.4 31.9 40.1 49.4 66.0

    107.2 11.6

    b 2530 - 3 2720 7.5 8 2600 2.7 3

    8 2680 5.9 v1

    w m

    B

    2750 8.7 2620 3.6

    2820 11.4 2710 7.1

    cn

    a Lee (1945). (Calculated on the Basis of Dry Weight and of Alcohol-Insoluble Solids)

  • 74 FRANK A. LEE

    in carotene which are recorded on the basis of total solids, and the con- stancy of the carotene values when expressed on the basis of alcohol- insoluble solids. The relative constancy of the results when expressed on the wet basis indicates that as far as carrots are concerned, the loss of soluble solids is closely compensated by the uptake of water.

    4. THIAMIN Fellers et al. (1940) reported that little thiamin was lost during

    the preparation for freezing of peas and spinach but that lima beans and asparagus lost 54% and 26% respectively, The latter decrease in the vitamin content was attributed to a longer blanching period for the latter vegetables. Moyer and Tressler (1943) noted a small drop in the thiamin content of asparagus and peas as a result of blanching.

    Lee and Whitcombe (1945) found that no significant differences in any of the B vitamins resulted from blanching peas and snap beans in distilled water, tap water rendered safe for drinking with chlorine, hard water, or potable water containing dissolved iron.

    Guerrant et al. (1947) investigated the effect of duration and tem- perature of blanching on the vitamin content of certain vegetables previous to preservation by canning. Four representative vegetables (peas, green beans, lima beans, and spinach) were studied with respect to the amount of vitamin retained after being blanched at different temperatures for varying periods of time. Both water and steam blanch- ing were employed.

    When computed on the basis of 100 grams of sample, peas, spinach, and lima beans were found to have retained consistently less thiamin after water blanching as the temperature and the duration of the blanch were increased, Under the most adverse conditions of water blanching employed in these studies, peas, lima beans, and spinach retained only 66, 55 and 338 respectively, of their preblanched thiamin content, whereas under the more favorable conditions of blanching, from 82 to 97% of the vitamin was retained. Loss of vitamin was accompanied also by considerable loss of soluble solids. The loss of solids from spinach, however, did not account for the severe loss of the vitamin from this vegetable. Relatively small losses of thiamin resulted from the steam blanching of the three vegetables.

    Lamb et al. (1948) performed retention experiments on commercially blanched peas using rotary and tubular blanchers. In general, the reten- tion of thiamin during blanching was found to parallel the retention of ascorbic acid, but differences between different blanching treatments were not so pronounced. The data indicate that blanching procedures giving maximum retention of ascorbic acid can also be expected to give

  • THE BLANCHING PROCESS 75

    maximum retention of thiamin, provided processing conditions remain constant. These authors found [as shown by Lee (1945)] that retention values calculated on the basis of alcohol-insoluble soIids represent more nearly the actual retention obtained than do those based on total solids. Retention values calculated on the basis of alcohol-insoluble solids were lower in every instance than those calculated on the basis of total solids.

    Feaster et al. (1949) concluded that when a rotary blancher of the drum type was used, the blanching time exerted a definite effect on thiamin retention in standard and extra standard No, 4 sieve canned sweet peas. A 4$$-minute blanch at 190OF. to 200OF. was found to be superior to an 18%-minute blanch for retention of this vitamin. The quality of the canned peas was essentially the same with either blanch. Data are given in Table XII. Holmquist and associates (1954) reported that steam-blanched peas showed no significant difference in retention of thiamin when contrasted with those blanched in the conventional water blancher,

    5. RIBOFLAVIN

    Guerrant et d. (1947) included riboflavin in their experiments. Under the conditions studied, riboflavin retention during the water blanching of peas, spinach, and lima beans was also affected more markedly by increasing the period of blanch than by increasing the temperature of blanch. Under the more severe conditions of blanching, peas, lima beans, and spinach were found to have retained only 50, 57, and 274: respectively, of their former riboflavin contents. These severe losses of vitamin were somewhat paralleled by severe losses of soluble solids. However, under the more favorable conditions of blanching, riboflavin retention ranged from 90 to loo%, the percentage varying with the different vegetables. Green beans showed good retention of riboflavin after water blanching. All four vegetables lost relatively in- significant amounts of this vitamin as the result of steam blanching.

    Holmquist et al. (1954) concluded that steam-blanched peas showed no significant difference in the retention of riboflavin when contrasted with those blanched in the conventional water blancher.

    6. NIACIN

    Guerrant et al. (1947) concluded that lima beans sustained appreci- able losses of niacin during all conditions of blanching. Increasing the period of blanching had a more adverse effect on niacin retention than did increasing the temperature of blanch. Only about 60% of the original content was retained when lima beans were water blanched for 8 min- utes at 93OC. ( 2 0 0 O F . ) . Holmquist et d. (1954) concluded that steam-

  • 76 FRANK A. LEE

    blanched peas showed no significant difference in the retention of ribo- flavin when contrasted with those blanched in the conventional water blancher.

    7. ASCORBIC ACID (VITAMIN C )

    Much more information is available in the literature on changes in ascorbic acid (vitamin C ) content in vegetables during the blanching process than for the other vitamins. Tressler and co-workers (1937) found that approximately one-third of the ascorbic acid of lima beans is lost during blanching according to the usual process followed in pre- paring the beans for freezing. This loss was found to be materially reduced by cutting the blanching time in half. This is illustrated in Table IX.

    TABLE IX Effect of Blanching on Ascorbic Acid Content of Fordhook Lima Beans a

    Sieve size Ascorbic acid content diameter

    of opening period Blanching

    Fresh basis Dry basis ~~~ ~

    Inch Seconds Mg. per gram Mg. per gram

    Pass 15/32 0 .46 3.1 Pass 15/32 30 .33 2.5 Pass 15/32 45 .30 2.4 Pass 15/32 60 .2a 2.2 Pass 15/32 150 .24 2.1

    a Adapted from: Tressler et al. (1937).

    Jenkins et al. (1938) found that during blanching a portion of the ascorbic acid is lost, and that this loss becomes greater as the blanching period is prolonged from 60 seconds to 153 seconds in water at 93OC. (200OF.). This is illustrated in Table X.

    TABLE X Effect of Length of Blanch in Water at 93C. on Ascorbic Acid Content of Peaso

    Ascorbic acid

    Fresh basis Dry basis ( mg./g. ) (mg.fg.1

    Total solids Time

    ( sec. )

    0 60 85

    128 153

    22.5 0.25 22.2 0.21 23.1 0.20 21.8 0.17 21.8 0.16

    ~~

    1.11 .95 .a7 -78 .73

    a Jenkins et al. (1938 ).

  • THE BLANCHING PROCESS 77

    Melnick and associates (1944) concluded that hot water blanching has the disadvantage of extracting soluble nutrients from vegetables. Loss of soluble components during steam blanching was not significant. Steam blanching was, therefore, the preferred method. Results on ascor- bic acid losses obtained by these authors are shown in Table XI.

    TABLE XI Influence of Various Blanching Procedures upon Ascorbic Acid in Green Beans a

    Ascorbic acid values (mg./100 g.)

    Reduced Dehydroascorbic Total Sample Treatment

    ~~

    Raw beans, untreated Steam blanch, 1 min. Steam blanch, 3 min. Steam blanch, 5 min. Steam blanch, 10 min. Hot water blanch, 1 min. Hot water blanch, 3 min. Hot water blanch, 5 min. Hot water blanch, 10 min.

    18.3 17.1 17.1 18.5 18.7 18.4 17.1 15.3 15.1

    4.3 5.4 5.8 3.4 3.1 4.0 3.6 3.7 3.1

    22.5 22.5 22.9 21.9 21.8 22.4 20.7 19.7 18.2

    a Adapted from: Melnick et al. ( 1944).

