the relation of reducing value and extent oe‘ … · the relation of reducing value and extent...

THE RELATION OF REDUCING VALUE AND EXTENT OE‘ BROWNING TO THE VITAMIN C CONTENT

OF ORANGE JUICE EXPOSED TO AIR

BY M. A. JOSLYN, G. L. MARSH, AND AGNES FAY MORGAN

(From the Fruit Products Laboratory and the Laboratory oJ Household Science, University of California, Berkeley)

(Received for publication, December 28, 1933)

Orange juice exposed to atmospheric oxygen will gradually decrease in its reducing value towards iodine and also towards 2,6dichlorophenolindophenol (1, 2). If the exposure is continued, browning occurs and increases in intensity. The following studies were undertaken to determine whether any change in vitamin C was involved in the browning of orange juice. All the samples of browned orange juice or concentrate that we have examined were either very low in iodine-reducing value or had a negligible titration. However, treatment of orange juice with oxygen or decolor- ization with bone char resulted in loss of reducing value without any noticeable browning. The oxygen-treated juice browned even under an atmosphere of carbon dioxide.

It is known that dried fruits unprotected from oxidation during drying and storage lack vitamin C and become brown. The amount of SOz necessary to prevent loss of vitamin C is about the same as that necessary to prevent browning (3, 4). This may mean merely that deterioration of color and loss of vitamin C are both due to oxidation.

The evolution of CO2 from sterile orange juice during browning (5) may be due to the decomposition of hexuronic acid and the fact that Hirst and Zilva (6) recently reported the production of a brown amorphous material from oxidized ascorbic acid lends some support to the possibility of vitamin C being involved in browning.

A secondary object of this study was to compare the iodine titration with the indophenol titration as an indication of the changes in vitamin C content of orange juice during oxidative deterioration over a long period.

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18 Vitamin C Content of Orange Juice

EXPERIMENTAL

Preparation and Storage of SamplesIn our tests two lots of navel orange juice and one lot of Valencia orange juice were prepared. Frozen Valencia orange juice, of sp. gr. 1.0559 and an acidity as citric acid of 1.18 per cent, held at -17’ for 6 months, was thawed, mixed with diatomaceous earth, and filtered bril- liantly clear by suction. Sodium benzoate (0.2 per cent) was added to the clear juice to preserve it during storage at room temperature. About 12 liters of the juice were stored in a cotton- plugged 19 liter glass carboy. This lot constituted Series V. The navel orange juice was freshly extracted, one lot, Series N, of specific gravity 1.0487 and an acidity of 0.692, being filtered and stored as above; and another lot, Series N-A, of specific gravity 1.0582 and acidity of 0.86 per cent, being merely strained through cheese-cloth and not filtered. During storage hesperidin precipi- tated out of the filtered navel orange juice, but not the strained. During the 1st week the juices were aerated for about half an hour on two occasions and they were stirred as samples were withdrawn for titration or for storage.

Immediately after preparation, thirty-six portions of the juice containing 75 cc. each were withdrawn and stored in 120 cc. glass jars at -17”. No precautions were taken to avoid oxidation of the juice in storage as it was reported (7, 8) that vitamin C potency of freshly extracted juice was well retained during storage at - 17’ even though air was present over and in the juice. At the intervals shown in Fig. 1 further lots of juice were withdrawn and stored at -17” for subsequent vitamin C assay. Sample 6, Series V, and Sample 5, Series N-A, not shown in the graph, were withdrawn after 64 and 53 days and had an iodine titration of 2.5 and 3.5 respectively. Although the vitamin C potency of frozen, fresh orange juice exposed to air may not decrease appreciably over a storage period of several months, it was found that in partly oxidized juice a noticeable decrease in iodine-reducing value did occur.

During storage at room temperature the decrease in iodine- reducing value was followed by titration of 50 cc. of the juice with 0.01 N iodine solution, starch being used as indicator. The end- point chosen was that when the starch-iodide color persisted for 15 to 30 seconds; this was sharp in freshly filtered juice but not in


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Joslyn, Marsh, and Morgan 19

the cloudy juice. However, as the juice browned the end-point was indefinite and reproducible to but f0.5 cc. Filtration of the juice noticeably reduced its iodine-reducing value. Thus, on navel orange juice the titration was reduced from 38.0 to 30.0 and on Valencia juice from 32.9 to 25.7.

