[Advances in Food Research] Advances in Food Research Volume 17 Volume 17 || Oxidation Systems in Fruits and Vegetables– their Relation to the Quality of Preserved Products

Download [Advances in Food Research] Advances in Food Research Volume 17 Volume 17 || Oxidation Systems in Fruits and Vegetables– their Relation to the Quality of Preserved Products

Post on 22-Mar-2017

216 views

Category:

Documents

3 download

Embed Size (px)

TRANSCRIPT

<ul><li><p>OXIDATION SYSTEMS IN FRUITS AND VEGETABLES- THEIR RELATION TO THE QUALITY OF PRESERVED PRODUCTS </p><p>BY </p><p>D. R. HAISMAN~ F. AYLWARD* AND </p><p>Fruit and Vegetable Preservation Research Association, Chipping Campden, England </p><p>I. Introduction . . . . . . 11. Oxidizing Enzyme S </p><p>A. Peroxidase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................... 9 </p><p>I. Clycolate Oxidase . . . . . . . . . . . . . . . . . . . . . . 14 </p><p>. . . . . . . . . .20 </p><p>. . . . . . . . . . . . . . . 22 </p><p>E. Antioxidants . . . . . . . . . . </p><p>'Present address: Department of Food Science, The University, Reading, Berks. England. +Present address: Unilever Research Laboratory, Colworth House, Sharnbrook, Beds., England. </p><p>1 </p><p>OXIDATION SYSTEMS IN FRUITS AND VEGETABLES- THEIR RELATION TO THE QUALITY OF PRESERVED PRODUCTS </p><p>BY </p><p>D. R. HAISMAN~ F. AYLWARD* AND </p><p>Fruit and Vegetable Preservation Research Association, Chipping Campden, England </p><p>I. Introduction. . . . . . . . . . . . 11. Oxidizing Enzyme Systems </p><p>A. Peroxidase.. . . . . . . . . B. Pseudo-Peroxidases . . . C. Catalase . . . . . . . . . . D. Cytochrome Oxidase . E. o-Diphenol Oxidase . F. p-Diphenol Oxidase . G. Ascorbate Oxidase . . I H. Amine Oxidases . . . . I. Glycolate Oxidase . . I J . Oxidation Mechanisms . K. Correlation between Enzyme Activity and Food Deterioration </p><p>111. Respiratory and Other Enzymes A. Respiration . . B. Fermentation . . . . . . . . . . . . . . C. Respiratory Enzymes and Food Deterioration D. Pectic Enzymes. E. Chlorophyllase . . F. Enzymes of Amino Acid Metabolism </p><p>IV. Oxidative and Other Changes in Lipids . A. Degradation of Lipids . B. Lipoxygenase . C. Autoxidation of Lipids D. Decomposition of Hydroperoxides E. Antioxidants . F. Lipid Oxidation in Relation to Food Quality </p><p>2 7 8 9 </p><p>10 10 11 12 13 13 13 14 18 19 19 20 20 21 22 22 23 24 26 </p><p>,27 28 </p><p>131 31 </p><p>'Present address: Department of Food Science, The University, Reading, Berks. England. +Present address: Unilever Research Laboratory, Colworth House, Sharnbrook, Beds., England. </p><p>1 </p></li><li><p>2 F. AYLWARD AND D. R. HAISMAN </p><p>V. Thermal and Other Environmental Factors Modifying Enzyme Activity . . . . . 34 A. Thermal Inactivation -General Principles . . . . . . . . . . . . . . . . . . . . . . . . . .35 B. Thermal Inactivation of Oxidizing Enzymes-Experimental Data . . . . . . 39 C. Enzyme Action at Low Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47 D. Effects of pH and Ionic Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47 E. Effects of Water. . . . . . . . . . . : . . . . . . . . . . . . . . .................... .48 F. Multiple Molecular Forms of Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . t.48 G. Adsorption of Enzymes on Natural Substrates . . . . . . . . . . . . . . . . . . . . . . . .50 </p><p>VI. Regeneration of Enzyme Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53 A. Test Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Heat Treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54 C. Storage Conditions .......................... .56 </p><p>H. Specific Inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53 </p><p>VII. Research Needs . . . . . . . . . . </p><p>B. Biochemical ......................... .58 C. Enzyme Systems: Substrates-Primary and Secondary Reactions . . . . . . 58 D. Plant Lipids: Oxidation Mechanisms and Reaction Products . . . . . . . . . . . 59 E. Plant Components Inhibiting or Modifying Enzyme Activity. . . . . . . . . . 59 </p><p>G. New Methods of Preservation ....................................... .60 References ......................................................... 61 </p><p>F. Heat Resistance of Enzymes . . . . . . . . . . ........................ .60 </p><p>I. INTRODUCTION </p><p>The fruit and vegetable canning, quick-freezing, and dehydration industries can be considered from the economic standpoint in terms of a chain stretching from farm to consumer and including growers and processors and groups concerned with transport, storage, and distribution. The scientific and technological problems of the industry must be considered in the same context. </p><p>During the growth of the canning industry, and later of the quick- freezing and dehydration industries, the importance of careful control of postharvest and processing conditions became well recognized, and scientific and technological investigations sponsored by indus- trial and other groups were directed to the production of preserved foods of good quality. </p><p>Not so much attention was given to the changes in quality of food after leaving the factory, but