    Lee and Whitcombe (1945) concluded that no significant differ- ences in ascorbic acid content resulted from blanching peas and snap beans in distilled water, tap water (chlorinated), hard water, or potable water containing dissolved iron. Guerrant et al. (1947) investigated the effect of duration and temperature of blanching, both water and steam, on the ascorbic acid content of four vegetables. The vegetables used were peas, green beans, lima beans, and spinach, as described under thiamin, The ascorbic acid content of all four vegetables was adversely affected by both methods of blanching, irrespective of temperature or duration of blanch. However, the effect of steam blanching was con- siderably less marked than that of water blanching. Except where green beans were blanched at 71OC. (160OF.) for 1 or 3 minutes, the general consequence of increasing the blanching temperature on ascorbic acid retention was less severe than that of increasing the duration of blanch. When computed on the basis of 100-gram samples, peas, green beans, and lima beans retained approximately 40% of their original ascorbic acid content as the result of the most severe blanching operations, whereas under comparable conditions of blanching and computation of data, spinach was found to have retained approximately 20% of its orig- inal content of this vitamin. However, under the most favorable condi- tions of blanching, ascorbic acid retention ranged from 72 to 93%, de- pending on the vegetable. Loss of asccrbic acid from peas during the

  • 78 FRANK A. LEE

    course of blanching does not always parallel loss of weight (moisture and soluble solids), but the proportion of vitamin loss usually exceeds that of loss of weight. The practice of blanching successive batches of peas in the same blanching water was not found to be warranted on the basis of greater ascorbic acid retention.

    Guerrant and Dutcher (1948) found that green beans retained little of their ascorbic acid when blanched for 3 minutes at 60, 6 6 O , and 71OC. ( 140, 150, and 160F.), whereas when blanched at 82", 88" and 93C. (180, 190, and 200OF.) for the same period, they retained a high percentage of this vitamin. The decrease in reduced ascorbic acid could not be accounted for in the blanching water. The data suggest that an enzyme system is involved. Additional data showed the reduced ascorbic acid content of green beans to be less stable during a 5-hour storage period at room temperature, when blanched at 66OC. (150OF.) than when blanched at 93OC. (200OF.), or when the beans were unblanched. While there was an appreciable loss in total ascorbic acid, a more significant change was the increase in the dehydro form of the vitamin at the expense of the reduced form, The data further show the im- portance of high temperature blanching in promoting maximum reten- tion of ascorbic acid. Similar results were obtained by Robinson et d. (1949) in work carried on independently.

    Moyer and associates (1949) in confirmation of Guerrant and Dutcher (1948) concluded that when blanching only is considered from the standpoint of preventing losses of vitamin C, a short, high- temperature blanch yields more nutritious peas. These authors noted that the results of vitamin C analysis show that the losses of this nutrient are due to the leaching action of hot water and to enzymatic destruc- tion. Which of these agents caused the greater loss of vitamin C was estimated by observation whether or not this loss persisted during hold- ing. When the destruction persisted on holding after blanching, enzyme action was considered to be the chief cause of the vitamin C loss. In view of the fact that the amount of ascorbic acid oxidase found in the peas after blanching did not always coincide with the continued destruc- tion of the vitamin on holding, it seems, therefore, that the present test for ascorbic acid oxidase cannot be considered as a reliable criterion of adequate blanching.

    Lamb et al. (1948) made a study of the retention of vitamins in unblanched peas, and in peas blanched in rotary and tubular commercial blanchers. Fancy No. 3 sieve peas were canned unblanched, after wash- ing for 3 minutes in water at 49OC. (120OF.) and after blanching 3 minutes at 88OC. (190OF.). Retention of ascorbic acid after processing was 96, 80, and 53% respectively on the basis of dry solids, and 92, 69,

  • THE BLANCHING PROCESS 79

    and 43% respectively on the basis of alcohol-insoluble solids, for the unblanched peas. However, the quality of the unblanched peas and the peas washed with warm water was unsatisfactory for the following reasons: ( I ) The cans buckled; (2) The brine was markedly cloudy and much starch material had sloughed off into the brine; (3) The flavor was slightly bitter or viny. The fact that they retained a higher vitamin content was overcome by the unacceptable quality.

    Retention experiments were performed on commercially blanched peas using rotary and tubular blanchers. The average retention of ascorbic acid after processing was 72% when results were calculated on the basis of dry solids (excluding added sugar and salt) and 63% when results were calculated on the basis of alcohol-insoluble solids. Slightly higher retentions were obtained on extra standard peas blanched in a rotary blancher for 4 minutes a t 88OC. ( 1 9 0 O F . ) than were obtained on peas blanched in a rotary blancher for 4% minutes at the same temperature. Retention values were obtained on extra standard peas blanched 5 minutes a t 88OC. (190OF.) and for 1% minutes a t 96OC. (205OF.) and on fancy peas blanched 454 minutes a t 88OC. (190OF.) and 134 minutes a t 96OC. (205OF.) in rotary blanchers. Approximately 5% higher retention of ascorbic acid was obtained on the extra standard and approximately 10% higher retention was obtained on the fancy peas blanched at the higher temperatures and shorter time. The quality of the peas blanched by the two procedures was indistinguishable except for the slightly greener color of the fancy peas blanched at the higher temperature. As was described in connection with the work of these authors under thiamin, calculation of the vitamin content on the basis of alcohol-insoluble solids seemed to represent more nearly the actual retention of vitamin obtained.

    Feaster et al. (1949) concluded that when a rotary blancher of the drum type was used, the blanching time exerted a definite effect on ascorbic acid retention in standard and extra standard No. 4 sieve canned sweet peas. A 436-minute blanch at 88OC. to 93OC. ( 19O0F. to 200F.) was found to be superior to an 1856-minute blanch for retention of this vitamin. The quality of canned peas of these maturities was essen- tially the same with either blanch. The ascorbic acid content of the extra standard peas appears to be more readily affected by blanching time than that of standard grade peas, The data are given in Table XII.

    Holmquist et nl. (1954) reported that steam-blanched peas showed greater retention of ascorbic acid than those blanched in the conven- tional water blancher.

    Ascorbic acid retention in steam-blanched peas, with and without detergent wash, also was compared with retention in hot-water-blanched

  • TABLE XI1 Influence of Blanching Time on Vitamin Retentions in Canned Sweet Peas (No. 4 Sieve Size) a

    Ascorbic Acid Thiamin Total Solids Description Wt. Per 100 Per Retention Per 100 Per Retention Per 100 Per Retentionc

    percan grams can ( X ) grams can ( X ) grams can ( X )

    Raw 346b 18.9 65.3 100 0.344 1.19 100 20.03 69.4 100 Blanched 18% min. 346 12.9 44.6 68 0.235 0.81 68 19.13 66.2 95 Brine added 245 None None 4.40 11.0 Canned, 18% min. blanch:

    STANDARD SWEET PEAS

    Drained solids 364 7.1 25.8 40 0.064 0.23 20 18.05 65.6 94 Drained liquid 227 8.7 19.8 30 0.063 0.14 12 6.28 14.3

    Raw 352b 19.2 67.5 100 0.340 1.19 100 20.24 71.2 100 Blanched 4% min. 352 19.0 67.0 99 0.319 1.12 94 21.44 75.4 105 Brine added 240 None None 4.48 11.7 Canned, 4% min. blanch:

    *!

    ?

    Drained liquid 233 11.4 26.6 39 0.113 0.25 22 7.00 16.3 +- E Raw 368b 23.2 85.4 100 0.329 1.21 100 19.25 70.7 100 M

    Blanched 18% min. 368 13.8 50.8 60 0.233 0.86 71 17.69 65.0 92 Brine added 220 None 4.48 9.9 Canned, 18% min. blanch:

    Drained solids 359 9.6 34.5 51 0.102 0.37 30 19.38 69.5 97 3 EXTRA STANDARD SWEET PEAS

    Drained solids 375 6.7 25.1 29 0.086 0.32 26 15.39 57.5 81 Drained liquid 214 8.4 18.0 21 0.087 0.19 16 6.30 13.5

    Raw 362b 23.5 85.0 100 0.346 1.25 100 19.63 71.2 100 Blanched 4% min. 362 17.9 64.8 76 0.335 1.21 97 19.68 71.2 100 Brine added 230 None None 4.48 10.3 Canned, 4% min. blanch:

    Drained solids 363 8.1 29.4 35 0.101 0.36 29 16.78 60.8 86 Drained liquid 229 9.8 22.4 26 0.101 0.23 18 7.24 16.5

    a Feaster et al. ( 1949). &This assumes that the weight of the peas does not change during blanching. per cent total solids in raw peas retained in blanched peas or drained solids of canned peas.

  • THE BLANCHING PROCESS 81

    peas. The tests showed that 0.25% Nacconol wash (at various times and temperatures) preceding the steam blanch had no deleterious effects on ascorbic acid retention.

    8. CHLOROPHYLL

    It has been said that blanching sets the color in vegetables to be frozen. This is a descriptive but a none too scientific expression. I t is hard to see how the green chlorophyll pigment or the yellow pigments could be set by the heating process. It is possible that during this treat- ment combinations of chlorophyll with complex substances are altered, which, together with physical changes of the tissues, could perhaps account for the more intense green color.

