Stereospecific positional distribution of fatty acids and frying properties of winged bean (Psophocarpus tetragonolobus DC) and soybean (Glycine max.) oils

Abstract

To make appropriate utilization of the winged bean (WB) seeds, lipids were extracted from the matured seeds along with that of soybeans (SB) as comparison and subjected to lipase hydrolysis and frying experiment. Following the silica gel TLC plate and GC-FID analyses, winged bean oil (WBO) had > 15 % behenic acid (C22:0) compared to soybean oil (SBO) (˂0.5%). C22:0 together with palmitic (C16:0), stearic (C18:0) and lignoceric (C24:0) acids constituted 42 % of the saturated fatty acid (SFA) esters at α-(sn -1/3) positions. At the sn-2 position, C16:0 (3.2%), C18:0(2.6%) and C22:0(2.3%) were the main SFA esters, whereas linoleic (47.46%) and oleic (39.24 %) formed the unsaturated fatty acids (UFA) esters. In SBO, the main unsaturated fatty acid (UFA) at α-(sn -1/3) were oleic (C18:1n-9), linoleic (C18:2n-6) and linolenic (C18:3n-6) acids, constituting 24.50, 43.2 and 5.6%, respectively of the total esters. At the β-(sn-2) position, C18:2n-6 (74.7 %), C18:1n-9 (14.6%) and C18:3n-6 (8.6%) acids were the dominant UFA, with palmitic (17.5 %) and stearic (7.1 %) acids being the major SFA at the same positions. Physicochemical characteristics of used WBO and SBO following a 16 h frying process generated 1.43 and 8.87 %, and 8.02 and 12.13%, respectively, of polar compounds and total oxidation value. The presence of about 13 % of C22:0 at the sn-1/3 positions coupled with its high solid fat content and oxidative stability could make WBO a potentially good raw material for the development of functional foods, trans-fat free margarines and a better frying fat compared to soy oil that could be prone to oxidation.

Keywords: Stereospecific, hydrolysis, rancidity, soybean oil, winged bean oil