Changes in α- and ß-amylase during Storage of Sweetpotato Lines with Varying Starch Hydrolysis Potential

TLDR: In this article, four lines of staple-type sweetpotato (Ipomoea batatus) were evaluated for changes in α- α- and s-amylase activity and reducing sugars (by HPLC) at harvest, after curing, and at intervals during 180 days of storage.

Abstract: Staple-type lines of sweetpotato (Ipomoea batatus (L.) Lam.) do not sweeten significantly upon cooking as com- pared to the traditional-type lines. Four lines exhibiting distinct differences in sweetness after cooking were evaluated for changes in α- α- and s-amylase activity and reducing sugars (by HPLC) at harvest, after curing, and at intervals during 180 days of storage. The traditional cultivar 'Jewel' and staple-type line 'Sumor' displayed high a- and s-amylase activities, which rose from low levels at harvest to peak level s ≈ ≈ 90 days into the storage period. Staple-type lines '99' and '86' displayed significantly lower a- and s-amylase activities. By using polyclonal sweetpotato s-amylase antibody and western blot following native- and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, it was confirmed that a lower level of s- amylase synthesis existed in '99' and '86'. Quantitatively, 'Jewel', 'Sumor', and an additional staple-type line, 'HiDry', had 361,374, and 365 µg s-amylase protein per gram of fresh storage root tissue, respectively, while '99' and '86' possessed <60 and 12 µg·g -1 , respectively. In raw roots, individual (glucose, fructose, and sucrose) and total sugar concentrations were significantly higher in 'Jewel' than in 'Sumor', '99', or '86'. Only trace amounts of maltose were found in raw roots of any line. Sucrose, glucose, and fructose concentrations decreased with baking in all lines except '86', in which they increased. There was substantial maltose produced by baking 'Jewel' and 'Sumor', but only trace amounts found in baked '99' and '86'. Sweetpotato germplasm can be separated into four general classes based on initial sugar concentration and changes during cooking: 1) low sugars/low starch hydrolysis, 2) low sugars/high starch hydrolysis, 3) high sugars/low starch hydrolysis, and 4) high sugars/high starch hydrolysis. At least two mechanisms may confer the lack of starch hydrolysis and subsequent sweetening in staple-type sweetpotato: 1) inhibition of s-amylase synthesis, and 2) a nonenzyme mediated mechanism.