230 Chilled foods 4 Chill storage time(days) Fig. 9.3. Effect of cooking and storage atmospheres on woF development in chicken breasts(O-O) cooked and stored in air; (x-x)cooked in nitrogen, stored in air; )cooked in air, stored in nitrogen,([A]-[AD cooked and stored in nitrogen. atmosphere reduced the TBa values and sensory scores for WOF intensity as compared with those cooked in air and stored in either nitrogen or air(Fig. 9.3) Autooxidation or oxidative rancidity is by no means confined to meat and meat products. Dairy products and fatty fish are also highly susceptible. Migration of copper into cream on churning can initiate the oxidative sequence of reactions causing rapid flavour impairment. Buttermilk has a high proportion of unsaturated phospholipids, particularly phosphatidylethanolamine, that can bind metal ions in a prooxidative fashion, and the presence of a metal-phospholipid complex 1-water interface facilitates lipid hydroperoxide formation. Fish fats contain a high proportion of n-3 polyunsaturated fatty acids, which are vulnerable to oxidation by atmospheric oxygen leading to deteriorative changes. Despite this, rancid flavours only appear to affect the acceptability fattier species such as trout, sardine, herring and mackerel; and even then, trout and gutted mackerel oxidize at temperatures above 0oC whereas herring remains relatively unaffected. Castell(1971)has suggested that in fish, oxidized lipids become bound in lipid-protein complexes rather than forming carbonyl compounds associated with rancid flavours. The lipid-protein complexes also ontribute to the toughened texture of poorly stored fish. Competing demands for vailable oxygen from microorganisms and enzymes, which differ between species, may also influence whether oxygen is available for autooxidation. In trout, reports of lipoxygenase activity in the skin tissue have suggested the
atmosphere reduced the TBA values and sensory scores for WOF intensity as compared with those cooked in air and stored in either nitrogen or air (Fig. 9.3). Autooxidation or oxidative rancidity is by no means confined to meat and meat products. Dairy products and fatty fish are also highly susceptible. Migration of copper into cream on churning can initiate the oxidative sequence of reactions causing rapid flavour impairment. Buttermilk has a high proportion of unsaturated phospholipids, particularly phosphatidylethanolamine, that can bind metal ions in a prooxidative fashion, and the presence of a metal–phospholipid complex at an oil-water interface facilitates lipid hydroperoxide formation. Fish fats contain a high proportion of n3 polyunsaturated fatty acids, which are vulnerable to oxidation by atmospheric oxygen leading to deteriorative changes. Despite this, rancid flavours only appear to affect the acceptability of fattier species such as trout, sardine, herring and mackerel; and even then, trout and gutted mackerel oxidize at temperatures above 0ºC whereas herring remains relatively unaffected. Castell (1971) has suggested that in fish, oxidized lipids become bound in lipid-protein complexes rather than forming carbonyl compounds associated with rancid flavours. The lipid–protein complexes also contribute to the toughened texture of poorly stored fish. Competing demands for available oxygen from microorganisms and enzymes, which differ between species, may also influence whether oxygen is available for autooxidation. In trout, reports of lipoxygenase activity in the skin tissue have suggested the Fig. 9.3. Effect of cooking and storage atmospheres on WOF development in chicken breasts. (❍—❍) cooked and stored in air; ( —) cooked in nitrogen, stored in air; (■— ■) cooked in air, stored in nitrogen; ([]—[]) cooked and stored in nitrogen. 230 Chilled foods
Non-microbiological factors affecting quality and safety 231 potential to initiate lipid oxidation by providing a source of initiating radicals German and Kinsella 1985). A complicating factor in the assessment of the significance of oxidation to the quality of fish is that many products distributed chilled have previously been frozen, particularly for example, herring, to sprea seasonal availability 9.3.2 Pink discoloration in meat products Discoloration in foods is a common problem which can take many forms and be ssociated with a wide range of chemical reactions: biochemical or enzymic browning is considered later in this chapter. Pink discoloration in cooked meats br is a long-standing and all too common problem affecting manufacturing, retailing food service and domestic sectors and is often interpreted as undercooking. The problem is particularly evident with sliced meats, reformed roast products, pasties and casseroles. Various causes of pinking have beer identified and these are indicated in Table 9. 1 on the basis of the pigment type thought to be involved. Maga(1994)has reviewed the causes and factors affecting pink discoloration in cooked white meats L Myoglobin is a monomeric globular haem-protein found in all vertebrates hich together with haemoglobin give rise to the red colour of meats. The amount of myoglobin varies from species to species, tissue to tissue and is affected by a wide range of environmental factors. As indicated in Table 9. 