Microwave irradiation effects on the structure, viscosity, thermal properties and lubricity of soybean oil

Soybean oil is a highly valuable agricultural commodity for the United States. To further add value to soybean oil, chemical and physical modifications, as well as additives, have been extensively used to change the oil characteristics and properties, broadening the potential industrial applications. Heat treatments such as heat-bodying have been implemented to change soybean oil properties, but no research has studied the effects of microwave-irradiation on soybean oil structure and properties.Soybean oil (SBO) was heat-bodied (HB) or microwave-irradiated (MI). HB and MI (200–250 °C for 20–60 min) oil had similar Gardner bubble viscosity (B–C range). SBO that was HB or MI had increased viscosity compared with untreated SBO. 1H NMR analysis showed no oxidation occurred for all treatments. However, HB and MI oil formed a cyclic ring structure with polymerization that most likely contributed to the increased viscosity. Pour point decreased from −9 °C for the untreated SBO, −15 °C for the HB, and −18 °C for the MI despite viscosity increases. Pour point anomaly is likely due to triacylglyceride cyclic ring formation. Pressurized DSC analysis showed higher oxidative stability for HB oil with even higher stability for MI oil. Compared with untreated SBO, HB and MI oil increased friction coefficient and decreased film percentage, whereas MI oil tended to leave larger wear scratches on the ball and disk during friction measurements. MI oil improved SBO cold-flow behavior, but reduced its potential as a lubricant.