216 Novel food packaging techniques In general, CA influences flavour quality by reducing loss of acidity, starch to sugar conversion, and biosynthesis of aroma volatiles, especially esters Retention of ascorbic acid and other vitamins results in better nutritional quality, including antioxidant activity, of fruits and vegetables when kept in their optimum CA(Kader, 2001). However, little information is available on the effectiveness of controlled atmospheres or modified atmosphere packaging(C MAP)on nutrient retention during storage. The influence of CA/MAP on the antioxidant constituents related to nutritional quality of fruits and vegetables including vitamin C, carotenoids, phenolic compounds, as well as glucosinolates will be reviewed here 11.6.1 Vitamin C Vitamin C is one of the most important vitamins in fruits and vegetables for human nutrition. More than 90% of the vitamin C in human diets is supplied by the intake of fresh fruits and vegetables. Vitamin C is required for the prevention of scurvy and maintenance of healthy skin, gums and blood vessels. Vitamin C, as an antioxidant. reduces the risk of arteriosclerosis. cardiovascular diseases and some forms of cancer(Simon, 1992). Ascorbic oxidase has been proposed as he major enzyme responsible for enzymatic degradation of L-ascorbic acid (AA). The oxidation of AA, the active form of vitamin C, to dehydroascorbic acid (dha)does not result in loss of biological activity since dha is readily re- converted to l-AA in vivo. However, DHA is less stable than AA and may be hydrolysed to 2, 3-diketogulonic acid, which does not have physiological activity (Klein, 1987)and it has therefore been suggested that measurements of vitamin C in fruits and vegetables in relation to their nutritional value should include both aa and dha The vulnerability of different fruits and vegetables to oxidative loss of AA varies greatly, as indeed do general quality changes. Low pH fruits(citrus fruits are relatively stable, whereas soft fruits(strawberries, raspberries)undergo more rapid changes. Leafy vegetables(e.g. spinach) are very vulnerable to spoilage and AA loss, whereas root vegetables(e.g. potatoes) retain quality and AA for many months(Davey et al., 2000). Fruits and vegetables undergo changes from the moment of harvest and since l-aa is one of the more reactive compounds it is particularly vulnerable to treatment and storage conditions. In broad terms, the milder the treatment and the lower the temperature the better the retention vitamin C, but there are several interacting factors that affect Aa retention (Davey et al., 2000). The rate of postharvest oxidation of AA in plant tissues has been reported to depend upon several factors such as temperature, water content, storage atmosphere and storage time(Lee and Kader, 2002) The effect of controlled atmospheres on the ascorbate content of intact fruit has not been extensively studied. The results vary among fruit species and cultivars, but the tendency is for reduced O2 and/or elevated CO2 levels to enhance the retention of ascorbate(Weichmann, 1986; Kader et al., 1989).A reduction in temperature and of O2 concentration in the storage atmosphere haveIn general, CA influences flavour quality by reducing loss of acidity, starch to sugar conversion, and biosynthesis of aroma volatiles, especially esters. Retention of ascorbic acid and other vitamins results in better nutritional quality, including antioxidant activity, of fruits and vegetables when kept in their optimum CA (Kader, 2001). However, little information is available on the effectiveness of controlled atmospheres or modified atmosphere packaging (CA/ MAP) on nutrient retention during storage. The influence of CA/MAP on the antioxidant constituents related to nutritional quality of fruits and vegetables, including vitamin C, carotenoids, phenolic compounds, as well as glucosinolates will be reviewed here. 11.6.1 Vitamin C Vitamin C is one of the most important vitamins in fruits and vegetables for human nutrition. More than 90% of the vitamin C in human diets is supplied by the intake of fresh fruits and vegetables. Vitamin C is required for the prevention of scurvy and maintenance of healthy skin, gums and blood vessels. Vitamin C, as an antioxidant, reduces the risk of arteriosclerosis, cardiovascular diseases and some forms of cancer (Simon, 1992). Ascorbic oxidase has been proposed as the major enzyme responsible for enzymatic degradation of L-ascorbic acid (AA). The oxidation of AA, the active form of vitamin C, to dehydroascorbic acid (DHA) does not result in loss of biological activity since DHA is readily re￾converted to L-AA in vivo. However, DHA is less stable than AA and may be hydrolysed to 2,3-diketogulonic acid, which does not have physiological activity (Klein, 1987) and it has therefore been suggested that measurements of vitamin C in fruits and vegetables in relation to their nutritional value should include both AA and DHA. The vulnerability of different fruits and vegetables to oxidative loss of AA varies greatly, as indeed do general quality changes. Low pH fruits (citrus fruits) are relatively stable, whereas soft fruits (strawberries, raspberries) undergo more rapid changes. Leafy vegetables (e.g. spinach) are very vulnerable to spoilage and AA loss, whereas root vegetables (e.g. potatoes) retain quality and AA for many months (Davey et al., 2000). Fruits and vegetables undergo changes from the moment of harvest and since L-AA is one of the more reactive compounds it is particularly vulnerable to treatment and storage conditions. In broad terms, the milder the treatment and the lower the temperature the better the retention of vitamin C, but there are several interacting factors that affect AA retention (Davey et al., 2000). The rate of postharvest oxidation of AA in plant tissues has been reported to depend upon several factors such as temperature, water content, storage atmosphere and storage time (Lee and Kader, 2002). The effect of controlled atmospheres on the ascorbate content of intact fruit has not been extensively studied. The results vary among fruit species and cultivars, but the tendency is for reduced O2 and/or elevated CO2 levels to enhance the retention of ascorbate (Weichmann, 1986; Kader et al., 1989). A reduction in temperature and of O2 concentration in the storage atmosphere have 216 Novel food packaging techniques