Mechanism of DDMP-saponin degradation and maltol production in soymilk preparation

•DDMPs-saponins were degraded during grinding of soybeans with lipoxygenase activity.•Lipoxygenase activities on DDMPs-saponins could be mimicked using radicals.•Treatments with radicals decreased DDMPs-saponins and inhibited maltol production.

The degradation mechanism of 2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one (DDMP) conjugated saponins into group B, E saponins, and maltol production during soymilk preparation was studied by using lipoxygenase (LOXs)-deficient soybean variety ('L-Star') and purified DDMP-saponin βg. Experiments were performed under practical soymilk preparation and model reaction system. DDMP-saponins (DDMPs) degraded by both heat and radical (LOXs, AAPH, and DPPH) treatments but maltol was produced only by heat degradation. DDMPs in raw soymilk prepared from 'Fukuyutaka' (common variety) were completely degraded whereas DDMPs in 'L-Star' raw soymilk remained. When DDMP-saponin βg was treated with AAPH radical, a compound that has one oxygen atom attached at DDMP-moiety, which was identified by LC-MS/MS analysis, was detected but gradually degraded into group B saponin Bb. After treating DDMP-saponin βg with DPPH-radical, some compounds which seem to be dehydrogenated DDMP-βg, and group E saponin Be were produced. These results suggest that competitive degradation of DDMPs between heat treatment and LOXs activities affected the combination of group B, E saponins, and maltol contents during soymilk preparation. Thus, appropriate processing conditions should be considered for controlling the flavor and health promoting characteristics, which are derived from saponin components of soybean base foods.