Improved freeze-thaw stability of o/w emulsions prepared with soybean protein isolate modified by papain and transglutaminase

Oil-in-water (o/w) emulsions stabilized by soybean protein isolate (SPI) and enzyme-modified soybean protein isolate (SPI + TGase, SPIH, SPIH + TGase) were subjected to three freeze-thaw cycles, and changes in the molecular weight distribution, secondary structure contents, particle size, oiling off (OF), creaming index (CI) and microstructures were investigated. The emulsions exhibited different freeze-thaw tolerance stabilities, which were dependent on the type of enzyme used to prepare the SPI as well as the cycle number of freeze-thaw treatments. Differential scanning calorimetry suggested that emulsion destabilization was mainly due to water crystallization. After three freeze-thaw cycles, the emulsion stabilized by SPIH + TGase showed a decrease of 78.89% in CI and 72.35% in OF compared with SPI. Correspondingly, the particle size decreased by 70.76% (in water) and 88.99% (in 1% SDS). The improvement in the freeze-thaw stability could be largely attributed to the increased molecular flexibility due to the increased alpha-helix and random coil contents after enzymatic hydrolysis and the formation of gel-like network structures after the addition of TGase cross-linking. The observation of the microstructure also indicated that SPIH + TGase was an effective food emulsifier. These findings could be of great importance for the formulation of food grade emulsions with excellent freeze-thaw stability.