FIQ. 1. Rate of reduction in iodine-reducing value in orange juice exposed to air. The letter and figure notations on the curves with the exception of the curve numbers represent the series and sample number, respectively. V-l, V-2, V-3, V-4, V-5 represent Samples 1 to 5 of Series V (Val- encia orange juice); N-l, N-2, N-3, Samples 1 to 3 of Series N (navel orange juice); and N-lA, N-2A, N-3A, N-4A, Samples 1 to 4 of Series N-A (strained navel orange juice).

The rate of oxidation of the reducing matter by air as shown in Fig. 1 is more rapid in the filtered navel orange juice than in the Valencia juice, but is somewhat slower for cloudy navel orange juice. Filtration may remove some protect.ive substance from the juice. Bobh the filtered and cloudy navel orange juice had a higher


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initial reducing value than did the Valencia juice. This is typical of the juices of samples of the two varieties of fruit that we have examined. The rate of oxidation, of course, depends on the actual concentration of oxygen in the juice.

In the Valencia orange juice, browning began to occur when its iodine-reducing value had decreased from 26.0 to 12.9 and the juice was markedly brown 1 week later. Browning started in Series N-A when the iodine value was reduced from 38.5 to 20 and t,he juice was fairly brown 7 days later. Only the last sample of Series N was noticeably brown.

In addition to these three lots the following were prepared: a lot of filtered Valencia orange juice decolorized with bone char, Series V-X; and a lot of 10 year-old black (7: 1) concentrate diluted with an equal volume of water, Series V-Y. Both of these prep- arations had 0 iodine titration.

Biological Test&--The basal diet used was a modification of that suggested by Sherman, La Mer, and Campbell (9).

A test period of 60 days was used for these tests. All fruit juice doses were fed daily by pipette so that no doubt could arise as to ingestion of the full amounts prescribed. The data from the animal assays from the various juices are completely summarized in Table I.

Examination of Samples-After storage at - 17’ for 140 days after withdrawal of the first sample, samples of the various lots of juice were analyzed with the results shown in Table II. The iodine titration was carried out in the presence of 1 cc. of concentrated HCl. This partly bleached the brown color, especially near the end-point, so that this was more readily determinable and was sharper. This reduced the iodine value by 1 cc. but variations in amount of acid added from 0.5 to 1.5 had no not,iceable effect on the volume of iodine reduced.

It is remarkable that, even after allowing for the decrease in iodine value by addition of acid, there is a marked decrease in the t,itration from the initial values shown in Fig. 1. This is slightly more pronounced in the clear than in the cloudy juice and rela- t,ively greater for the partially oxidized than for the freshly packed juices. Apparently some oxidation of the iodine-reducing constit-

1 We are indebted to Gwendolyn Jones and Marjorie C. Brown for the care of the animals and the administration of the doses.


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TABLE I

Antiscorbutic Value by Biological Test of Orange Juices Exposed lo Varying Amounts of Oxidation -

I No. of nimalr 1-

1 -

Inter- dhxml miti of .itamin

hitial Final , per cc.

sm. om.

V 1 311 432 319 386 10.0

2 335 379 6.6 334 279

3 317 318 3.3 4 315 214 1.5

333 245 349 436

5 320 239 326 212

6 312 206 327 195

N 1 353 482 10.0 320 411

2 337 358 6.6 334 395

3 356 346 3.3 N-A 1 323 440 10.0

2 322 448 10.0 4 305 368 5.0 5 320 207

317 184 v-x 337 212 1.5

323 181 314 437

V-Y 325 197 L-J* 321 381 10.0

332 554 O-J* 366 423 10.0

350 486 342 499

F-V’ 319 464 10.0 328 451 326 466

F-N* 315 319 10.0 332 435 319 487

Conk01 329 227

* Series L-J represents freshly extracted lemon juice; Series O-J, freshIf extracted orange juice; Series F-V, froxcn Valencia orange juice, two other samples; Series F-N, frozen navel orange juice, two other samples.

