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.

Protein and polysaccharide are used as many food function including emulsifying and stabilizing agent. Some food emulsions are in low pH such as sorbet ice cream, soft drink, salad dressing etc. According to our previous study was found that fish gelatin (FG) and sodium alginate (AL) mixture provide the information to be a new food emulsifier including at low pH condition (pH 3.5). In addition the study of interfacial rheology has been interested to interpret the formation and stabilization of food emulsion. Then the objective of this study was to investigate the relationship between emulsifying characteristics and interfacial shear rheology of acidified emulsion which was stabilized by FG and AL mixture, compared with a commercial gum arabic (GA). The mixtures of FG and AL were separately prepared with phosphate-citrate buffer at pH 3.5 and 1 %w/v of total concentration. The ratio of FG:AL was 100:0, 80:20, 50:50, 80:20, and 0:100 by weight. They were mixed at 24±2 oC for 90 minutes and kept at the same temperature for 24 hours to reach equilibrium. The emulsion was prepared by adding 1.5 g of olive oil in 100 ml of mixture and homogenized by high speed homogenizer at 10,000 rpm for 10 min at 24±2 oC. The emulsion stabilized with 20:80 showed the best stability after 14 days. The result could be supported by low interfacial tension, high bulk viscosity, and electrostatic repulsion (minus value of electrophoretic mobility). For interfacial shear rheology (storage modulus, G' and loss modulus, G''), G' of 20:80 seem wider liner viscoelastic region which mean more resistance to fracture at interface. While the magnitude of G' and different value of G' and G'' were not difference. In addition, the interfacial shear viscosity of 20:80 also showed high resistance to flow at low shear rate (0.001 - 0.1 s-1). Keywords: Fish gelatin, Sodium alginate, Emulsion, Interfacial shear rheology