it was gradually recognized that changes1 in quality could and did take place, especially with frozen and dehydrated foods. Realization of the extent of such changes has led to various investigations, notably those sponsored by the Western Utilization Research Laboratory of the United States Department of Agriculture. </p></li><li><p>PLANT-TISSUE OXIDATION SYSTEMS 3 </p><p>Quality changes may result from three main types of reaction within the foodstuffs or between the food and its environment: (a) microbiological; (b) enzymic, and (c) chemical nonenzymic changes. This review is concerned primarily with the effects of enzymes. </p><p>It is well known that active enzyme systems can spoil fruits and vegetables even at subzero temperatures and low moisture levels. For this reason most vegetables and some fruits which are to be pre- served by canning, freezing, or dehydration are given a preliminary heat treatment (for instance, blanching in boiling water) to inactivate the enzyme systems in the tissues. The blanching operation reduces the level of infection by microorganisms, and may improve color and flavor by expelling volatile degradation products formed during the postharvest interval (Adam et al., 1942). Blanching is omitted with certain products, such as onions and peppers and other strongly flavored materials, in which enzymic deterioration either does not occur or has no noticeable effect on quality (Makower, 1960). </p><p>The literature on the effect of enzymic activity on the quality of foods has been comprehensively reviewed by several workers. Joslyn (1949) examined the evidence for the enzymic nature of flavor changes, the effectiveness of blanching, and the tests available for measuring its efficiency. He noted that the best indicator for blanch- ing efficiency was peroxidase activity, and believed that until more was known about the enzymic systems involved in off-flavor forma- tion, technological developments would be limited to improvements in techniques of peroxidase estimation. From a survey of the experi- mental evidence, he concluded that there was no doubt of the activity of enzymes in frozen foods. In a later publication Joslyn (1951) dealt with enzyme activity in the dried state and in concentrated solutions, drawing parallels with the situation in frozen tissues. In more recent reviews Joslyn (1961, 1966) has emphasized that procedures for the control of enzymic activity during the preparation and freezing of fruits and vegetables are largely empirical; further biochemical re- search is required to define postharvest changes and changes during processing and storage. </p><p>Leeson (1957) reviewed the inactivation of enzymes by heat, the effect of residual enzyme activity on the quality of fruits and vege- tables, and evidence for the regeneration of enzyme activity. He emphasized that in devising tests of blanching efficiency the possi- bility of enzyme regeneration should not be overlooked. McConnell (1956) tabulated data on the heat resistance of enzymes in fruits and vegetables, and showed that in high-temperature short-time process- </p></li><li><p>4 F. AYLWARD AND D. R. HAISMAN </p><p>ing, enzyme inactivation may take longer than the destruction of microorganisms. Board (1961) surveyed the effect of enzymes on the stability of canned foods; in describing the changes caused by en- zymes he gave examples of nonenzymic catalysis such as the effect of copper and hemochromogens on the destruction of ascorbic acid and concluded that the mechanisms of the deteriorative changes were still obscure, but that both enzymic and nonenzymic reactions may take place. </p><p>The investigations of the Western Regional Laboratory, to which reference has already been made, were initiated in 1948 and covered not only fruits and vegetables but also other frozen products. The research program covered the behavior of frozen foods within the extremes of time and temperature which might be encountered in commercial practice. The main aims of the investigation were to establish tolerable deviations from ideal conditions for different products, to identify and improve critical operations, to seek techno- logical improvements ensuring greater tolerances, and to establish tests of product quality. The results were published from 1957 onward, starting with a definition of the problem (van Arsdel, 1957); surveying the quality of frozen soft fruits (Guadagni et al., 1957a,b,c, 1958, 1960; Guadagni, 1957; Guadagni and Nimmo, 1957a,b, 1958; Guadagni and Kelly, 1958), poultry products (Hanson and Fletcher, 1958; Hanson et al., 1959; Klose et al., 1959), vegetables (Dietrich et al., 1957, 1959a,b, 1960, 1962; Boggs et al., 1960), and liquid products (Hanson et al., 1957; McColloch et al., 1957), and covering relations between temperature history and product quality (van Arsdel and Guadagni, 1959) and bacterial population (Michener et al., 1960). </p><p>In frozen fruits the degree of browning and percentage loss of ascorbic acid (based on content of ascorbic acid, dehydroascorbic acid, and diketogulonic acid) were found to be closely related to overall quality. In peaches, partial inactivation of the oxidizing