Soybean hulls by-product: extraction, characterization and antioxidant properties

During the processing of soybeans, hull is generated as the main by-product of the oil industry, which represents between 6-8% of the whole grain.
The objective of this study was to evaluate the potential as a source of high value-added products in terms of dietary fiber, phenolic compounds and antioxidant capacity of hulls generated from soybean processing.
Soybean hulls were characterized in terms of moisture, protein, lipids, ash, dietary fiber, phenolic content (Folin-Ciocalteau) and antioxidant activity (ABTS assay).
Ultrasound assisted extraction (UAE) to obtain extracts with high antioxidant activity were carried out according to a simple factorial experimental design of 3 levels with 2 factors, whose independent variables were: extraction time (15-30-60 minutes) and power (54-108-180 watts). An ethanol-water mixture (75:25) was used as solvent, with a ratio of 50 mg CH/mL.
Extraction yield, total phenolic content, and antioxidant activity (ABTS assays) of the extracts were considered as response variables.
The UAE yields ranged between 6.6-8.8% and a positive correlation with the increase in extraction time and power was observed.
The total phenolic content of the extracts ranged between 16.2-19.9 mg GAE/g extract, and followed the same trend as the extraction yield. The values obtained in the extracts were significantly higher (p < 0.05) than in the hull (3.4 mg GAE/g).
The antioxidant activity of the extracts ranged between 80.8-96.3 µmol Trolox/g extract, higher values (p < 0.05) than the hull (16.1 µmol Trolox/g).
In conclusion, UAE allowed intensifying the extraction of compounds with antioxidant activity. Taking into account the abundance of this by-product, it is important to develop efficient methods to achieve extracts with a high content of polyphenols and compounds with antioxidant activity. Likewise, the residue resulting from the extractions is rich in fiber and carbohydrates, which also allows its potential use in the food industry.