Starch Digestibility and Antioxidant Activity of Different Varieties of Yam during Steaming

In this study, we investigated the effects of steaming time on the physicochemical properties, bioactive components and antioxidant properties of different yam varieties (Tiegun, Huai and Glutinous). The results showed that after steaming treatment, the contents of total starch and resistant starch in the three varieties of yam significantly decreased (P < 0.05), while the contents of digestible starch (fast digestible starch and slow digestible starch) and apparent amylose significantly increased. Meanwhile, the water-holding capacity of yam starch increased and its solubility decreased. Among these varieties, the contents of total soluble phenols and total flavonoids in Huai yam steamed for 15 min were the highest, which were 1.04 and 0.83 mg/g, respectively, and it had the strongest antioxidant capacity with Fe³⁺ reducing power of 165.56 μmol/g, 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging percentage of 58.15% and hydroxyl radical scavenging percentage of 82.26%. Scanning electron microscopic (SEM) analysis showed that the starch granules of steamed yam presented an irregular massive structure with a rough and cracked rough surface. Taken together, steaming time had pronounced effects on the quality characteristics and bioactive components of all the yam varieties. Yam steamed for 15 min had the highest contents of total soluble phenolics and total flavonoids and strongest antioxidant activity. Yam steamed for 20 min had the highest content of digestible starch. The microstructure of yam was damaged by steaming. These results show that moderate steaming can improve yam starch digestibility and antioxidant capacity.