Effect of roasting after sugar-soaking on the level of volatile compounds, total polyphenol, total flavonoid, and isoflavones in black soybean (Glycine max (L.) Merr.)

Thermal process with various sugars on black soybean before roasting process affects the levels of volatile compounds, total polyphenol, total flavonoid, and isoflavones. Black soybeans (Glycine max (L.) Merr.) were pre-soaked in 10, 30, and 50 g/L solutions of five types of monosaccharide (glucose, fructose, arabinose, xylose, ribose) for 2 h, followed by drying (50 °C/20 min) and roasting (200 °C/20 min) to determine the level of volatile compounds and isoflavones, and the antioxidant activity. Volatile compounds and isoflavones were analyzed by GC−MS and HPLC, respectively. Of the 22 volatile compounds identified in black soybean headspace, 2,5-dimethylpyrazine was the most abundant (1790 μg/kg). Pre-soaking in 50 g/L glucose gave the highest content of total volatile compounds (67.9% above control), while 10 g/L arabinose gave the highest content of total isoflavone (18.6% above control). With the increase in glucose, fructose, and ribose concentration, the contents of total volatile compounds, isoflavone, polyphenol, and flavonoid increased (p < 0.05) but decreased for xylose and ribose solutions (p < 0.05), except for the total polyphenol. This study provides practical information on the changes in the volatiles, isoflavones, and antioxidants in roasted black soybeans after soaking in five monosaccharide solutions.