Stability of pea DDMP saponin and the mechanism of its decomposition

DDMP saponin can be converted to saponin B by the loss of its DDMP group (2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one). The stability of DDMP saponin from pea was investigated under various conditions (temperature, ethanol concentration, pH). DDMP saponin in water was observed to be unstable at acidic and alkaline pHs, and to have an optimal stability around pH 7. In water, DDMP saponin became unstable at temperatures >30 °C. The presence of ethanol, however, had a stabilizing effect on the DDMP group. The loss of the DDMP group at 65 °C could be completely prevented at >30% (v/v) ethanol. The breakdown reaction of DDMP saponin and the subsequent formation of saponin B was modelled using a multi-response modelling approach and was found to be best described by a first-order reaction. The activation energy was estimated to be 49 kJ/mol, indicating a chemical reaction with moderate temperature dependence. A mechanism of DDMP saponin decomposition is proposed, consisting of a fast protonation or deprotonation, followed by a rate-determining step in which maltol is the leaving group.