The pH-dependent protection of α-galactosidase activity by proteins against degradative enzymes during soymilk in vitro digestion

• Gastric pH 2.0 and below inactivate α-galactosidase.
• Digestive enzymes contribute to α-galactosidase activity loss at low pH.
• Soy peptides and proteins are protective of α-galactosidase.

α-Galactosidase (α-GAL) dietary supplement is used to alleviate flatulence discomfort caused by raffinose family oligosaccharides (RFOs), but reported efficacy varies among consumers. This study investigated the hypothesis that factors such as stomach acid, gastrointestinal (GI) enzymes (pepsin and pancreatin), and food proteins could affect the activity of α-GAL against RFOs. Strong acidic conditions (pH < 2.0) and GI proteases were found detrimental to α-GAL whereas soy protein could protect α-GAL. In soymilk samples with 1 and 5 mg/mL protein, α-GAL retained 60 and 100% of activity (P < 0.05) throughout the entire digestion (pepsin + pancreatin, 3 h) at pH 2.5 whereas α-GAL was completely inactivated within 1 h of pepsin digestion in water. Isolated soy protein was more protective of α-GAL than soymilk protein. The results were corroborated by SDS–PAGE and the release of galactose as evidenced with the thin layer chromatography.