Relationship between Starch Gelatinization Properties and β-Amylase Activity of Various “Soggy-Type” Sweet Potato Cultivars Produced in Japan

Abstract

Sweet potato (Ipomoea batatas (L.) Lam) is a widely cultivated crop valued for its nutritional and functional properties. In Japan, consumers particularly prioritize the sweetness and texture of baked sweet potatoes. Among the free sugars in cooked sweet potatoes, maltose is predominant, generated by β-amylase-mediated hydrolysis of gelatinized starch during heating. This study investigated the relationship between starch gelatinization properties and β-amylase activity in ten soggy-type sweet potato cultivars (Aikomachi, Annouimo, Beniharuka, Benimasari, Fukumurasaki, Himeayaka, Karayutaka, Kenroku, Silksweet, and Tamayutaka) and one intermediate-type standard cultivar (Kokei 14), all cultivated in the same farm. We evaluated free sugar content, starch gelatinization characteristics, amylopectin chain-length distribution, and β-amylase activity. A positive correlation was observed between the proportion of DP13-24 chains in amylopectin and the onset (T_o) and peak (T_p) gelatinization temperatures, as measured by differential scanning calorimetry (DSC). Himeayaka, with the highest maltose content, also showed the highest β-amylase activity. In contrast, Silksweet had similar enzyme activity but lower maltose levels. These findings indicate that starch gelatinization behavior is key to the sweetness and texture of cooked sweet potatoes. Principal component analysis grouped the cultivars into three categories: typical soggy-type, slightly soggy-type, and near intermediate-type. These insights can guide the breeding and selection of cultivars aligned with consumer preferences.