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.