Effects of processing and storage on antioxidant efficacy in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 379

were lost. During cabbage cooking by steaming, losses are smaller, i.e. 21±23%
ascorbic acid and 10% total phenols. The Trolox antioxidant activity decreased
by 5±20% (Podsedek et al., 2008). Phenolic compounds enhanced the
antioxidant capacity of peas, due to phenol±protein interaction. Hydroxy-
cinnamic acids (such as ferulic, coumaric and caffeic acids), were the most
active ingredients (Pi-Jen and Chen-Hue, 2006).
Hydrolysis of phenolic antioxidants bound as glycosides could proceed
during boiling, but aglycones usually have almost the same molar activity as the
respective glycosides so that the antioxidant functionality is not significantly
affected.
15.6.2 Changes during baking
In the course of baking, food materials are heated to temperatures higher than
150±200 ëC. Such a high temperature is only on the surface, but less than 100 ëC
inside the material. Therefore, the decomposition of phenolic antioxidants is
much more intensive in the surface layer than in the core. Oxidized products of
phenolic substances can react with amine and thiol groups of proteins present in
baked foods. Both types of oxidation products react with proteins. During
baking, Maillard reactions proceed (Manzocco et al., 2001). Phenolic anti-
oxidants oxidize into free radicals and partially further to quinones. The basis for
Maillard reaction can be either sugars or ascorbic acid and proteins, or oxidized
lipids and proteins. The resulting brown products act as synergists of phenolic
substances by forming free radicals, or partially by binding the prooxidative
metals, such as iron and copper, into undissociated inactive complexes.
Garlic juice heated to 130 ëC for two hours showed strong antioxidant activity
because the formation of thiacremonone (2,4-dihydro-2,5-dimethyl-thiophen-3-
one) by secondary decomposition of garlic sulfur compounds. Thiacremonone
has stronger antioxidant properties than -tocopherol, ascorbic acid, and BHA
(Guk et al., 2007).
15.6.3 Changes during microwave heating
Microwave cooking is a modern, frequently used procedure for food prepara-
tion. In the case of microwave heating, lipids are oxidized rapidly because of
close contact with oxygen so that the oxidation in a microwave oven is faster
than in case of a hot-air oven. The time necessary for microwave cooking to
reach the desired temperature, and the time of heating is, however, much shorter
than in a hot-air oven. Therefore, the changes of lipid by oxidation processes and
antioxidant destruction are not much different from the conventional hot-air
oven, at least under conditions of food cooking (Table 15.1) or may be even
lower. Phenolic antioxidants offer satisfactory protection of the lipid fraction
(DostÂlov et al., 2005). Almost no changes took place after 8±10 min heating of
canola oil stabilized with BHA, BHT or citric acid (Vieira and Reghitano-
d'Arce, 2001).