    Magoon and Culpepper (1924) quoting Willstatter and Stoll noted that when green leaves are heated in water, the chloroplasts become swollen and distorted, or may even burst, and the green color becomes more or less diffused throughout the cell. In the fresh green leaves the chlorophyll is in a colloidal state, but when the temperature is raised by the scalding in hot water, the chlorophyll passes into a true solution in the waxes within the cells.

    Chlorophyll is insoluble in water, and therefore, does not leave the cells unless the cell walls are ruptured or destroyed. It seems, therefore, that scalding does not bring the color to the surface of the green vege- tables, and, since cooling merely hardens the cell waxes, plunging the freshly scalded vegetable into cold water does not bring about any changes in the chlorophyll which make it more resistant to chemical transformation by the heat of the subsequent sterilization process.

    Mackinney and Weast (1940), however, found contrary to the con- clusions of Willstatter and Stoll, that the plastids, originally turgid and arranged around the periphery of the cell, become shrunken and clumped in the center of a mass of coagulated protoplasm. The analogy of grape and raisin roughly illustrates this difference. With material sub- jected to low-temperature blanching and with living sections, great care must be exercised since even slight pressure on the cover glass results in the discharge of the contents of the plastids into the cell which becomes uniformly green throughout,

    These observations are best made with chlorophyll-rich material, such as spinach leaves. Similar results, however, were obtained with string beans. After one hour in water at 100C. (212OF.) the plastids of the bean were recognizable though deformed in shape. They were still the only pigmented bodies on the microscope slide. These authors concluded that Willstatter and Stolls results are incorrect except pos- sibly with leaves of high oil content. They further noted that these

  • 82 FRANK A. LEE

    observations do not apply to unblanched frozen material. In the case of several different species of fresh leaves that were subjected to freezing, the chloroplasts ruptured. None were observed intact.

    Although the temperature of boiling water 100OC. (212OF.), is usually employed for the blanching of vegetables, other temperatures are sometimes recommended. In the canning of spinach, Thomas (1928) obtained a patent, in which the vegetable was wilted at the maximum temperature of 71C. (160OF.). It was said that at this temperature the formation of pheophytin from chlorophyll would be insufficient to appre- ciably affect the natural color of the vegetable, It is true that spinach processed in this fashion does yield a product which has a slightly better color in the area of the stems after canning, but one must con- sider that the high temperatures of sterilization used in the cooking process will most certainly affect the chlorophyll. A disadvantage of the lower temperature of blanch is to be found in the incomplete release of tissue gases. In the No. 10 cans, this can be a problem.

    Mackinney and Weast (1940) found that a substantial part of the chlorophyll in frozen-pack peas and string beans is converted to pheo- phytin. In simple cases, where the vegetable is treated for successive times in water at various temperatures, the formation of pheophytin is an interrelated function of time and temperature. Translated into indus- trial blanching practice, this step in itself is not prolonged to the point where serious impairment of color is obtained. This happens during sub- sequent handling prior to consumption. Since blanching in many cases does have a beneficial effect on color retention, it was suggested that an adequate blanch must remove a large proportion of those volatile and water-soluble components which would react with chlorophyll dur- ing subsequent cooking.

    The evidence for pheophytin formation lies not only in the spectro- scopic data but also in the behavior of ethereal solutions to hydrochloric acid and to dilute alkali, and finally upon the cleavage products obtained on hot saponification, namely, chlorine and rhodin g.

    The frozen-pack string bean, on cooking, has between 60 and 85% of its chlorophyll converted to pheophytin regardless of pretreatment. The canned beans examined apparently contained no unchanged chlorophyll.

    Frozen-pack peas, after cooking, may still appear bright green if they were adequately blanched previous to freezing. A minimum

    blanching temperature for 2 minutes is approximately 75OC. ( 167OF. ) . If inadequately blanched, the cooked pea contains about 80% of its green pigment in the form of pheophytin; if adequately blanched, the value is between 50 and 60% after cooking.

    It was found upon analysis immediately after thawing, that blanch-

  • 83 THE BLANCHING PROCESS

    ing of peas for 2 minutes a t 100OC. (212OF.) resulted in the formation of only 7% of pheophytin. When these peas were cooked, they had a bright green color.

    Dutton et al. (1943) investigated the effects of length of blanch in flowing steam on the chlorophyll content of spinach processed by dehy- dration. Blanching and dehydration was shown to have an effect on the chlorophyll content. Unblanched spinach showed a conversion of 26% of its chlorophyll to pheophytin during the process of dehydration. However, material blanched for 2 minutes followed by dehydration showed a conversion of 44% of its chlorophyll to pheophytin. Blanching for 4 minutes gave 46% and blanching for 6 minutes gave 50% of the chlorophyll converted to pheophytin.

    Talburt and Legault (1950) made a study of the effects of blanching on dehydrofrozen peas. They concluded that the conversion of chloro- phyll to pheophytin is a good index of color deterioration and is correlated closely with subjective color. Conversion of chlorophyll during blanching increased with increasing blanch time, ranging from 2% for a 38-second blanch to 8% for 120 seconds. During drying the losses of chlorophyll were slight where reductions in weight did not exceed 50% of the raw weight.

    Crude fiber and sugar data indicated progressive leaching losses with increasing blanch time. Organoleptic data from the test immedi- ately after processing showed that neither the blanching times nor the weight reductions employed had much effect on the flavor or sweetness of dehydrofrozen peas.

    After 6 and 12 months of storage at -23OC. (-10OF.) characteristic flavor, objectionable off-flavor, and sweetness were not seriously affected by the blanching time or the extent of drying when the samples were assayed for organoleptic quality.

    9. SULFUR COMPOUNDS

    Diemair and Koch (1940) studied the separable sulfur compounds and their importance in the preservation of vegetables. During process- ing of vegetables volatile sulfur compounds split off. The amount of volatile sulfur compounds released depends on the type of vegetable and the blanching process. The volatile sulfur compound dissolves in the liquid and can be determined by means of a distillation procedure. With asparagus, peas, and kohlrabi little is released, so that the pre- served products contain more than the fresh. More volatile sulfur is released by spinach and beans, but with open-kettle processing, enough is lost so that these final canned products contain less than the fresh.

  • 84 FRANK A. LEE

    10. ENZYMES

    A considerable amount of work has been done on enzymes in frozen vegetables, and their destruction during the blanching process. How- ever, no single enzyme or combination of enzymes has been conclusively proven to be the cause of the development of off-flavor in unblanched material.

    Diehl and co-workers (1933) concluded that enzymes can cause off- flavors even when foods are frozen. Table XI11 shows the inactivation of catalase after various times of blanching.

    TABLE XI11 Relation of Catalase Activity to Index of Heat Penetration in Alderman Peasa

    Index of Index of Index of Scalding Catalase penetra- Catalase penetra- Catalase penetra-

    reaction tion b reaction (seconds) reaction c tion b tion

    Temp. 212F. Temp. 190F. Temp. 160F.

    10 +++ .13 ++++ 0.15 20 ++ .54 +++ 0.28 25 + .60 30 - .92 ++ 0.38 40 - 1 .oo + 0.84 50 - 1 .oo - 1.00 60 - 1.01 +++ .12 75 + .34

    _ _ -

    90 + .36 105 - .86

    Q Diehl et al. (1933). &The index of penetration is the ratio of twice the width of the darker green

    penetration area of heated pea tissue (determined by measuring across the inner face of the cotyledon) to the entire diameter of the cotyledon.

    c I n larger peas +; in smaller peas -.

    Joslyn and Marsh (1938) concluded that the scalding period for vegetables to be frozen should be long enough to inactivate enzymes responsible for deterioration yet not long enough to soften the texture to such an extent that further cooking will make the vegetable too soft to be palatable or to alter the flavor undesirably.

    The qualitative test for catalase may occasionally be deceptive in determining the proper blanching time, and it is not applicable for all products. The peroxidase test may be used as an additional index of proper blanching in some cases. Phenolase test, for vegetables and fruits which discolor, is the most definite index of adequate blanching.

    Campbell (1940) concluded that the qualitative test for catalase

  • THE BLANCHING PROCESS 85

    activity in scalded cut corn is not sufficiently reliable to warrant its use as an index of scalding efficiency. The peroxidase test for adequacy of scalding, carried out with full knowledge of its limitations, appears to be a useful index in the scalding of cut corn.

