Food processing involves changes in structural integrity of the plant material and this produces both negative and positive effects. When the negative and positive effects counterbalance each other, no change in the antioxidant activity occurs.115 The antioxidant activity is diminished owing to inactivation of antioxidant compounds caused by oxidation, for example, by enzymes (polyphenoloxidase and others) or leaching into the cooking water. Both negative changes have a greater impact on the water-soluble antioxidants, vitamin C, flavonoids and phenolic acids, than on the lipid-soluble antioxidants, carotenoids and tocopherols. The positive effects of food processing include transformation of antioxidants into more active compounds, such as the deglycosylation of onion quercetin,106 as well as an increase in the antioxidant activity owing to inhibition of enzymes.81 Peeling and juicing result in substantial losses of carotenoids, anthocyanins, hydroxycin-namates and flavanols as the fruit and berry skins and vegetable peels are very rich in these antioxidant compounds. However, the antioxidant activity of fresh fruits and berries is comparable to that of their processed products such as juices and wine.4,29,111 Also the antioxidant activity in tomato juice was comparable to that of fresh vegetables in most studies.52,114
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