21

cc.

1.5 3.0 1.5 3.0 3.0 4.0 8.0

10.0 5.0

10.0 7.0

10.0 1.5 3.0 1.5 3.0 3.0 1.5 1.5 3.0 4.0

10.0 3.0 5.0

10.0 10.0

1.0 2.0 1.0 2.0 3.0 1.5 2.0 3.0 1.0 2.0 3.0

4 1 4 3 4 4 2 1 4 2 4 2 4 1 4 1 4 3 3 4 4 2 2 2 2 2 5 5

13 12 1G

3 8

10 9

11 18 12

O”L.

2.0 1.1 0.7

-0.9 10 -1.9 -2.1

1.4 -2.4 -3.3 -3.3 -4.2

2.1 1.5 0.3 1.0

-0.1 2.0 1.9 1.0

-3.8 -4.7 -3.9 -3.6

2.0 -4.0

1.0 3.5 0.9 2.2 2.G 2.4 2.0 2.3 0 1.7 2.8

-3.6 -

days

60 60 60 60 60 53 42 60 34 34 32 31 60 60 60 60 60 60 60 GO 30 28 32 39 60 32 60 GO 60 60 GO GO 60 60 60 60 60 29

0 0 7 0 5

14 6 0

20 12 22 15

0 0 4 0 5 0 0 0

21 21 11

6 0

16 2 0 5 0 0 2 2 0 6 0 0

17

-


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uents had occurred. Analyses 2 months later showed but rela- tively little decrease in iodine value, an equilibrium in most cases having been reached. Nelson and Mottern (7) report that the average values of the Tillmans (10) indophenol titer in orange juice

TABLE II

Analyses of Orange Juice after Freezing Storage

Series mph NO.

3

_

Indo- I phenol titer l

Color index

-

.- V 20.8

13.5 6.5

N

N-.4

38.3 25 1.8 25.5t 3.3 21.2 25 3.8 15.6 6.7

9.1 25 9.1 6.0 11.5 3.9 25 17.0 (22.511 3.0 38.0 70 2.1 27.0 0.8 30.0 70 1.0 39.0

43.1 10 0.9 7.0 3.2 34.7 15 1.8 4.7 4.8 11.5 20 9.5 2.5 16.0 54.6 25 1.8 9.8 2.2 44.6 25 1.8 5.4 3.3 24.8 25 7.0 2.7 6.2 12.3 25 13.5 2.3 12.0 0.9 25 17.0 1.0 27.0 0.05 1 0.1

40 7.0 5 1.1 5 1.3

15 2.8 20 3.0

- -

* Impure indophenol approximately 0.0005 M; titration against 10 cc. of juice.

t La Motte indophenol, 0.001 M; titration against 5 cc. of juice. $ Estimated. $ Diluted 1: 100. 1) Diluted 10: 100.

1 2 3 4 5 6 1 2 3 1 2 3 4 5

19.5 13.5 6.1 1.9 1.5 0.8

23.5 19.0 6.35

34.1 25.3 12.9

7.3 1.2 0.3 0.0

1.5 0.9

21.5 16.8

4.7 34.0 22.8 12.6 6.5 1.2

v-x V-Y0

VI

-

Analyzed after 140 days Analyzed after 221 days _

frozen with head space of nitrogen, oxygen, and air after storage for 1 year at -17” were 7.1, 6.7, and 6.6 cc. of approximately 0.01 N 2,6-dichlorophenolindophenol per 3 0 cc. of juice. Apparently a slight amount of oxidation had occurred.


