Effect of pH and Pepsin Limited Hydrolysis on the Structure and Functional Properties of Soybean Protein Hydrolysates

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Abstract

Effects of limited enzymatic hydrolysis with pepsin on the functional properties and structure characteristics of soybean proteins were investigated. Hydrolysates with different incubation time (10 to 900 min) were prepared. Results showed that SPI hydrolyzed for 60 min exhibited the best emulsibility and the ability of resisting freezing/thawing. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis proved that pepsin can degrade glycinin but had little effect on the α’ subunit of β-conglycinin. The structure unfolding reached the largest extent after incubation for 60 min and the soluble and flexible aggregates were formed. After 120 min, glycinin was degraded totally and β-conglycinin formed insoluble aggregates. Moreover, 2 methods were applied for the deactivation of pepsin to obtain final hydrolysates at pH 2.0 and 7.0, respectively. The structure analysis revealed that the unfolding extent and structure characteristic were different in these 2 conditions. When adjusting the pH value from 2.0 to 7.0, the unfolding protein molecular would reaggregate again at pH 7.0 due to the charge neutralization, and the hydrodynamic diameter and λmax absorbance decreased compared to pH 2.0. Moreover, some of the insoluble aggregates formed at pH 2.0 became soluble at pH 7.0, because of the salt-in phenomenon.

Practical Application

Globulin, mainly including components of glycinin and β-conglycinin, makes up approximately 90% of soybean proteins. Due to the compacted globular conformation of glycinin, the functional properties of soybean proteins are limited. Hence, methods to modify glycinin structure and improve its functional properties are a very large technological challenge to expand the use of soybean proteins. In this study, we found that pepsin could effectively degrade glycinin and after pepsin treatment, the soybean protein showed better solubility at around isoelectrict point (pH 4 to 5), offering a potential use in the dairy industry for the manufacture of products such as yogurt.

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