The changes in texture and sweetness of steamed storage roots of sweetpotato cultivars induced by the storage of raw roots at low temperature in relation to cell structure and starch metabolism

摘要

The changes in texture and sweetness of steamed storage roots during the storage of raw roots for 8 weeks at 5-7 ℃ (the low temperature) were investigated compared with the storage at 13-16℃ (the normal temperature) about 3 cultivars with different storability,"Beniazuma","Tamayutaka" and "Kokei No. 14" in relation to cell structure and starch metabolism. The storage of raw roots of "Beniazuma" and "Tamayutaka" at the low temperature remarkably decreased the starch content and induced rapid change in the texture of steamed ones,while "Kokei No. 14" roots did not show these changes by the same storage treatment. The cell structure of steamed roots after the storage for 4 weeks at the low temperature was different between "Tamayutaka" and "Kokei No. 14". The activity of a-amylase largely increased during 1-4 weeks of storage at the low temperature only in "Beniazuma" and "Tamayutaka",not in "Kokei No. 14",suggesting the relation of this enzyme to the different changes in texture of steamed root during the storage at low temperature. On the other hand, the sweetness of steamed roots of the three cultivars moderately increased by the storage at low temperature. However, the changes in activity of P-amylase could not completely account for this increase in sweetness. HPLC analysis revealed the rapid and drastic increase in sucrose content in the steamed root after the storage at the low temperature. The metabolism from starch or other carbohydrate to sucrose is thought to play important roll in the changes in texture and sweetness of steamed root.