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Our indophenol titrations were made first with 2 ,&dichlorophenolindophenol prepared according to the methods given by Tillmans et al. (10). We experienced difficulty in preparing a pure quinonechloroimide and the final product after washing with half- saturated salt solution still contained about 30 to 50 per cent in- soluble matter. A solution of this preparation in ~/15 phosphate buffer at pH 7.0 was used to titrate 10 cc. portions of neutralized orange juice in tests made on August 14, 1933. Preliminary in- vestigations indicated that the volume of the dye solution reduced by the juice varied with the speed of titration, being markedly lower when the dye was slowly added to the juice or solutions of ferrous or titanium sulfate. Tillmans et al. (10) had reported that the titer was practically the same in air, in an atmosphere of Nz, in acid juice, and in juice brought to pH 7.0. However, for our indophenol preparation this was not the case. The rate of titration was the most important variable. Similar results were obtained with indophenol prepared and purified according to the method of Clark (11). Recently Harris and Ray (12) specified the necessity for rapid titration with the dye solution. We found that the dye solution was reduced slowly at first and then more rapidly by the Valencia juice, but rapidly from the first by t(he navel orange juice. A preparation of indophenol purchased on the market gave us the same marked variation in titer with rate of titration. The end- point in the titration was more difficult to gage in the oxidized and browned samples than in the freshly packed juice. The values given in the fourth column of Table II show the relative difference between the various lots of juice.

The indophenol titration made after 221 days showed a surpris- ing and unaccountably low titration value for navel orange juice. It is given here merely for the purpose of comparison.

The color of the juice was determined in a Lovibond tintometer with red slides No. 200, yellow slides No. 210, and maple sirup slides No. 52. The latter were adapted from the standardized caramel-glycerol solutions of Balch (13). It is noticed that although the yellow shades remained fairly constant during browning, there was a very marked increase in the reds. The browned juices were pink to red when examined visually in the tintometer tube. The increase in the reds paralleled the decrease in the iodine titer.


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Since Harris and Ray had recently report,ed that t.he indophenol titer was more specific for ascorbic acid in plant products when the titration was made in acid solution, we repeated the indophenol titer using their technique with slight variations. The juices were adjusted to pH 2.5 with HCl instead of trichloroacetic acid. In preliminary tests we found that varying the pH of the juice from 3.9 to 0 had but little effect on the dye titer in freshly prepared juice. However, t,itration of the dye with the juice resulted in a sharper end-point and more readily duplicable results in all cases except the markedly browned samples where the fading of the red color of the dye was masked by the reddish tinge of the juice. The rate of titration had appreciable but not as marked an effect as in the Tillmans titer. The volume of juice brought to pH 2.5 by addition of 0.3 cc. of concentrated HCl per 50 cc. of juice testing pH 3.9 to 4.0 necessary to reduce 5 cc. of an unbuffered water solution of 2,6-dichlorophenolindophenol (purchased from La Motte) is shown in the tenth column of Table II. The results of duplicate determinations agreed very closely except for Samples 5 and 6, Series V, and Sample 5, Series N-A, where, because of interference of the color of the sample, the results are accurate only to *l.O cc.

It was found that the indophenol reagent was reoxidized on standing in the juice so that the titrated samples became intensely blue in color. Preliminary titrations of unneutralized orange juice with buffered dye reagent gave sharper end-points than the unbuffered indicator of Harris and Ray.

It was pointed out by Szent-Gyorgyi (14) that iodine reduction is not necessarily specific for hexuronic acid as other iodine-reducing substances are also present. Mottern et al. (2) reject the iodine titration because of a residual titration in completely air- oxidized orange juice and for other reasons. Harris and Ray (12) have recently shown that iodine titration in fresh orange juice measures an appreciable amount of material in addition to hex- uranic acid. Indophenol was introduced as a convenient and a more specific oxidant by Zilva (15) and the 2,6-dichlorophenolindophenol by Tillmans (lo), and the latter dye was largely used by subsequent investigators. However, the oxidation-reduction


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potential of 2,6-dichlorophenolindophenol at pH 7.0 is 0.217, and increases to 0.335, at pH 5 as compared with 0.535, for iodine (16). Harris and Ray (12) reported that Tillmans’ method was but little less specific than the iodine reduction and broke down when tested by titration values given by aerated orange juice. They recom- mend titration with an unbuffered dye solution in acid solutions at pH 2.5. Since the oxidation-reduction potential of indophenol increases with decrease of pH, it is difficult to see how decreasing the pH of foods makes the titration more specific for vitamin C un-