    Morse (1949) ran tests to determine the effectiveness of triphenyl- tetrazolium chloride as an indicator for blanching. As a result of these tests, he concluded that: Examination of the data collected shows that the enzyme system responsible for the reduction of triphenyl-tetrazolium chloride to triphenyl formazan is more easily inactivated by heat than either the catalase or peroxidase systems. Since the basis for use of an enzyme test for determination of adequate blanching of dried or frozen foods is the inactivation of the most heat stable enzyme, it is unlikely that the dehydrogenase test has much application to the blanching of dehydrated or frozen food.

    Cobey and Manning (1953) discussed catalase versus peroxidase as an indicator for adequacy of blanching of frozen vegetables, Peroxidase is a heat resistant enzyme, and allows a greater margin of safety in blanching than catalase on most vegetables. A positive-result peroxidase test does not always indicate that the vegetable is underblanched. A few vegetables often have a positive peroxidase reaction after proper blanch- ing, such as snap beans and asparagus. A positive catalase reaction on asparagus, snap beans, carrots, broccoli, cauliflower, kale, peas, spinach, and squash indicated that the product was underblanched and would deteriorate during storage.

    It was found that samples with a positive-reaction peroxidase test on snap beans, squash, asparagus, spinach, and kale held up well in storage if the catalase test on these vegetables was negative. Further blanching of these items was not necessary for quality retention during storage, and a continued blanch sufficient to obtain a negative peroxidase reaction on many of these would overblanch them, causing a loss of color, flavor, and nutritional value.

    Another disadvantage of the peroxidase test is the regeneration of peroxidase during storage. Properly blanched vegetables that had nega- tive catalase and peroxidase reactions soon after blanching may have a positive peroxidase and a negative catalase reaction after freezing and storage, as was pointed out by Campbell, and by Joslyn and Marsh. The peroxidase test should not be depended upon as an indicator after storage of a product because of the tendency toward reactivation of this enzyme.

    Masure et al. (1953) found that a peroxidase test used on Brussels sprout samples served as a satisfactory index of adequacy of blanching for this commodity and offered a good margin of safety in the blanching operation.

  • 86 FRANK A. LEE

    It was further observed that the addition of a 0.5 to 3% solution of hydrogen peroxide to the cut surfaces of Brussels sprout halves brought about a rapid development of a pinkish-orange color in underblanched materials. The extent of the color development was found to parallel roughly the peroxidase content of the samples, Because of the parallel- ism between peroxidase content and color development by hydrogen peroxide, it was suggested that the latter be used instead of the peroxi- dase test as a simple, rapid test for adequacy of blanching in Brussels sprouts to be preserved by freezing.

    Cruess (1947) made a survey of ten varieties of frozen vegetables from the market of the San Francisco-Berkeley area. These packages were, in general, found to be rather seriously underblanched. Many of these samples had developed hay-like odors and flavors, in some cases rendering the products almost inedible. The author recommended that most of the vegetables, particularly peas, snap beans, and lima beans be blanched until the peroxidase test is negative.

    McColloch et al. (1952) found that surface localized pectic enzymes were inactivated by blanching. Pectic enzymes in ripe Pearson and San Marzano variety tomatoes occur in greatest quantity near the surface of the fruit; therefore a large proportion of such enzyme activity may be destroyed prior to crushing by passing whole fruit through a steam or hot water blanch. Such treatment can improve the retention of pectic substances during processing, and tomato products of higher consistency may be obtained.

    Wagenknecht and associates (1952) postulated that the enzymes lipoxidase and lipase and the large increase in acid number of the lipids were the causative agents for chlorophyll breakdown during the frozen storage of the raw peas. Lee (1954) studied the crude lipids prepared from raw asparagus and raw lima beans which had been held in frozen storage at -18OC. ( O O F . ) for extended periods of time. The crude lipids from these two vegetables were low in peroxide numbers even after long storage. He cited references which indicated that these vege- tables are low in lipoxidase, and which could, in turn, account for the low peroxide numbers.

    The method making use of the time necessary to inactivate catalase and allowing an additional 50% of the inactivation time as a safety factor, has produced good results for vegetables to be frozen,

    Ill. SPECIAL TECHNIQUES FOR BLANCHING

    1. STEAM PRESSURE Woodroof et al. (1948) reported that scalding vegetables under 10.

    pounds of steam pressure resulted in products which were not graded high on flavor.

  • THE BLANCHING PROCESS 87

    2. ELECTRONIC BLANCHING Moyer and Holgate (1947) concluded that the use of a stream of

    refrigerated air seems to be more suitable for cooling electronically blanched vegetables than fluming or water spraying methods.

    a. Weight Changes

    Although snap beans were water-blanched for a shorter period of time than lima beans, the weight loss was greater. Small weight losses of electronically blanched snap beans and lima beans were due either to moisture vaporization or exudation of cellular juices. Weight losses in blanching are regained by cooling in flumes or under water sprays. Weight increases after water cooling may be attributed to water adher- ing to the vegetables or to an increased water-holding capacity of vegetable colloids. Moisture uptake varies with method of blanching and type of vegetable. Air cooling resulted in weight loss due to the drying action of the air stream. Drying, in this case, might be overcome by atomizing water into the cold air before cooling vegetables.

    Moyer and Stotz (1945) discussed the use of radio-frequency power for the blanching of vegetables, Making use of a radio frequency of 150 megacycles, experiments were conducted on the heating of cabbage with an oscillator having an output of 750 watts. At this high frequency there is little tendency to arc if the vegetable is tightly packed into the container. In testing the effectiveness of dielectric heating, heads of cabbage were cut into slices an eighth of an inch thick, and the shredded material packed into Peters-type cartons commonly used for the freezing of vegetables. A carton of shredded cabbage was placed between two copper electrodes mounted in an electric air oven. An oven temperature of 100C. (212OF.) was used to prevent condensation of moisture on the electrodes and to counteract heat losses from the carton by radiation. A heating period of two to three minutes was sufficient to raise the temperature of 180 grams of cabbage to 99OC. (210OF.) as indicated by a spirit-filled thermometer inserted in the carton.

    As an indication of the small nutrient loss that may be expected in blanching with radio-frequency power, the ascorbic acid contents of raw and of water-, steam-, and electronically blanched cabbage samples were determined. The blanching periods were in each case of minimal dura- tion to insure a negative catalase test. See Table XIV.

    The same lot of shredded cabbage was used for all three blanching operations, and 35-gram portions were taken for analysis to ensure adequate sampling. The water and steam treatments were carried on simultaneously, hence only a single analysis of the raw material sufficed as a reference. Since the electronic blanching was performed an hour later and raw shredded cabbage loses ascorbic acid on standing, a second

  • 88 FRANK A. LEE

    TABLE XIV Loss of Ascorbic Acid during Blanching by Boiling Water, Steam,

    and Radio-Frequency Power a

    Blanching Ascorbic acid Loss on time content blanching Sample

    ( minutes ) ( mg. / g . ) ( % )

    Raw I 0.38 Stearn-blanched 2.5 0.26 32 Water-blanched 0.75 0.23 40 Raw I1 0.34 Electronically blanched 2.5 0.33 3

    a Moyer and Stotz (1945).

    raw sample was analyzed immediately prior to radio-frequency applica- tion. The effect of storage on these samples was not discussed. This was considered in the following later paper.

    Moyer and Stotz (1947) found it was feasible to blanch a variety of vegetables by electronics with a favorable retention of ascorbic acid and carotene. Unless adequate provision can be made for rapid cooling of the blanched material in the carton, it would seem necessary to adopt some other methods of applying radio-frequency power such as by placing the material on a belt and, immediately after blanching, remov- ing the heat in a refrigerated air stream to avoid deterioration in storage.

    In all cases, all of the electronically blanched samples of peas after six months storage at -23OC. (-1OOF.) were poorer in flavor than those water- or steam-blanched. Deterioration in vitamin C content of potatoes was noted when they were blanched electronically for 3 minutes a t 100C. (212OF.) and cooled in the carton at -23OC. (-10F.). Under these conditions a 64% loss of ascorbic acid resulted, when con- trasted with the samples analyzed immediately after radio-frequency blanching of 7% loss of ascorbic acid.

    Samuels and Wiegand (1948) carried out blanching experiments on cut corn and Elberta peaches using a radio-frequency generator. The equipment used was a Mann-Russell Electronic Heater, 3 kilowatts, 3300 B.T.U. output per hour and operated at 27 megacycles.

