Abstract
The object of this study was to detect the substances which caused foaming of thermally oxidized soybean oil in deep fat frying.Soybean oil was subjected to thermal oxidation at 200°C in the presence of air for various lapse of time. The column chromatographic method was used to fractionate the foaming oil thus obtained. Silicagel (20g) treated with 60ml of n-hexane containing 5% i-propyl ether served as the adsorbent. Thermally oxidized soybean oil samples (0.51g) were eluted with 100ml of 15% i-propyl ether in hexane to yield first fraction (F1); 100ml of 60% i-propyl ether in n-hexane to yield the second fraction (F2) and with 100ml of ethyl ether to yield the third fraction (F3). Soybean oils which were heated at 200°C for 2, 4, 6 and 10hr, respectively, in the presence of air showed increase of fractions F2 and F3, while a decrease was seen in fraction F1 in the respective oil. The contents of F1, F2 and F3 in soybean oil heated for 10hr at 200°C were 38, 35 and 25% respectively.The physical and chemical characteristics, mean molecular weight and foaming tendencies of each fraction thus obtained were measured. The measurements of foaming tendency were carried out by the gas pumping method through a sieve-plate which would be suitable for the purpose for treating such small quantities of samples. The results showed that the foaming of soybean oil was caused by the oxidative polymerized fractions therein.The reduction of carbonyls and epoxy groups to hydroxyl in F2 and F3 with Sodium Borohydride caused more foaming, and the elimination of hydroxyl group caused less foaming, indicating that the polar fractions do exert effect on the foaming tendencies but would actually vary by the quantity and or variety of the polar groups formed.
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