Effect of endogenous glycosidase on stability of steroidal saponins in Taiwanese yam (Dioscorea pseudojaponica yamamoto) during drying processes

The effect of different drying methods on steroidal saponins, furostanol and spirostanol glycosides, in Dioscorea pseudojaponica Yamamoto, were studied. The yams were either steam-blanched or not blanched before drying. Results showed that solar and shaded drying without prior blanching would cause significant changes of yam saponins; hot air drying caused slight variation, and other drying methods showed almost no change. A similar phenomenon was observed in a model system containing crude yam saponin extract; it indicated that higher temperature and light exposure had very little effect on the stability of yam saponins during drying. On the other hand, the endogenous glucosidase in yam might play an important role in altering the saponin structures during drying. This was confirmed by a model experiment; a crude furostanol glycoside 26-O-β-glucosidase prepared from yam was shown to remove the glucose at position C-26 of the furostanol glycoside structure and change it to the respective spirostanol glycoside.