    Cut corn blanched in boiling water gave a 35% loss of ascorbic acid compared to as low as a 10% loss by blanching with radio-frequency applications using air as a medium. Both the water-blanched and elec- tronic blanched material were cooled in running water for 15 seconds.

    The loss was further reduced 10% in both types of blanching by cooling in a -29OC. (-20OF.) air blast for five minutes before pack- aging. Cut corn blanched with radio frequency and cooled in a -29OC.

  • TABLE XV Effect of Blanching with Radar Oven, Boiling Water, and Steam on Ascorbic acid^ Content of Vegetablesb

    Ascorbic acid in R

    Method of m carrots Spinach Peas Green beans Broccoli r ~ * blanching Amt. Amt. Amt. Amt. Amt. ( mg. (mg.1 Retained Retained 3 E ( a )

    Retained (mg.1 Retained (mg.1 Retained (mg.1 1009.) ( % ) loop.) ( % ) l0Og.) ( a ) l0Og.) l00g.)

    r j

    ; 00

    Fresh 4.42 - 14.9 - 18.0 - 10.0 - 93.0 - Radar 4.50 100 14.6 98.0 22.0 100 10.5 100 95.0 100 Boiling water 1.62 36.7 3.5 23.5 16.7 92.8 7.3 80.0 81.4 87.5 Steam 3.56 80.4 5.6 37.6 18.1 100 9.8 98.0 87.0 93.5 rn

    =Expressed on a fresh moisture basis. b Proctor and Goldblith ( 1948).

    0 3 W

  • 90 FRANK A. LEE

    (-2OOF.) air blast for 5 minutes gave practically no ascorbic acid loss. After nine months storage at -18OC. ( O O F . ) , the samples were rated

    by organoleptic tests. From these data it can be stated that cut corn blanched in an air medium with radio frequency and cooled in a -29OC. (-20OF.) air blast for five minutes equals boiling-water blanched cut corn in flavor and color.

    Blanching peaches by radio frequency is effective but has a dele- terious effect on the volatile fruit flavors. Application of any appreciable amount of heat results in a cooked flavor,

    Proctor and Goldblith (1948) conducted experiments using an oven heated only by the emanations of a magnetron tube generating micro- waves at a frequency of approximately 3000 megacycles having a wave- length of 10 cm. with a power input of 2000 watts to the food. The effects of blanching with the radar oven, with boiling water, and with steam on the ascorbic acid content of vegetables are given in Table XV. The boiling water and steam blanch (Table XVI) are those recom- mended by Tressler and Evers (1947).

    TABLE XVI Optimal Times Needed to Blanch Vegetables in Radar Oven, Boiling

    Water, and Steam=

    Optimal time for blanching Radar oven

    ( S W . ) Boiling water b Steam b ( min. )

    Vegetable ( min. )

    Spinach Carrots Peas Green beans Broccoli

    20 25 25 20 30

    2.5 3.0 1.0 2.0 3.0

    3.5 3.0 2.0 2.0 4.0

    a Proctor and Goldblith ( 1948). b From Tressler and Evers ( 1947).

    It is apparent that practically no loss was sustained in ascorbic acid content by any of the several vegetables when subjected to radar treat- ment for the time periods necessary to raise the temperatures of the vegetables sufficiently to inactivate their enzymes. Steam-blanching of the same material was far more efficient than hot water in this respect but less efficient than the radar treatment for some commodities, notably spinach and carrots.

    A comparison of the ascorbic acid contents of a different 100-gram sample of fresh, unblanched spinach and a 100-gram sample of radar-

  • THE BLANCHING PROCESS 91

    blanched spinach from the same lot was separated, after blanching, into its liquid and solid portions before vitamin assay gave the following results :

    Ascorbic acid ( mg. )

    Fresh, unblanched spinach Juice, radar-blanched Solids, radar-blanched

    38.20 2.13

    32.38

    This comparison shows that only a small amount of ascorbic acid was contained in the juice, the major portion being in the solids.

    A like comparison was made with a 100-gram sample of fresh, un- blanched spinach and a 100-gram sample of spinach from the same lot blanched in boiling water. In this case vitamin assays were made not only on the water-blanched solids but on the blanching water used, with the following results:

    Ascorbic acid ( mg. )

    Fresh, unblanched spinach Blanching water Solids, water-blanched

    38.20 28.30 9.00

    The retention of ascorbic acid in the solids of the radar-blanched spinach was strikingly greater than that in the solids of the water- blanched spinach. Almost all the ascorbic acid apparently lost by spinach solids in the water-blanching procedure may be accounted for in the blanching water, which would normally be discarded.

    3. SPECIAL TREATMENTS Holmquist et al. (1954) concluded that an overall analysis of the

    blanching operation shows that it accomplishes two things simul- taneously. ( I ) The desirable actions of enzyme inactivation and of washing to remove dirt, bacteria, and entrapped air; (2 ) The undesir- able action of leaching out a portion of the essential nutrients in the natural product. This was discussed further by Holmquist et al. (1955).

    Consideration of these factors led to the view that blanching might better be done in two steps. One step would perform the function of washing. The other step would remove the gases and inactivate the enzymes, A greater retention of nutrients would thereby be effected.

    The present overall industry conclusion is that the conventional hot- water blancher still accumulates a high level of flat sour organism contamination.

  • 92 FRANK A. LEE

    In the course of the bacteriological surveys, samples of peas entering the steam blancher and the water blancher were tested for thermophilic flat sours, all being found negative. The contamination of the peas leav- ing the blancher was measured by plate counts upon the washing from the peas and by a processed tube test. There is a significant difference between the spore counts of peas leaving the water blancher in both the 1949 and 1950 seasons (Table XVII). Of the samples taken from

    TABLE XVII Summary of Distribution of Spore Counts of Samples of Peas Leaving Steam

    and Hot-Water Blanchers in Two Different Yearsa

    Per cent of total samples

    Spore count 1949 1950 per 303 can

    Water Steam Water Steam blancher blancher blancher blancher

    ~ ~~ ~~

    0 2 69 25 85 1-30 2 24 35 12

    31-60 2 4 10 3 61-100 16 2 20 0

    1014300 36 0 10 0 300+ 41 0 0 0

    Total samples 39 89 20 34

    a Holmquist et al. (1954).

    the steam blancher during the 1949 and 1950 seasons, 69 and 85% respec- tively, showed no spore count from the washings. This compares with 2% and 25% showing no spore count in the water-blanched samples for the same years. Also, a significant difference was found between the flat sour contamination of peas blanched in the steam bIancher and those conventionally water-blanched (Table XVIII ) . It can be seen that very high contamination resulted from waier-blanching.

    TABLE XVIII Results of Processed Tube Tests Made on Samples of Peas while Emerging

    from Steam and Hot-Water Blanchers a

    Total number Number of tubes Per cent of tests positive positive

    Steam blancher 1949 147 1950 96

    1949 153 1950 102

    Hot-water blancher

    4 I

    119 42

    3 I

    78 41

  • THE BLANCHING PROCESS 93

    One of the chief objections to steam-blanching of peas has been the viny- or grassy-like off-flavor in the final product as compared with peas conventionally water-blanched. Preliminary tests confirmed that a good hot-water wash prior to steam-blanching eliminated the off -flavor. But this practice essentially introduces a dual blanching operation with the same potential, undesirable bacteriological problems as now exist in conventional hot-water blanchers. Therefore, it was considered highly desirable to remove the material carrying the off-flavor by means of a suitable detergent in cold water. No significant raw- or viny-, or grassy- like off-flavor remained after washing with detergent for 1 min. at 24OC. (75OF.). Nor did any of the detergents used produce significant off-flavors in the peas. No significant difference was noted between Nacconol NR and Nacconol SX in concentrations of 0.10% and 0.25% Detergent washing at 71OC. ( 1 6 0 O F . ) appeared to be more effective in eliminating the off-flavors than detergent washing at 24OC. (75OF.). The difference, however, was not sufficient to justify a 71OC. ( 1 6 0 O F . ) wash.

    Treating peas with sodium hexametaphosphate in the preliminary wash to improve the skin texture also significantly reduced the viny- or grassy-like off -flavors. Tests were made using various concentrations of sodium hexametaphosphate at both 7 5 O and 16O0F. for '/2 min. and 1 min. intervals. With the same concentration, the hot-water wash was more effective than the cold-water wash. But increasing the concentra- tion of sodium hexametaphosphate in the cold-water wash from 0.3 to 1.0% effectively softened the skins. Since there was no significant dif- ference between the sodium hexametaphosphate-washed peas and the detergent-washed peas, the studies were made on the basis of a l-min. 24OC. (75OF.) detergent of sodium hexametaphosphate wash prior to steam blanching.

