Antioxidants present in food undergo chemical changes during technological processing . Literature devoted to this is generally limited to losses in described antioxidants during single processing events, without assessing changes in their biological activity. Most research is based on model experiments where synthetic antioxidants are added to a product.
Technological processing may include many drastic thermal and hydrothermal processes such as pasteurisation, sterilisation, blanching, concentrating by evaporation, drying, extrusion, microwave heating as well as culinary processing such as cooking, roasting, stewing and frying. New processing methods such as high-pressure processing and high-intensity pulsed electric field treatment are also considered drastic .
Technological processes mentioned above may lead to inconvenient changes in a product matrix (Table .1) and in antioxidant stability (Table .2). Among them are antioxidant oxidation, complexing with other food compounds, enzymatic modifications and changes from active antioxidant into pro-oxidative form.
Most changes in food compounds, especially lipids and proteins, are caused by oxidative reactions. These reactions are especially dangerous when a product is subjected to contact with oxygen at increased temperature and in the presence of light. In oxidative stress conditions, reactive oxygen species and free radicals are produced by numerous biochemical pathways. Antioxidants, as they are especially sensitive to oxidation, have been commonly accepted as indicators of processing damage. Oxidation of food compounds concerns both types of compounds: those soluble in water and those soluble in lipid phase. Also, microbiological and enzymatic processing during fermentation may lead to changes in bioactive compounds. However, keeping the high antioxidant activity of antioxidants is a complex problem and the analysis is hard to perform. Negative effects due to food processing have led to the concept of food reconstitution. The idea is to add natural antioxidants, usually of the same origin, to a product to cover losses that are a result of processing. It is generally assumed that processed products have a lower nutritive value and lower health-protecting capacity than fresh ones.
Oxidative processes in fat are especially dangerous to antioxidants and their stability. They may lead to chain reactions started by lipid radicals ROO โข or RO โข , lipid hydroxides ROOH and lipid dioxylans. Singlet or triplet oxygen may be the reason for oxidative reactions. The latter changes phenols into inactive quinones. The best-known example of such reaction is tocopherol oxidation by hydrogen peroxide in the presence of Fe 2+ ions. Tocopherols change