1 myoglobin can be present in several forms, some of which can impart a red or pink residual colour to the meat even after cooking. Recent work has indicated that over 80% of instances of pinking are due to nitrosomyoglobin arising from nitrate contami- nation and its subsequent bacterial reduction to nitrite( Brown et al. 1998) 9. 4 Characteristics of biochemical reactions Biochemical reactions are catalysed by specialized proteins called enzymes They are highly specific and efficient catalysts, lowering the activation threshold so that the rate of reaction of thermodynamically possible reactions is Table 9.1 Pigment types and causes giving rise to pink coloration in meat products Brown et al. 1998) Cause of pink discoloration Oxymyoglobin Low temperature cooking Nitrosomyoglobin Nitrite contamination directly or from reduced nitrate nitrogen oxides in ovens Carboxymyoglobin Carbon monoxide in ovens; gamma-irradiation Reduced denatured myoglobin High pH, slow cooking, high salt and availability of educing agents
potential to initiate lipid oxidation by providing a source of initiating radicals (German and Kinsella 1985). A complicating factor in the assessment of the significance of oxidation to the quality of fish is that many products distributed chilled have previously been frozen, particularly for example, herring, to spread seasonal availability. 9.3.2 Pink discoloration in meat products Discoloration in foods is a common problem which can take many forms and be associated with a wide range of chemical reactions: biochemical or enzymic browning is considered later in this chapter. Pink discoloration in cooked meats is a long-standing and all too common problem affecting manufacturing, retailing food service and domestic sectors and is often interpreted as undercooking. The problem is particularly evident with sliced meats, reformed roast products, pasties and casseroles. Various causes of pinking have been identified and these are indicated in Table 9.1 on the basis of the pigment type thought to be involved. Maga (1994) has reviewed the causes and factors affecting pink discoloration in cooked white meats. Myoglobin is a monomeric globular haem-protein found in all vertebrates which together with haemoglobin give rise to the red colour of meats. The amount of myoglobin varies from species to species, tissue to tissue and is affected by a wide range of environmental factors. As indicated in Table 9.1 myoglobin can be present in several forms, some of which can impart a red or pink residual colour to the meat even after cooking. Recent work has indicated that over 80% of instances of pinking are due to nitrosomyoglobin arising from nitrate contamination and its subsequent bacterial reduction to nitrite (Brown et al. 1998). 9.4 Characteristics of biochemical reactions Biochemical reactions are catalysed by specialized proteins called enzymes. They are highly specific and efficient catalysts, lowering the activation threshold so that the rate of reaction of thermodynamically possible reactions is Table 9.1 Pigment types and causes giving rise to pink coloration in meat products (Brown et al. 1998) Pigment type Cause of pink discoloration Oxymyoglobin Low temperature cooking Nitrosomyoglobin Nitrite contamination directly or from reduced nitrate; nitrogen oxides in ovens Carboxymyoglobin Carbon monoxide in ovens; gamma-irradiation Reduced denatured myoglobin High pH, slow cooking, high salt and availability of reducing agents Non-microbiological factors affecting quality and safety 231
232 Chilled foods dramatically increased. The specificity of enzymes for a particular substrate is indicated in the name, usually by attachment of the suffix -ase' to the name of the substrate on which it acts: for example, lipase acts on lipids, protease on proteins. The catalytic activity of enzymes is highly dependent on the conformational structure of the protein, and many of the characteristics of enzyme-catalysed reactions result from the influence of the localized environment. Heat, extremes of acidity of alkalinity, and high may denature the enzyme, causing impairment or loss of activity. Enzyme inhibitors and activators that bind either reversibly or irreversibly may act by causing changes in conformational structure or acting directly at the active site The temperature at which denaturation takes place is often a reflection of nvironmental conditions that the enzyme naturally operates in. For most enzymes from warm-blooded animals, denaturation begins around 45.C, and by 55C rapid denaturation destroys the catalytic function of the enzyme protein; enzymes from fruit and vegetables are generally denatured at higher temperatures(70-80oC); and some microbial enzymes, e.g. lipases and proteases, can withstand temperatures in excess of 100C( Cogan 1977) In the living cell, enzymes catalyse a vast array of reactions that taken together constitute metabolism. In the cellular environment, control and coordination of enzyme activity is achieved by means of feedback mechanisms and compartmentalisation. Disruption which occurs at the time of slaughter or harvest may necessitate