    The specific gravity of peas blanched by steam is higher than that of those blanched by boiling water. The leaching of a smaller amount of water-solubles from the peas is one factor in explaining why their density is higher after steam-blanching than after hot-water blanching. It seems, therefore, that steam-blanched peas were superior in many respects to peas blanched in the conventional water blancher.

    Nielsen et al. (1943) reported that blanching in a solution of sodium hexametaphosphate brings about tenderization of vegetables so treated. It was stated that temperatures above 93OC. ( 2 0 0 O F . ) might not be economically feasible because of inversion of this chemical to sodium dihydrogen phosphate.

    Holmquist and associates (1948) showed by laboratory and field studies that the addition of sodium hexametaphosphate in amounts chemically equivalent to the amounts of calcium and magnesium in the blanch water will result in a definite softening of the skins of peas. It

  • 94 FRANK A. LEE

    is indicated that addition of amounts of sodium hexametaphosphate in excess of the quantity chemically equivalent to the hardness results in further softening of the peas. The addition of an excessive amount of sodium hexametaphosphate results in a corresponding degree of over- softness of the peas, and cloudiness to viscous brines. Careful control of the concentration of sodium hexametaphosphate in the blanch water must be maintained in order to obtain satisfactory results.

    While the experimental work shows that the use of sodium hexameta- phosphate may be of considerable value when blanching peas in hard water, it also indicates that careful control of the concentration of this compound is necessary in order to avoid undesirable conditions such as excessive cloudiness, viscosity, or even gelling of the brine. These con- ditions may occur if the concentration of sodium hexametaphosphate in the blanch water becomes too high.

    I t should be mentioned that it has been found that blanching water having a hardness of more than 10 grains, calculated as calcium car- bonate ( CaCO,), will result in marked toughening of the skins of peas (see Table XIX). These data indicate that the use of sodium hexameta-

    TABLE XIX Experimental Pack of Alaska Peas Blanched 5 Minutes at 200F. in Water

    Containing Various Amounts of Sodium Hexametaphosphate a

    Sodium hexametaphosphate in blanch water (S) Effect on texture

    Control Firm with tough skins 0.1 Very slightly more tender

    0.2 Considerably more tender than controls

    0.3

    0.3 ( in brine)

    than controls Soft in texture, but not objectionable since liquor still reasonably clear Soft and mushy, brine gelatinous

    a Holmquist et al. (1948)

    phosphate in blanch water or brine resulted in marked softening of the pea skins and cotyledons. Because of the extreme softening which took place in the sample to which sodium hexametaphosphate had been added to the brine, it was decided to forego any further tests by this method of treatment (Table XX).

    Lee and Whitcombe (1945) concluded, aside from the fact that vegetables blanched in hard water showed significant increases in cal- cium content, that the use of different types of potable water for blanch-

  • THE BLANCHING PROCESS 95

    TABLE XX Experimental Packs of Prince of Wales Type Peas Using Sodium Hexametaphosphate,

    Sodium Carbonate, and Sodium Bicarbonate in Blanch Water (16 g. CaCO, hardness) a

    Texture rating

    4 = firm Blanch water treatment 0 = soft Brine

    Control None 4 Normal 0.1% Sodium hexametaphosphate 334-4 Normal 0.2% Sodium hexametaphosphate 3 Normal 0.5% Sodium hexametaphosphate l?i Viscous 0.1% Sodium hexametaphosphate 3?1 Normal 0.2% Sodium hexametaphosphate 3 Normal 0.3% Sodium hexametaphosphate 1 Viscous 0.08% Sodium carbonate 4 Normal 0.16% Sodium carbonate 3:: S1. dark 0.4% Sodium carbonate 3 Dark brine 0.1% Sodium bicarbonate 4 Normal 0.25% Sodium bicarbonate 3?1 Normal

    a HoImquist et al. ( 1948).

    ing resulted in no significant differences in vitamin or mineral content of the blanched, stored, or cooked peas and snap beans. The types of potable water used were: distilled water, hard water (500 p.p.m. tem- porary, 100 p.p.m. permanent), tap water rendered safe by the addition of chlorine (0.25 p.p.m.) and water containing iron (1 p.p.m.).

    IV. RECENT STUDIES ON FUNDAMENTAL CHANGES DURING FROZEN STORAGE OF UNBLANCHED MATERIAL

    Recently, investigations have been conducted to determine the causes for the development of off -flavors, colors, and aromas when unblanched and underblanched vegetables are held in frozen storage. Joslyn and associates found that acetaldehyde and ethyl alcohol increased during the storage of unblanched and underblanched peas.

    In unblanched or underblanched fresh peas stored at -17OC. ( 1F.), acetaldehyde accumulated during storage. In fresh peas held under anaerobic conditions, acetaldehyde also increased. Blanching at times and temperatures sufficient to inactivate enzymes responsible for; off-flavor formation resulted in reduction in acetaldehyde formation. Peas which were underblanched before freezing showed an increase in their acetaldehyde content after defrosting and storage under anaerobic conditions (Table XXI) ,

    Ethyl alcohol formation and accumulation paralleled that of alde-

  • 96 FRANK A. LEE

    TABLE XXI Effect of Blanching ( 2 Minutes) and Freezing-Storage at -17C. for

    10 Years on the Aldehyde Content of Peasa

    Blanching temperature Aldehyde content ( " C . ) (mg. % )

    No Blanch 3.58

    Blanched 2 minutes 60.0 71.0 76.7 82.2 87.8 93.3

    100.0

    2.47 1.60 1.40 1.69 1.25 1.16 1.06

    a Joslyn and David (1952).

    hyde but occurred at faster rates so that the alcoho1:aldehyde ratio increased. On anaerobiosis the alcohol content increased five- to eight- fold in comparison with a two- to threefold increase in aldehyde con- tent (Table XXII).

    TABLE XXII Effect of Blanching ( 2 Min.) and Freezing-Storage at -17C. for

    10 Years on the Ethyl Alcohol Content of Peasa

    Blanching temp. Ethyl alcohol content ( "C.) (mg. % 1

    No blanch 154.2 60.0 46.9 71.0 16.9 76.7 21.9 82.2 15.8 87.8 14.5 93.3 14.8

    100.0 16.1

    a Joslyn and David ( 1952).

    The accumulation of volatile aldehydes in the tissue of raw and underblanched peas was reported by Joslyn et al. Arighi, Joslyn, and Marsh (1936) found that the total quantity of aldehyde present decreased with decrease in residual catalase activity, being least in peas which retained flavor and highest in off-flavored samples. Joslyn et al. (1938) and Joslyn and Bedford (1940) confirmed these observations, and found that high aldehyde content in peas that were unblanched or underblanched correlated with residual catalase and peroxidase activity

  • THE BLANCHING PROCESS 97

    as well as with off-flavor formation. Acetaldehyde was identified as the chief aldehyde component, but acetyl methyl carbinol, diacetyl as well as other aldehydes were found. The only alcohol which accumulated was ethyl alcohol.

    The carboxylase activity of broccoli tissues was investigated by Buck and Joslyn (1956) to determine the role of pyruvic carboxylase in the production of volatile aldehydic and ketonic compounds which might serve as precursors of the off-flavors developing in underscalded frozen broccoli. Broccoli carboxylase was found to catalyze synthesis of acetoin and diacetyl chiefly from added pyruvate and to a much smaller extent from acetaldehyde, unlike pea and wheat germ carboxylase. Acetalde- hyde inhibited broccoli carboxylase activity. In frozen broccoli, both inhibition by and restricted diffusion of acetaldehyde would favor pro- duction of acetoin. The concentration of acetaldehyde, acetoin, or diacetyl was not related to the organoleptically objectionable formation of off-flavors, whereas ethyl alcohol content was related to extent of off-flavor. These data are given in Table XXIII.

    TABLE XXIII Acetaldehyde, Ethyl Alcohol, Acetoin, and Diacetyl Formation during

    Storage of Frozen Broccoli Shoots a

    Blanching Storage Products, mg. / 1000 g. broccoli tissue 212F. period Minutes 0F. Acetaldehyde Ethyl alcohol Acetoin Diacetyl

    0 0 0 2 months 1 2 months 1 2 months 4 2 months 0 6 months 0 1 year 0 7 years 1 7 years 2 7 years 3 7 years 4 7 years

    0.56 1.40 0.87 0.67 0.45 0.97 0.93 2.30 1.43 0.96 0.75 0.77

    31.1 30.6 27.1 12.0 9.8

    18.2 26.3

    133.0 37.3 24.2 15.7 14.2

    0 0 0 0 0 0 0 0 0 0 0 0

    0 0.072 0 0 0 0 0 0.42 0.25 0.251 0.03 0

    a Buck and Joslyn (1956).

