Frying temperatures and minor constituents of oils and fats

Two important classes of minor constituents of oils and fats are tocopherol s and sterols. Both these classes are biologically active and they also aff ect the stability and performance of an oil at elevated temperatures. Tocop herols are phenolic antioxidants that react with free radicals and their co ncentration is reduced signifantly when the oil is heated. alpha-Tocopherol is lost faster during deep-fat frying than the beta, gamma and delta homol ogues. in the presence of stronger antioxidants, natural or synthetic, loss es of alpha-tocopherol can be eliminated. Unchanged phytosterols naturally present in vegetable oils are believed to be beneficial for the health. Dep ending on the chemical structure, phytosterols may act as prooxidants or an tioxidants. Sterols with an ethylidene group in the side chain have been fo und effective in retarding polymerisation at temperatures similar to those of deep-fat frying. Under unfavourable conditions (high temperature, presen ce of air) oxidation products are formed from sterols and a marked increase in the oxidation rate of the fat is observed. Oxidation products of the ma in phytosterols, beta-sitosterol and stigmasterol, are: hydrocarbons (3,5-d iene and 3,5,22-triene), mono-, di- and triunsaturated ketosteroids (4-en-3 -one, 3,5-dien-7-one, 3,5,22-trien-7-one), 5,6-epoxy derivatives, 3,7-diols and pregnane derivatives. Other minor constituents which may affect the ra te or degradation of unsaturated triacylglycerols at high temperatures are squalene, pigments and phospholipids. Squalene and phospholipids have both been reported to retard the degradation of unsaturated fatty acids under si mulated frying conditions. High chlorophyll levels were found to increase t he rate of tocopherol decomposition and formation of polymers in rape seed oil heated at 180 degrees C.