TABLE III

Relation of Calculated to Actual Vitamin C Content

Series

V

N

N-A

1

i 4 5 6 1 2 3 1 2 3 4 5

.nternstional Relative nit8 of vita- iodine Nos., nin C per cc. original titer

10.0 6.6 3.3 1.5

10.0 8.3 3.7

10.0 10.0

5.0

10.0 10.0 10.0 7.7 6.9 4.9 5.0 3.1 2.9 3.3 1.0 1.4 1.7 0.6 1.2 1.0 0.04 0.8

10.0 10.0 10.0 8.1 7.8 G.7 2.7 2.2 2.0

10.0 10.0 10.0 8.2 7.4 6.7 5.2 3.8 3.5 3.5 2.1 1.8 0.9 0 35 0.8

T Relative

iodine Nos. fter 140 day torageat -Ii

.elative indc ,henol value Harria titer:

Relattii;; red

10.0 4.7 2.0 1.1

10.0 5.0 0.9

10.0 10.0 2.6 1.3 1.0

less this selectively affects the reducing potential of the substances present in solution (cf. (17)).

A comparison between the units of vitamin C actually found and the relative units calculated from the iodine and Harris indophenol titration values is shown in Table III. The relative values were calculated by setting the iodine value of the first sample in each series equal to 10. In view of the decrease in iodine-reducing value after 140 days storage at - 17”, as shown in titrations made at that time, and from the fact that the feeding tests were concluded at about that time, the relative iodine values are shown for the


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original and the latter titrations. The agreement between the actual vitamin content and that calculated from the iodine titration is surprisingly close for the Valencia juice and fairly close for the navel orange juice. There is but little choice, if any, between the relative values based on the iodine titration and the indophenol titration of Harris and Ray. If anything, the former fit the facts somewhat better.

Loss in vitamin C accompanies decrease in reducing value of juice during oxidation and both occur at about the same rate. Also during this oxidation, t,he juice gradually browns. By using

TABLE IV

Calculated Ascorbic Acid Content of Juice

Scriea

V

N

N-A

.

Siamplt? NO.

Mg. ascorbic acid per cc.

From Htrrris titer From II titer

I 0.355 0.342 2 0.175 0.237 3 0.102 0.107 4 0.052 0.033 5 0.043 0.026 6 0.030 0.014 1 0.366 0.413 2 0.244 0.334 3 0.073 0.112 1 0.532 0.598

2 0.355 0.444 3 0.189 0.226 4 0.097 0.128 5 0.043 0.021

0.333 0.167

0.100

0.333 0.333

0.167

the relative increase in red as an index the values in the seventh column of Table III were obtained. It is interesting to note that these values are also in close agreement with the vitamin C content of orange juice. In view of the variation in yellow tint, in Series N the calculated relative red tints are probably not as good as for the Valencia juice. The agreement is not as good as could be expected.

The probable ascorbic acid contents of the juices calculated from the indophenol titrations and from the iodine titrations obtained after 140 days freezing storage arc shown in Table IV. For this


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purpose the indophenol was standardized against lemon juice (12).

The fifth column in Table IV shows the ascorbic acid content of the samples estimated from the feeding data in Table 1 on the assumption that 1.5 cc. of orange juice contains 0.5 mg. of ascorbic acid, i.e. the minimum protective dose of 0.5 mg. found by Hirst and Zilva (6).2 Although the agreement between the estimated ascorbic acid content and that found by indophenol titration was good for Valencia juice, the latter was markedly higher in navel orange juice. It would seem that navel orange juice should contain appreciably more vitamin C than Valencia juice; 1 cc. of navel orange juice being the equivalent of about 1.5 cc. of Valencia juice. The values calculated from the iodine titration on Valencia juice are in fair agreement with the actual but this is not so for navel orange juice.

