Effects of processing and storage on antioxidant efficacy in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 382

Pan frying is the traditional way of frying. A thin layer of fat is heated to
about 180±200 ëC for 2±10 min. The fried material is heated only on the surface
so that the antioxidants losses, expressed in the whole food mass, are only
moderate. Phenolic substances present in frying oil are, however, oxidized to a
high degree as air oxygen has easy access not only to the surface, but also to thin
lower layers. The technology of deep fat frying is discussed in more detail in
another chapter.
The impact of air oxygen is lower in deep fat frying as the layer of frying oil
is thick (100±200 mm), and oxygen can enter only through the interface of
frying oil with air. Therefore, the rate of oxidation in frying oil is relatively low,
and in comparison with pan frying, antioxidants are more important for their
inhibition of lipid polymer formation (RÂblov et al., 1999), and of course,
antioxidants do polymerize, too. The stability of synthetic or natural antioxidants
depends on their non-volatility and activity/temperature dependence.
Special antioxidants were reviewed from the standpoint of their application in
frying oil (Boskou and Elmafa, 1999). The access of oxygen may be prevented
by addition of small amounts of polysiloxane. It forms a thin layer on the
interface between air and frying oil so that the diffusion of air oxygen to
polymeric acid and antioxidants in frying oil is inhibited.
Fish balls (made of comminuting fish meat) were coated with zein and stored
at 4 ëC. An addition of BHA, BHT, and PG to the coating improved the stability.
The last mentioned antioxidant was most effective in this system (Lian-Sun et
al., 2009).
Losses of phenolic antioxidants in frying oil are higher in materials
containing more lipids, such as meat or fish, as they are partially lost by
diffusion in frying oil. Losses of antioxidants are much lower in the case of
frying potatoes or vegetables as another technological procedure is applied.
Potatoes have a high percentage of water, which is evaporated in hot frying oil,
and replaced by frying oil during the process. Therefore, losses of frying oils by
absorption into the fried material are relatively high, and the absorbed oil should
be often replaced by fresh oil (Pokorn , 2002), so that its degree of oxidation is
lower than in case of meat. The concentration of antioxidants in frying oil is thus
not so much affected by frying, and in frying French fries, the frequently
replenished frying oil is used for a very long time.
Frying is often performed without any oil to reduce the energy content. Its
flavour is different from products fried in oil. Such processes are more similar to
roasting. The lipid fraction (DostÂlov et al. 2005) is relatively stable,
nevertheless, tocopherols and other natural antioxidants suffer great losses.
15.7 Changes of antioxidant functionality during drying
Drying can proceed under ambient temperature (air drying) or more often, under
heating. Vacuum drying is more expensive. Water present in food is converted
to steam, and is replaced by air so that the lipids are oxidized, and natural