steps being taken to prevent the subsequent action of enzymes(blanching of vegetables is a good example); or the activity of enzymes may be enhanced if they improve product quality, as in the case of conditioning of meats, where protease activity is used to break down muscle fibres to develop full flavour and tenderness The rate of enzyme-catalysed reactions increases with substrate concentration but only up to a limit (maximal activity) at which the enzyme is saturated with substrate. Further increases in substrate concentration do not increase the rate of reaction. The rate of reaction increases with temperature in the same way as chemical reactions up to an optimum temperature for activity. At temperatures above this, denaturation of the enzyme protein takes place and activity is lost. At hill storage temperatures, the activity of enzymes in most foods is low, but there are notable exceptions. Enzymes in cold-blooded species may be adapted to be active at cold temperatures. In cod, lipase activity at ooC shows a marked lag phase before maximal activity is achieved and the rate of activity decreases to oc and increases to a maximum at -4C Enzymes from different sources, although catalysing conversion of the same substrates to the same reaction products, may have different characteristics in terms of rate of reaction, or pH or temperature optima, depending upon thei rigin.In a chilled pasta salad composed of cooked pasta, onion, red and green peppers, cucumber, sweetcorn, mushrooms and vinaigrette dressing, shelf-life was limited by browning of either the sweetcorn or the mushrooms depending on the holding temperature(Gibbs and williams 1990). Holding the salad at storage temperatures between 2C and 15C showed that the temperature characteristics
dramatically increased. The specificity of enzymes for a particular substrate is indicated in the name, usually by attachment of the suffix ‘-ase’ to the name of the substrate on which it acts: for example, lipase acts on lipids, protease on proteins. The catalytic activity of enzymes is highly dependent on the conformational structure of the protein, and many of the characteristics of enzyme-catalysed reactions result from the influence of the localized environment. Heat, extremes of acidity of alkalinity, and high ionic strength may denature the enzyme, causing impairment or loss of activity. Enzyme inhibitors and activators that bind either reversibly or irreversibly may act by causing changes in conformational structure or acting directly at the active site. The temperature at which denaturation takes place is often a reflection of the environmental conditions that the enzyme naturally operates in. For most enzymes from warm-blooded animals, denaturation begins around 45ºC, and by 55ºC rapid denaturation destroys the catalytic function of the enzyme protein; enzymes from fruit and vegetables are generally denatured at higher temperatures (70–80ºC); and some microbial enzymes, e.g. lipases and proteases, can withstand temperatures in excess of 100ºC (Cogan 1977). In the living cell, enzymes catalyse a vast array of reactions that taken together constitute metabolism. In the cellular environment, control and coordination of enzyme activity is achieved by means of feedback mechanisms and compartmentalisation. Disruption which occurs at the time of slaughter or harvest may necessitate steps being taken to prevent the subsequent action of enzymes (blanching of vegetables is a good example); or the activity of enzymes may be enhanced if they improve product quality, as in the case of ‘conditioning’ of meats, where protease activity is used to break down muscle fibres to develop full flavour and tenderness. The rate of enzyme-catalysed reactions increases with substrate concentration but only up to a limit (maximal activity) at which the enzyme is saturated with substrate. Further increases in substrate concentration do not increase the rate of reaction. The rate of reaction increases with temperature in the same way as chemical reactions up to an optimum temperature for activity. At temperatures above this, denaturation of the enzyme protein takes place and activity is lost. At chill storage temperatures, the activity of enzymes in most foods is low, but there are notable exceptions. Enzymes in cold-blooded species may be adapted to be active at cold temperatures. In cod, lipase activity at 0ºC shows a marked lag phase before maximal activity is achieved and the rate of activity decreases to 0ºC and increases to a maximum at 4ºC. Enzymes from different sources, although catalysing conversion of the same substrates to the same reaction products, may have different characteristics in terms of rate of reaction, or pH or temperature optima, depending upon their origin. In a chilled pasta salad composed of cooked pasta, onion, red and green peppers, cucumber, sweetcorn, mushrooms and vinaigrette dressing, shelf-life was limited by browning of either the sweetcorn or the mushrooms depending on the holding temperature (Gibbs and Williams 1990). Holding the salad at storage temperatures between 2ºC and 15ºC showed that the temperature characteristics 232 Chilled foods
