Impact of particle size fractions on composition, antioxidant activities, and functional properties of soybean hulls

This research investigated potential soybean hull utilization in high value products by determining the amount of proteins, dietary fiber, minerals, phenolics, antioxidant activities, as well as functional properties of soybean hull samples in five different size fractions from 20 to 140 mesh. The contents of protein, ash, soluble sugar, and most amino acids increased with decreasing sizes. This study first reported the quantities of bound phenolic contents and antioxidant activities, soluble sugars, and amino acids in soybean hulls. The statistically significant increases were found with decreasing particle sizes in the free phenolic contents (1.81 to 3.24 mg/g) and antioxidant activities (1.51 to 1.95 µmol/g). In general, free phenolic contents were slightly higher compared to bound phenolic contents, whereas the free antioxidant activities were considerably lower than bound antioxidant activities for the same fraction, which suggested that a portion of antioxidant activities were not released during neutral extraction. The extracted protein from soybean hulls showed similar banding patterns with soybean products as demonstrated by gel electrophoresis. The content of water-soluble solids increased significantly with decreasing size (15.08 to 23.71%). By contrast, water-holding capacities decreased significantly with decreasing particle sizes (666.72 to 337.47%). The final viscosity peak of soybean hulls increased as the sizes increased. The trend of final peaks from blends appeared to be related to their water-holding capacities. In addition, the correlation tests disclosed statistically significant relationships (p < 0.05) between selective properties of soybean hulls. Our results suggested that soybean hulls are a valuable source for functional foods and industrial applications.