Native soy protein is known to possess poor interfacial activity compared to flexible proteins. Acidification could alter its structure in a way that improves its interfacial activity. This study aimed to investigate the effects of seven acids, including hydrochloric, acetic, ascorbic, lactic, malic, citric, and tartaric acid on the oil-water interfacial properties and oil-in-water emulsifying properties of soy protein isolate (SPI) at pH 3.0. The aqueous solutions of 1.5 %(w/v) SPI were adjusted to pH 3.0 with different acids, and the solution without acidification (pH 8.0) was used as a control. The zeta potential of acidified SPI solutions and SPI emulsions were positive value while it was negative value for control. The particle sizes of acidified SPI solutions were between 19-83 m (107 m for the control). The particle size of acidified SPI emulsions were 0.4 m for control, acetic, ascorbic, lactic, and malic, 1.2 m for citric and tartaric, while 20.7 m for HCl. The interfacial pressure between soybean oil and the acidified SPI solutions were between 12.7-15.4 mN/m, while SPI-control was significantly higher than that at 19.4 mN/m. The acidified SPI solution had the meaningful values of interfacial shear rheological parameters and showed viscoelastic layer, but the control was almost no viscoelastic layer. All acidified SPI emulsion showed much higher emulsifying activity index (130-158 m2/g) than the control (111 m2/g). The appearance of emulsion looked no difference over the time when observed by eyes. The evaluation of emulsion stability by the changing of particle size distribution within 40 days showed that the control and HCl was no change, while the others tended to increase particle size.

• Peerapong Ngamnikom(Department of Marine Food Science and Technology, Gangneung-Wonju National University, Institutes of Green Bio Science and Technology, Seoul National University)
• Moojoong Kim(Institutes of Green Bio Science and Technology, Seoul National University)
• Jong Heon Kim(Graduate School of International Agricultural Technology, Seoul National University)
• Donghwa Chung(Institutes of Green Bio Science and Technology, Seoul National University, Graduate School of International Agricultural Technology, Seoul National University)