    In this article, while it was noted by the authors that the amount of ethyl alcohol found was related to the extent of off-flavor, it is obvious that this substance, per se, in the amounts formed could have little actual influence on the flavor of the broccoli.

    Lee (unpublished data, 1947) found that pea slurries extracted with organic solvents gave lipid materials which differed in taste and appear-

  • 98 FRANK A. LEE

    ance depending on the source. Those from blanched peas were mild and pea-like ir. character. Those obtained from raw peas, of the same lot, were off-flavor. Both of these samples of peas had been held in frozen storage at -18OC. ( O O F . ) for two years. This indicated that the lipid fraction might be responsible for the off-flavors which developed.

    This was investigated further by Lee and associates. Lee and Wagenknecht ( 1951 ) prepared the samples by lyophilization, followed by grinding and extracting in Soxhlet equipment with anhydrous, perox- ide-free, ethyl ether during this study for 48 hours, but in later work for 24 hours. Tests of the extracts obtained from raw peas showed them to be rancid in character with high peroxide numbers and high titratable acidity, The lipid from blanched peas gave zero peroxide and low acid values. They reported that rancidity of the lipid matter was one of the primary, if not the principal cause of the development of off-flavor in unblanched frozen peas during storage at -18OC. ( O O F . ) . The peas described in this test had been held in frozen storage for five years.

    These authors showed also that little change took place in the sugars and total nitrogen between the blanched and unblanched peas. This is illustrated in Table XXIV.

    TABLE XXIV Effect of Storage on Major Components of Frozen Thomas Laxton Peas0

    (A11 samples analyzed July, 1950) b

    Total Reducing Total Year Treatment solids sugars Sucrose Starch nitrogen

    harvested years ( 9) ( glucose ) (9) ( % ) ( % )

    1950 0 Unblanched 19.61 Negative 26.72 15.68 4.42 1950 0 Blanched 17.34 Negative 25.82 15.73 4.44 1945 5 Unblanched 21.26 Trace 26.64 18.85 4.59 1945 5 Blanched 16.85 Negative 22.84 18.12 4.62

    a Lee and Wagenknecht ( 1951 ). b All results expressed on the dry weight basis.

    Their results showing indications of rancidity of the extracted lipid matter are given in Table XXV.

    In addition to this, Wagenknecht et nl. (1952) showed that consider- able destruction of the chlorophyll takes place during the storage of frozen unblanched peas. The data are given in Table XXVI. The de- terioration in the lipid material in unblanched vegetables was shown to be progressive.

    1 PP and co-workers (1955) found that unblanched peas, corn, and snap beans showed definite development of off-flavor in frozen storage,

  • THE BLANCHING PROCESS 99

    TABLE XXV Peroxide Number and Acid Number of Crude Lipid Material Extracted

    from Thomas Laxton Peas a

    Year Storage Peroxide c Acid d harvested b years number number

    1950 0 Unblanched Negative 22.6 1950 0 Blanched Negative 23.0 1945 5 Unblanched 27.3 97.2 1945 5 Blanched Negative 18.6

    Treatment

    a Lee and Wagenknecht ( 1951 ) . b All samples analyzed July, 1950. c Millimoles of peroxide oxygen in 1 kg. of lipid. d Milligrams KOH (potassium hydroxide) in 1 g. of lipid.

    TABLE XXVI Effect of Blanching on Chlorophyll Content of Alderman Peas a

    Chlorophyll, nig. per 100 g. dry weight

    1950 crop 1945 crop Treatment of sample

    Unblanched 54.1 48.4 Blanched 68.8 61.7

    awagenknecht et al. (1952).

    which could be detected by a taste panel in from two to four weeks of storage. Furthermore, the crude lipid extracted from these unblanched vegetables showed a definite increase in acid after the vegetables were stored for one week. The rise continued during long storage, but the main increase took place in the early months of storage. The crude lipid extracted from frozen unblanched vegetables showed a positive test for peroxides after the vegetables had been held in frozen storage for periods of time as follows: peas, three weeks; snap beans, one month; sweet corn, three months. These results are shown in Figs. 1, 2, and 3.

    Lee (1954) found that corn, snap beans, and spinach were similar to peas in that high peroxide values and high or fairly high acid num- bers were noted when the crude lipids extracted from the unblanched frozen-stored vegetables were analyzed. Similar crude lipid prepared from asparagus gave a high acid number, but no peroxide value except in very old samples, whereas the crude lipid extracted from raw lima beans gave low peroxide and low acid values. It is interesting to note that asparagus and lima beans are low in lipoxidase, which is postulated as the cause of the development of the peroxides. This will perhaps expiain the failure of peroxides to appear in quantity.

  • 100 FRANK A. LEE

    raw

    I I I I I I I 2 3 4 5 6

    WECKS OF STORAGE AT OF 617.8OC)

    FIG. 1. Thomas Laxton peas; 1954 samples.

    Increase in acid number and peroxide number of crude lipid from

    0 _ _ _ _ _ 4- - - - - - - - - - -0- - - -- - - - - I 1 I 1 I t

    I 2 3 4 5 6 WEEKS OF STORAGE AT Oo F (-17.8O C )

    FIG. 2. Increase in acid number and peroxide number of crude lipid from raw Wade snap beans; 1954 samples.

    The crude lipid from corn which was blanched before having been placed in frozen storage showed a low peroxide value after I$/, years' storage. This suggests that nonenzymatic peroxidation is perhaps taking place.

    Lee et al. (1956) made a study of the chemicaI and organoleptic dif-

  • THE BLANCHING PROCESS

    a

    m . U

    r r

    2

    0 80-

    a 60- 2 . 0

    40.

    101

    I

    FIG. 3. Increase in acid number and peroxide number of cnide lipid from Golden Cross corn; 1952 samples.

    ferences in unblanched peas, vined previous to storage at -17.8OC. ( O O F . ) and those harvested from the same field and at the same time, which were stored at the same temperature in the pods. A control of vined and blanched material from the same lot was included in the study.

    It was found that peroxide values of the extracted crude lipids obtained from peas stored in the pods longer than 62 days were con- siderabIy higher than those found in the crude lipids extracted from peas which were vined previous to storage. Total sugars, reducing sugars, and sucrose were found to be higher in the peas stored in the pods than in those which were vined previous to storage. Greater chloro- phyll degradation took place in the peas which were stored in the pods than in those which were vined previous to storage,

    Peas stored at -17.8OC. ( O O F . ) unblanched in the pods retained reasonably good eating quality for a little over a month. After this period gradual deterioration became apparent. Unblanched vined peas started to decline in quality after about a week of storage.

    It was suggested that since the peas stored in the pods were injured the least previous to storage, perhaps areas in which peroxides are formed contain little material on which these peroxides can act, thus resulting in an accumulation of peroxides. The peroxides are perhaps intermediate in the formation of off-flavors. The fact that peas in the pods are really being stored in gas chambers may have a bearing on the observed sugar differences, and possibly on the peroxide and chloro- phyll differences as well.

    It can be seen from Table XXVII that the total sugars are higher

  • c

    w

    TABLE XXVII Sugar Contents of Frozen Thomas Laxton Peasa Held in Storage at -17.8"C., 1955 Harvestb

    Period of

    storage days

    0 38 83

    160 209

    Frozen in pods (shelled previous to analysis, pods discarded)

    Frozen after vining in commercial viner

    Blanched for 60 seconds in boiling water at 100C.

    ~~ ~~~

    Reducing Total Sucrose sugar

    ( W ) sugars

    ( % )

    Trace 5.28 5.28 0.09 6.01 6.10 0.29 5.80 6.09 0.30 5.94 6.24

    Reducing Total Sucrose sugar

    (9) ( I & ) sugars

    ( % )

    Trace 5.28 5.28 Negative 5.26 5.26 Trace 5.32 5.32 Negative 5.09 5.09

    Reducing Total 3 sugars Sucrose sugar 9 ( a ) ( % ) (%) 2

    E 9

    H

    Trace 4.28 4.28

    a Tenderometer reading 113. b Lee et al. ( 1956).

