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 characteristicsdramatically 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