Extraction of soybean hull pectin polysaccharide and its applications in soy protein-based gel foods

Pectin polysaccharides can be closely related to various physiological functions, particularly for the resistance to obesity, diabetes, and cardiovascular diseases. Therefore, pectin polysaccharides can be often used in gel foods, due to their excellent gelation property, water solubility, and low viscosity. Among them, there is up to 30% of soybean hull pectin polysaccharide (SHPP) in the soybean hull byproducts, more than three million tons of which are produced every year in China. This study aims to improve the utilization efficiency of soybean byproducts for the better application of SHPP in gel foods. A salt-assisted hydrothermal method was also applied to extract the SHPP from soybean hulls. Physicochemical properties of SHPP were characterized, including the extraction yield, composition, size distribution, and viscosity. Then, the soybean curd (one of the soy protein-based gel foods) was applied as a case study. Segregative phase separation was investigated under various concentrations (0-2.0%) of SHPP in these mixed systems. A correlation analysis was implemented on the microphase behavior and the macro-properties (e.g., rheological behavior, mechanical properties, water holding capacity, and sensory evaluation). Besides, the commonly-used commercial citrus pectin (CP) was used as a control in foods. The results showed that the extraction yield of SHPP was up to 17.95% ± 0.21%. The obtained SHPP mainly consisted of pectin, with a galacturonic acid content of 42.13% ± 2.05%. The mean size of SHPP dispersion was determined as (210.3±2.9) nm and the polydispersity index (PDI) was 0.466 ± 0.037. The extracted SHPP was better dispersed in water, due to the hydrophilic property and low shear viscosity. There was no microphase separation, when the 0-0.5% SHPP was added to prepare the soybean curd. The obtained soybean curds were pretty white in a better appearance, particularly with the viscoelastic network and excellent water holding capacity. Furthermore, the leaking water was visible around the soybean curd, as the SHPP concentration increased up to 1.0%. A significant (P<0.05) reduction was found in the viscoelasticity and water-holding capacity. The outstanding phase separation was observed when the addition of SHPP was up to 1.5%. There was a disordered microstructure with the enlarged pore of the network in the soybean curd, resulting in weak gel strength and low water holding capacity (46.2%±3.2%). Therefore, the optimal addition concentration of SHPP was 0.5% for the preparation of the soybean curd. A comparison was also made on the soybean curd that was prepared with the optimal CP concentration of 0.1%. The better texture properties and sensory scores were achieved in the color, texture, flavor, taste, and acceptability of the soybean curd prepared with 0.5% SHPP, compared with the 0.1% CP. Therefore, the SHPP can be expected to serve as a bioactive substance with various physiological functions in protein-based gel foods. The finding is conducive to the recycling of soybean byproducts in the whole bean food.