  • THE BLANCHING PROCESS 103

    by almost 1% in the peas stored in the pods than in those which were vined before placing in storage. Blanched peas show the smallest amount of total sugars. This latter result is to be expected because of losses of soluble materials during blanching. Another noteworthy result is the presence of detectable amounts of reducing sugars in the peas stored in the pods. Reducing sugars in the vined peas were either absent or present only in traces.

    Table XXVIII shows the observed differences in chlorophyll content.

    TABLE XXVIII Chlorophyll Content of Frozen Thomas Laxton Peas Held for 160 Days

    in Storage at -17.8"C.n.b

    Chlorophyll content

    Condition of sample mg./100 g. mg./100 g. %loss, fresh I& loss, dry fresh weight dry weight weight basis weight basis

    Blanched 9.42 41.77 0 0 Stored in pods (shelled

    previous to analysis, pods discarded) 0.04 26.95 35.9 35.5

    Raw, vined 6.87 31.44 27.0 24.7

    Q Lee et al. (1956). b 1955 Crop. Tenderometer reading 113.

    It has long been believed that developing off-flavors and off-odors in unblanched and underblanched vegetables during frozen storage, are caused by enzymes. Wagenknecht et al. (1952) postulated that lipoxidase and lipase were the causal agents in the observed deterioration noted in the lipid extracted from unblanched peas. Later, Siddiqi and Tappel (1956) presented data in support of this view, a t least insofar as lipoxi- dase is concerned. These authors favor the view that damage caused by autoxidation of the fat would be negligible in comparison with that caused by enzymatic oxidation at low temperatures of storage when one considers the low activation energy for lipoxidase (4.3 kcal./g. mole) compared to the activation energy (15.2 kcal./g. mole) for autoxidation.

    Wagenknecht and Lee ( 1956) showed that extensive peroxidation of the lipid matter of raw peas can be achieved by blending the peas with water in the Waring blender for 5 minutes. Blanched peas do not form peroxides under these conditions. Lipoxidase was shown to be capable of causing peroxidation of the lipid and also the destruction of chlorophyll in blanched peas.

    It is obvious that an understanding of the causes for the develop-

  • 104 FRANK A. LEE

    ment of off-flavors and off -aromas in unblanched and underblanched vegetables during storage at -18OC. ( O O F . ) is being sought, Those working on the development of aldehydes and alcohol during storage believe that respiratory enzymes are responsible for the development of off-flavors and odors. The others working on the changes in the lipid fraction believe that the answer to the problem is to be found in this group of compounds. The author of this review believes that the latter is the more likely of the two explanations, and that the present knowl- edge, while it is not complete, is indicative that this is the case. How- ever, both might be involved. More research on this problem is needed.

    Lee (1956) investigated the activity of sunlight on the extracted crude lipids of blanched and unblanched vegetables. Frequently, after varying periods of exposure to sunlight, the crude liquids from raw vegetables gave a more rapid increase in peroxide number than those crude lipids which had been extracted from the corresponding blanched samples, except in lima beans. The peroxide values usually reached higher peaks for unblanched material as contrasted with corresponding blanched material similarly exposed, However, in the case of the crude lipid extracted from lima beans, the reverse seems to be true.

    Lee (1955) showed that carotene drsticuZlgr reduces the activity of sunlight on oils otherwise obtained in the presence of chlorophyll. It is likely that the low peroxide values obtained for the action of sunlight on the crude lipids extracted from spinach is accounted for by the presence of large amounts of carotene present in this material.

    V. SUMMARY Considerable work has been done on the blanching of vegetables

    previous to preservation since Nicholas Appert found that a preliminary scalding was desirable in certain vegetables previous to bottling. For a long period there was much speculation and supposition, but little under- standing was achieved. Canned unblanched vegetables lack quality.

    In recent years, a great deal has been done, and many advances made. It is quite generally understood now that blanching as a pre- liminary treatment in the canning process (1 ) removes the tissue gases and ( 2 ) effects a shrinking of the material so that adequate fills can be had in the can. Enzyme inactivation is not so important, because it is likely that the heat used to remove the tissue gases will inactivate the enzymes. Should it not do so, any enzyme remaining in an active state would be inactivated by the cooking process.

    Blanching as a preliminary treatment in the freezing process ( I ) inactivates the enzymes in the tissues and (2) shrinks the material so as to conserve space in packing.

    Blanching is necessary in the dehydration process to inactivate the

  • THE BLANCHING PROCESS 105

    enzymes, because as in the case of preservation by freezing, no further cooking, previous to storage, is involved.

    Late in the 1920's and early in the 1930s it was realized that vege- tables to be preserved in freezing storage would require a heat treat- ment to inactivate the enzymes,

    While the process of blanching is necessary, it does lead to losses of nutrients and flavor. Much work has been done on the losses (and gains in certain instances ) of such inorganic substances as calcium, potassium, phosphates, and iron, as well as the losses of such organic materials as sugars and nitrogenous substances. Similar studies were made on the vitamins. Time and temperature blanching studies have revealed the best conditions for the retention of the maximum quantities of nutrients consistent with maintenance of desirable quality of the finished product.

    A great deal of effort has been spent on better understanding of the role of enzymes in products preserved by freezing and dehydration and the most effective methods for the inactivation of these enzymes. While many of the tests devised and used are for catalase and peroxidase, it has not been proved that either of these enzymes, if present in the active state in vegetables held in freezing storage, are effective in the develop- ment of off-flavor. The method making use of the time necessary to inactivate catalase, and then allowing an additional 50% of the inactiva- tion time as a safety factor while empirical, has produced good results.

    It is easy to test for catalase, m d the tests employed do not require expensive equipment or highly trained personnel, More recent work indicates that lipoxidase and lipase are responsibIe for the development of off-flavors in raw and underblanched vegetables during storage at -18OC. ( O O F . ) .

    During the blanching process, some chlorophyll is converted into pheophytin.

    The relative merits of water-blanching versus steam-blanching have been studied intensively. It seems that steam-blanching is the more effective of the two for the conservation of soluble nutrients. However, some authorities contend that under certain conditions, steam-blanching leaves some undesirable flavors. The present wide acceptance of steam- blanching in the vegetable field is evidence that some of the disad- vantages, at least, are not too important. The blanching of vegetables by means of steam pressure resulted in products which were not fully satisfactory, a t least a t the pressures tried.

    The blanching of vegetables by means of electronics was tried. While the used of radio frequency has interesting possibilities, much more work of an economic as well as of a scientific nature will have to be done before application will be possible. Vegetables which were blanched in packages by this means, with the intent to conserve nutri-

  • 106 FRANK A. LEE

    ents, cooled very slowly, and developed off-flavors. The possibility of blanching vegetables by this method on a belt, followed by spray or air-blast cooling was suggested.

    Recent work has been directed toward a better understanding of the changes taking place during storage at -18OC. ( O O F . ) of raw and underblanched vegetables.

    Some workers found that acetaldehyde and ethyl alcohol accumu- lated during the storage at -18OC. (OOF.) of unblanched and under- blanched peas. While acetaldehyde was the chief component of the aldehydes, acetyl methyl carbinol, diacetyl, and other aldehydes were found. Ethyl alcohol was the only alcohol found. However, the amounts found were insufficient to influence the flavor of the vegetable. The study on frozen broccoli resulted in the finding that while the amount of ethyl alcohol was found to be related to the extent of off-flavor, it is obvious that this substance in the amounts formed could have little actual influence on the flavor of this vegetable.

    Other workers studied the effects on the lipid materials extracted after the vegetables had been stored for extended periods of time. It was found that rancidification of the lipid materials, as evidenced by increases in the peroxide and acid numbers of the extracted lipids, had a great deal to do with the increase in off-flavors. This was further studied on a progressive basis, and increased effects were obtained as the storage time lengthened.

    Additional studies were carried on using samples of peas from the same lot which had been frozen raw in the pod, vined and packed raw, and the third group of samples packed following blanching. The peas in the pod maintained eating quality for a little over a month, while the raw vined peas showed deterioration after two weeks. However, acid values of the peas packed raw in the pods and those vined and packed raw, increased as fast and to the same extent. The peroxide values for the lipids extracted from the peas packed in the pods increased to a much greater extent than those which were put through the viner previous to packing. The peas stored in the pods were higher in sugar and lower in chlorophyll after storage than either the peas which were vined and packed raw, and those which were blanched.

    Considerable work will have to be done before all the problems are solved.

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  • THE BLANCHING PROCESS 107

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  • 108 FRANK A. LEE

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