Oxidative-antioxidative interactions in the processing of edible oils
This study investigates the interplay of naturally occurring pro- and antioxidants during deep-fat frying. The study has significance to the food industry because knowledge of these interactions are important for maintaining the sensory and nutritional quality of fried food.
Antioxidant protection was observed in an oil blend of sunflower, olive and butteroil when used for potato frying, reflecting the presence of butteroil. Fractionation of butteroil and incorporation of these fractions into heating studies showed enhanced stability in the liquid fraction. Carotenoids and tocopherols may be responsible for this protection since these natural antioxidants are present in butteroil and are known to migrate to the liquid phase during fractionation. When butteroil was blended with sunflower oil and stored at 37$\sp\circ$C no antioxidant activity was observed, suggesting that the high temperatures of frying may also be necessary in the observed protection, i.e. through the production of malliard reaction products.
When the oil blend was used to fry cod and analyzed for oxidation by polymer formation, the oil showed less polymers than expected, in comparison to the extensive oxidation reported by change in dielectric constant and fatty acid analysis. Subsequent studies suggest that polymers produced during the frying of cod may preferentially bind to the fish protein. When the cod was freeze-dried prior to frying, the fish was shown to protect the oil against oxidation. Lipid and non-lipid solid fractions of the fish exhibited antioxidant protection under similar conditions. This suggests that phospholipids and protein, the primary components of the lipid and solid fractions, respectively, may be responsible for this protection. Enhanced stability of sunflower oil was also observed when commercial amino acids and phospholipids were incorporated and heated at 185$\sp\circ$C.
The oxidative process of oils during deep-fat frying can be very complex. A wide variety of factors must be considered when determining the oxidative stability of a frying system. The competitive role of pro- and antioxidants potentially present in both the oils used for frying and the food substrates being fried can greatly influence the frying system and ultimately affect the quality of the food product.