Difference in Enzymatic Degradation of Starch between Potato Tubers with Different Specific Gravities during Steaming Treatment

Abstract

　The texture of steamed potatoes (whether mealy or firm) is determined by the specific gravity of potato materials, i.e., water content. Since water possesses extensive heat capacity, the endothermic nature of potato tubers due to the water content of the tubers may be related to the physical properties of the products of steaming. Additionally, endogenous enzymes may contribute to compositional changes during steaming. Here, we analyzed the endothermic capacity of and enzymatic starch degradation in steamed potatoes, particularly to low-specific-gravity (LSG) and high-specific-gravity (HSG) potatoes. The central temperature of the LSG potatoes increased more slowly than that of the HSG potatoes during steaming, implying that enzymes in the LSG potatoes are active for a longer time than those in the HSG potatoes. The amounts of starch degradation products in steamed potatoes indicate that the starch in the potatoes was degraded by steaming. Predictably, the increase in the amounts of starch degradation products was more pronounced in the LSG potatoes than in the HSG potatoes. Hence, the LSG potatoes lose substantial amounts of starch during steaming, because the enzymes remain active. Furthermore, fresh LSG potatoes displayed higher starch-hydrolyzing enzyme activities than fresh HSG potatoes. Meanwhile, thinly sliced potatoes exhibited reduced amounts of starch-degradation products during steaming, probably because of the rapid inactivation of endogenous enzymes. These results indicate that the control of the internal temperature of potato materials can alter the final starch content of steamed potatoes.