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 qualityyellowish 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 0.5
1% 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