In view of the fact that Bennett and Tarbert (18) claim that benzoate is involved in loss of vitamin C as measured by decrease in indophenol titer in juice exposed to air, our use of sodium benzoate to preserve the juices might be questioned. However, Morgan et al. (8) have shown by feeding tests that sodium benzoate has no effect on the vitamin C potency of bottled orange juice beverages protected from oxidation. Furthermore, the results of our experiments on the rate of oxidation and browning of orange juice which will be reported elsewhere definitely show that the rate of reduction in iodine-reducing value is independent of the presence of sodium benzoate and is merely a function of the amount of oxygen absorbed by the juice. Variations in amount of oxygen in the head space and their choice of fermenting juice as control may account for the results obtained by Bennett and Tarbert. Fermentation probably introduces some protective influence (re- ductase of the yeast) which prevents oxidation of the juice.

SUMMARY

1. Loss of vitamin C accompanies decrease in iodine-reducing and indophenol-reducing value of orange juice and occurs at about the same rate; the correlation of titer and antiscorbutic value is definitely better with Valencia than with navel orange juice.

2. The extent of browning parallels the extent of loss in vitamin

* Fresh orange juice is often reported to contain 0.6 mg. of ascorbic acid per cc.


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C so that either the latter is involved in browning or it is destroyed simultaneously.

3. Navel orange juice has a higher concentration of reducing substances than Valencia orange juice and probably has more reducing material other than ascorbic acid.

4. The decrease in reducing value of orange juice which occurred during freezing storage was more marked in the partly oxidized juice.

5. There is little choice between the indophenol titration and the iodine titration in estimating changes in vitamin C content during prolonged oxidation of orange juice.

6. The iodine titration is superior to the indophenol titration in practise as it is easier to carry out and can be more readily dupli- cated.

BIBLIOGRAPHY

1. Joslyn, hi. A., and Marsh, G. L., Science, 76, 82 (1932). 2. Mottern, H. H., Nelson, E. M., and Walker, R., .I. Assn. 0.f. Agric.

Chem., 16, 614 (1932). 3. Morgan, A. F., Field, A., and Nichols, P. F., 7. Agric. Research, 42, 35

(1931). 4. Nichols, P. I?., and Cruess, W. V., Ind. and Eng. Chem., 24, 649 (1932). 5. Wilson, C. P., Znd. and Eng. Chem., 20, 1302 (1923). 6. Hirst, E. L., and Zilva, S. S., &&hem. J., 27, 1271 (1933). 7. Nelson, E. M., and Mottern, H. H., Znd. and Eng. Chem., 26,216 (1933). 8. Morgan, A. F., Langston, C. I., and Field, A., Ind. and Eng. Chem., 26,

1174 (1933). 9. Sherman, H. C., La Mer, V. K., and Campbell, H. L., J. Am. Chem. Sot.,

44, 165 (1922). 10. Tillmans, J., Hirsch, P., and Remshagen, E., Z. Untersuch. Lebensmittel,

66, 272 (1923). 11. Gibbs, H. D., Hall, W. L., and Clark, W. M., Pub. Health Rep., U. 8.

P. H. S., suppl. 69, 24 (1928). 12. Harris, L. J., and Ray, S. N., Biochem. J., 27, 303 (1933). 13. Balch, R. T., Ind. and Eng. Chem., 22, 255 (1930). 14. Szent-Gyorgyi, A., Biochem. J., 22, 1387 (1928). 15. Zilva, S. S., Biochem. J., 21, 689 (1927). 16. Clark, W. M., The determination of hydrogen ions, Baltimore, 3rd edi-

tion, 683 (1928). 17. Green, D. E., Biochem. J., 27, 1044 (1933). 18. Bennett, A. H., and Tarhert, D. J., Biochem. J., 27, 1294 (1933).


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MorganM. A. Joslyn, G. L. Marsh and Agnes Fay

JUICE EXPOSED TO AIRVITAMIN C CONTENT OF ORANGE

AND EXTENT OF BROWNING TO THE THE RELATION OF REDUCING VALUE

1934, 105:17-28.J. Biol. Chem.

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