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

Abstract

Staple-type lines of sweetpotato [Ipomoea batatus (L.) Lam.] do not sweeten significantly upon cooking as compared to the traditional-type lines. Four lines exhibiting distinct differences in sweetness after cooking were evaluated for changes in α- and ß-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 ß-amylase activities, which rose from low levels at harvest to peak levels ≈ 90 days into the storage period. Staple-type lines `99' and `86' displayed significantly lower a- and ß-amylase activities. By using polyclonal sweetpotato ß-amylase antibody and western blot following native- and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, it was confirmed that a lower level of ß-amylase synthesis existed in `99' and `86'. Quantitatively, `Jewel', `Sumor', and an additional staple-type line, `HiDry', had 361,374, and 365 μg ß-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 ß-amylase synthesis, and 2) a nonenzyme mediated mechanism.