Effects of processing and storage on antioxidant efficacy in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 378

15.5.5 Changes under frozen storage
During frozen storage (below 0 ëC), water present in food is frozen. Air can thus
penetrate to the lipid components easily through channels left in food after water
freezing. Oxidation is often faster in spite of low temperature, than during
refrigeration, and losses of phenolic antioxidants become higher than on
chilling. Frozen food may be protected against losses of antioxidant activity by
application of phenolic antioxidants on the surface. The natural antioxidants,
mainly tocopherols, are then protected against losses caused by oxidation.
Polyunsaturated fatty acids, e.g. in fish, are protected against oxidation, too.
Hydrolytical changes are negligible if water is turned into ice. Fresh beef meat
was packed in a polypropylene film under frozen conditions. An addition of
rosemary resins enhanced the stability against oxidation (Nerin et al., 2006).
Red raspberries were frozen at ÿ30 ëC, and then stored at 4 ëC for three days and
one day at 18 ëC. Antioxidants remained stable, and the antioxidant activity
was unaffected, too (Mullen et al., 2002).
15.6 Changes of antioxidant functionality during heating
15.6.1 Changes during boiling
During boiling, food material is dipped in water, which is the medium of heat
transfer (Pokorn and Schmidt, 2001, 2003). The effect of phenolic antioxidants
depends on the access of oxygen. The concentration of oxygen in boiling water
is very low, and the temperature is also rather low compared to other culinary
operations at high temperature. Some synergists, such as ascorbic acid and
phosphates, are dissolved in the water phase, where the access of oxygen is slow,
but considerably easier than in boiled food. The degree of oxidation of less polar
or nonpolar antioxidants may be considered as negligible. However, some
polyphenolic antioxidants may be bound to food proteins during boiling, so that
the concentration of free phenolic antioxidants becomes lower. A small part of
polar antioxidants is dissolved in the water phase.
During steam cooking of broccoli, the contents of flavonoids and phenolic
compounds rose as they were liberated from insoluble complexes, while vitamin
C was not affected. By water boiling and steam cooking, the contents of -
carotene, lutein, and -tocopherol and -tocopherol increased, too. The cooking
has similar effect on the antioxidant activity, as well (GliszczyÂska- Ï wiglo et
al., 2006). Changes of antioxidants during cooking depend very much on the
composition of cooked food, e.g. they are different in case for pasta (Fares et al.,
2010) or beef muscle (Ahn et al., 2002).
Storage and pasteurization of green or black teas increased the optical
density, due to the formation of polymers, but decreased the content of catechins
(Manzocco et al., 1998). Phenolics are also bound to lipoproteins, but they still
exhibited the antioxidant activity corresponding to catechins (Vinson and
Dabbagh, 1998). Red cabbage may be prepared by conventional cooking.
During the process, 35.5±67.3% ascorbic acid and 33.1±54.3% of total phenols