Non-microbiological factors affecting quality and safety 233 260 265 70 275 280 Fig. 9.4. Organoleptic changes in chill-stored pasta salad in vinaigrette. Temperature dependence of the rates of browning of sweetcorn(●-● and mushrooms((-○) Gibbs and williams 1990) of the browning reaction, likely to be catalysed by the enzyme polyphenolox idase, were quite different in the mushrooms and sweetcorn(Fig. 9.4). In mushrooms, the rate of browning reaction appeared to be less temperature- sensitive than was the reaction in sweetcorn, such that at higher temperatures the shelf-life of the salad was limited by browning of the sweetcorn, and at lower temperatures by browning of the mushrooms. To prevent such changes or to predict the shelf-life as a function of temperature, the subtleties of the reactions causing the changes in visual appearance need to be known Enzymes in food may be endogenous, that is, they are present naturally in the tissues of the plant or animal that comprises the food. Many hundreds of enzymes fall into this category, though not all will have a significant effect on food quality. Exogenous enzymes in food may be added by the manufacturer to perform a specific function, such as papain for the tenderization of meat proteases for cheese ripening, or naringinase for the debittering of citrus juices particularly grapefruit juice. Enzymes may be present as a result of tion from one food to another when they are in contact;an example would be the migration of lipases from unblanched peppers n a pizza topping to the cheese if the appropriate tracy glycerols are available, lipolysis will result in soapy flavours. Alternatively, there may be contamination by extracellular enzymes from microorganisms such as lipases and proteases, where the organism may be destroyed by heat processing but the enzyme which is resistant to the heat treatment remains 9.5 Biochemical reactions of significance in chilled foods 9.5.1 Enzymic browning In fruits and vegetables, enzymic browning occurs due to damage such as bruising and preparation procedures of cutting, peeling and slicing. The
of the browning reaction, likely to be catalysed by the enzyme polyphenoloxidase, were quite different in the mushrooms and sweetcorn (Fig. 9.4). In mushrooms, the rate of browning reaction appeared to be less temperaturesensitive than was the reaction in sweetcorn, such that at higher temperatures the shelf-life of the salad was limited by browning of the sweetcorn, and at lower temperatures by browning of the mushrooms. To prevent such changes or to predict the shelf-life as a function of temperature, the subtleties of the reactions causing the changes in visual appearance need to be known. Enzymes in food may be endogenous, that is, they are present naturally in the tissues of the plant or animal that comprises the food. Many hundreds of enzymes fall into this category, though not all will have a significant effect on food quality. Exogenous enzymes in food may be added by the manufacturer to perform a specific function, such as papain for the tenderization of meat, proteases for cheese ripening, or naringinase for the debittering of citrus juices particularly grapefruit juice. Enzymes may be present as a result of ‘contamination’ by migration from one food to another when they are in contact; an example would be the migration of lipases from unblanched peppers in a pizza topping to the cheese where, if the appropriate triacy1glycerols are available, lipolysis will result in soapy flavours. Alternatively, there may be ‘contamination’ by extracellular enzymes from microorganisms such as lipases and proteases, where the organism may be destroyed by heat processing but the enzyme which is resistant to the heat treatment remains. 9.5 Biochemical reactions of significance in chilled foods 9.5.1 Enzymic browning In fruits and vegetables, enzymic browning occurs due to damage such as bruising and preparation procedures of cutting, peeling and slicing. The Fig. 9.4. Organoleptic changes in chill-stored pasta salad in vinaigrette. Temperature dependence of the rates of browning of sweetcorn (●—●) and mushrooms (❍— ❍) (Gibbs and Williams 1990). Non-microbiological factors affecting quality and safety 233
234 Chilled foods yellowish brown through to black pigments that are formed can appear very rapidly and are unappetizing. In the intact tissue the enzymes responsible, are separated from the sul However, when they are brought into contact as a result of damage, naturally ccurring phenolic compounds are enzymically oxidized to form yellowish quinone compounds (vamos-Vigyazo 1981). A sequence of polymerization reactions follow, giving rise to brown products such as melanins The extent of browning is dependent on the activity and amount of the polyphenoloxidase in the specific fruit or vegetable and the availability of substrates which may be catechol, tyrosine or dopamine amongst others, but there is al ways a requirement for oxygen. A number of approaches have been taken to prevent or retard enzymic browning. Reduction of the available oxygen concentration has been achieved via various approaches: vacuum packaging which retarded enzymic browning in potato strips(O Beirne