enzymes, followed by vacuum-packing in sealed containers and freezing, was sufficient to retard browning for a reasonable time after opening and thawing; complete inactivation of the enzymes by con- ventional heat processing caused serious flavor changes and increased leaching losses (Guadagni and Nimmo, 1957a). </p><p>In frozen vegetables, the loss of ascorbic acid was also found to be a useful index to quality, but the percentage retention of chlorophyll (considering chlorophyll-pheophytin ratios) was found to give the closest correlation with overall quality (Dietrich et al., 1959a,b). </p></li><li><p>PLANT-TISSUE OXIDATION SYSTEMS 5 </p><p>Chlorophyll losses were affected by the initial heat treatment given the product: the greater the loss during blanching, the lower the stability during storage. Walker (1964a,b) also found that the rate of change of chlorophyll during frozen storage increased when green beans were overblanched. Chlorophyll degradation in green snap beans was minimized by the use of high-temperature short-time blanching (Dietrich et al., 1959b). Water-blanching of Brussels sprouts was more effective than steam-blanching in that enzyme in- activation was faster and chlorophyll degradation less (Dietrich and Neumann, 1965). </p><p>The search for improved quality in preserved fruits and vegetables has led to proposals to reduce the period of heat processing to a minimum, and so retain to the maximum extent the natural char- acteristics of the food. It is extremely important to have precise estimates of the effectiveness of different types of heat treatments and process times on enzymes, as well as on microorganisms. Many authors have stressed that with high-temperature short-time processes enzymes, such as peroxidase, may be more difficult to destroy than microorganisms; enzyme inactivation may therefore be the deciding factor in assessing the efficiency of the process (e.g., McConnell, 1956; Leeson, 1957; Adams and Yawger, 1961; Yamomoto et al., 1962). </p><p>Deterioration of foods through enzyme action can lead to the development of off-flavors and also to marked changes in color and texture. Despite the efforts of many investigators over the past thirty years the enzymes responsible for quality deterioration have not been positively identified except in a few cases, mostly concerned with changes in texture. There is general agreement that where flavor is concerned several enzymic systems may be involved, working in sequence or simultaneously. </p><p>The problem is complicated by the fact that the substances respon- sible for off-flavors are also largely unknown. Many compounds which could be involved can be detected by taste at extremely low concen- trations, of the order of 1 part in lo9 (Lea and Swoboda, 1958), that is, at levels at which chemical isolation and identification are difficult. In some frozen vegetables a good correlation can be obtained between acetaldehyde content and off-flavor, so that acetaldehyde levels can be used as an index of quality deterioration (Gutterman et al., 1951; Lovejoy, 1952). Acetaldehyde is not the cause of the off-flavor, but appears as a reaction by-product, possibly from anaerobic glycolysis. Anaerobic conditions may exist in frozen tissues (Fuleki and David, 1963). Joslyn (1966) has noted that the odors and flavors formed during </p></li><li><p>6 F. AYLWARD AND D. R. HAISMAN </p><p>the storage of frozen unblanched vegetables resemble those of fresh vegetables held in oxygen-deficient atmospheres at room temperature, and that particular vegetables develop quite characteristic odors. </p><p>Progress has been made in identifying both the precursors and volatile products responsible for flavor and odor changes in certain products. Falconer et al. (1964) found that the violetlike off-flavor in dehydrated carrot was closely related to the oxidation of p-carotene, and was derived from p-ionone and other oxidation products. The formation of pyrollidone carboxylic acid from glutamine has been shown to cause bitter phenolic off-flavors in beet products (Shallen- berger and Moyer, 1958). </p><p>Gas-liquid chromatographic examination of the volatile compounds from stored frozen vegetables has yielded interesting results. Bengts- son and Bosund (1964) evaluated the volatile substances from stored frozen peas, and found that the compounds formed slowly during frozen storage resembled those found in rapid postharvest changes at ordinary temperatures. The main components were acetaldehyde, ethanol, and hexanal; the hexanal content showed promise as an indi- cator for off-flavor development. It was shown later (Bengtsson et al., 1967) that the hexanal concentration in the vapor over cooked frozen peas correlated well with the postharvest deterioration which had occurred. Off-flavor development during the first months of storage at -5C coincided with the formation of hexanal, but over longer periods the hexanal concentration decreased, suggesting that it may be un- reliable as a single quantitative indicator of quality. </p><p>Other workers have examined hexanal formation in different products. Thus, Whitfield and Shipton (1966), in their...</p></li></ul>