The effect of surfactants on the interfacial tension of frying fat

Interfacial tension (IFT) between fat and water was measured as a means of evaluating fats used for frying. The Du Nouy ring method was used with a tensiometer to obtain IFT. The IFT of donut frying fats, which were diluted 1:1 with unused fat, decreased with increasing frying time from 21 mN/m in unused fat to 5 mN/m in fat at the point of discard. To determine which individual chemical components in donut frying fat are responsible for the decreased IFT, various surfactants were added. The addition of surfactants to the water phase (phosphatidylcholine, sodium oleate, and sodium chloride) or oil phase (monoacylglycerols, diacylglycerols, and fatty acids) had varying effects on the IFT of soybean oil against water at 25 ± 0.1°C. The IFT decreased with increasing concentration of monoacylglycerols, sodium oleate, phosphatidylcholine, and sodium chloride. Sodium oleate was the most effective agent in lowering IFT, reducing it almost to zero at a concentration of 0.1%. At a concentration of 0.2%, monoacylglycerols lowered IFT of fresh soybean oil by 14% (monolinolein), 22% (monoolein), and 26% (monostearin). Egg lecithin lowered the IFT by 42%. These surfactants changed the IFT of fresh soybean oil by their adsorption at the oil/water interface. Although sodium chloride is not a surfactantper se, it lowered IFT by salting out surfactants from the aqueous phase onto the interface. The addition of diacylglycerols or fatty acids had virtually no effect on IFT.