and Ballantyne 1987); modified atmosphere packaging, e.g. for shredded lettuce and cut carrots (McLachlan and Stark 1985); the addition of an oxygen scavenger to the pack, which retarded enzymic browning and textural changes in apricot and peach halves(Bolin and Huxsoll 1989); and restricting oxygen diffusion into tissues by immersion in water, brine or syrup solutions. In contrast, high levels of oxygen (70-100%)have also been shown to reduce ascorbic acid breakdown, lipid oxidation and enzymic browning in cut lettuce probably as a result of increasing the total antioxidant capacity of the material (Day 1998). A more direct method to prevent enzymic discoloration is to use enzyme inhibitors, though this may conflict with the ' fresh'image of the product or be restricted by legislation Traditionally, the use of sulphite in the form of metabisul phite dips provided an effective means of preventing enzymic browning in many instances. With restrictions on the use of sulphite, alternatives have been sought. The pH optimum for phenolase activity is generally between pH 5 and 7. Reduction of the ph to less than 4 by the use of edible acids inactivates the enzyme. Citric acid and ascorbic acid dips retard browning by both a reduction in pH and complexation of copper which is essential for the enzyme to function. Levels of 10% ascorbic acid were shown to be effective for potatoes, and 0.5-1% for apples(O Beirne 1988). Phenolases from most fruits and vegetables are inactivated by heat (vamos-Vigyazo, 1981)but for salads and pre-pre vegetables heat treatment may not be an acceptable option owing concomitant changes in colour and texture 9.5.2 Glycolysis This is a key metabolic pathway of intermediary metabolism found in almost all living organisms. Changes that take place at the time of slaughter and harvest nfluence the route that substrates metabolized via this pathway subsequently follow. Diversion of the pathway to produce end-products of lactic acid in meat and ethanol in vegetables have marked consequences for the subsequent quality
yellowish brown through to black pigments that are formed can appear very rapidly and are unappetizing. In the intact tissue the enzymes responsible, generically referred to as ‘phenolases’, are separated from the substrate. However, when they are brought into contact as a result of damage, naturally occurring phenolic compounds are enzymically oxidized to form yellowish quinone compounds (Va´mos-Vigya´zo´ 1981). A sequence of polymerization reactions follow, giving rise to brown products such as melanins. The extent of browning is dependent on the activity and amount of the polyphenoloxidase in the specific fruit or vegetable and the availability of substrates which may be catechol, tyrosine or dopamine amongst others, but there is always a requirement for oxygen. A number of approaches have been taken to prevent or retard enzymic browning. Reduction of the available oxygen concentration has been achieved via various approaches: vacuum packaging which retarded enzymic browning in potato strips (O’Beirne and Ballantyne 1987); modified atmosphere packaging, e.g. for shredded lettuce and cut carrots (McLachlan and Stark 1985); the addition of an oxygen scavenger to the pack, which retarded enzymic browning and textural changes in apricot and peach halves (Bolin and Huxsoll 1989); and restricting oxygen diffusion into tissues by immersion in water, brine or syrup solutions. In contrast, high levels of oxygen (70–100%) have also been shown to reduce ascorbic acid breakdown, lipid oxidation and enzymic browning in cut lettuce probably as a result of increasing the total antioxidant capacity of the material (Day 1998). A more direct method to prevent enzymic discoloration is to use enzyme inhibitors, though this may conflict with the ‘fresh’ image of the product or be restricted by legislation. Traditionally, the use of sulphite in the form of metabisulphite dips provided an effective means of preventing enzymic browning in many instances. With restrictions on the use of sulphite, alternatives have been sought. The pH optimum for phenolase activity is generally between pH 5 and 7. Reduction of the pH to less than 4 by the use of edible acids inactivates the enzyme. Citric acid and ascorbic acid dips retard browning by both a reduction in pH and complexation of copper which is essential for the enzyme to function. Levels of 10% ascorbic acid were shown to be effective for potatoes, and 0.51% for apples (O’Beirne 1988). Phenolases from most fruits and vegetables are readily inactivated by heat (Va´mos-Vigya´zo´, 1981) but for salads and pre-prepared vegetables heat treatment may not be an acceptable option owing to the concomitant changes in colour and texture. 9.5.2 Glycolysis This is a key metabolic pathway of intermediary metabolism found in almost all living organisms. Changes that take place at the time of slaughter and harvest influence the route that substrates metabolized via this pathway subsequently follow. Diversion of the pathway to produce end-products of lactic acid in meat and ethanol in vegetables have marked consequences for the subsequent quality of the food product. 234 Chilled foods