Effects of processing and storage on antioxidant efficacy in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 373

Pericarp contains more water, therefore it is unstable, and the present pro-
oxidative enzymes are active. For this reason, it should be processed as quickly
as possible. The material contains similar antioxidants as oilseeds. If the
processing is careful, oils obtained by gentle pressing have agreeable flavour,
and can be directly used, without refining. They are called virgin oils, or extra
virgin oils in the case that the quality is extraordinary. The more widely used
virgin oil is produced from olives. The virgin and extra virgin olive oil contains
a group of related phenolic antioxidants.
Seeds are converted into meal prior to processing, allowing enzymes to come
in contact with oil. They are treated with steam in order to inactivate enzymes
and to cleave the lipoprotein complexes, so that antioxidants remain without any
perceptible change.
15.3.2 Expeller pressing and solvent extraction
Oil is isolated from the oilseed meal, usually by a combination of expeller
pressing and solvent extraction, most often with hexane. Nonpolar natural
antioxidants remain in the oil phase, while polar antioxidants remain in the
expeller cakes (Amarowicz et al., 2001) or in extracted meals (Schmidt and
Pokorn , 2005). Natural antioxidants of rice brans (tocopherols, -oryzanol)
were decomposed during storage and solvent extraction. The decrease could be
minimized by optimizing technological factors, such as heating time and
temperature (Yan and Gu, 2003).
15.4 Changes of antioxidant functionality during oil
processing
15.4.1 Changes during oil refining
The taste and flavour of crude oil as obtained by pressing and extraction, are
unacceptable for most consumers, and should be refined before consumption.
The classical process consists of several steps (Tailor, 2005); treating with acids
precipitates phospholipids; it is called degumming. Degummed oils are mixed
with an aqueous solution of sodium hydroxide or sodium carbonate to remove
fatty acids as water-soluble salts. Natural pigments are removed by bleaching
with a bleaching earth.
Impurities are removed by deodorization, i.e. oil is heated under very low
pressure and high temperature so that volatiles are distilled off. A part of
tocopherols is co-distilled. The losses of tocopherols may be as high as 30%, but
the stability may be improved by adding citric acid before the deodorization
(Greyt and Kellens, 2005).
Loss of tocopherols means loss of vitamin activity as well, therefore, -
tocopherol acetate is added to deodorized oil. The acetate is used instead of free
tocopherols as it is more oxidation-resistant than tocopherol, but has the same
biological activity in vivo. The disadvantage is that -tocopherol acetate has no