Alpha-amylase and soluble sugars in two zones of sweetpotato roots

Abstract

Sweetpotato [Ipomea batatas (L) Lam] is consumed worldwide as a staple or complementary food. The nutritive value of processed sweetpotato products may be compromised by size exclusion and peeling and heat processing. This study investigated the distribution of alpha-amylase activity, alcohol insoluble solids (AIS), total sugars, maltose, sucrose, glucoce and fructose in two zones of 'Beauregards' and 'Jewel' sweetpotato roots. Previously cured (10 days at 30 ºc and 90 ºc relative humidity), and stored (three and four months at 15 ºc and 85 ºc relative humidity) roots were classified into two size categories (large, 6-9 cm and small, 2-4 cm diameter). The cambium of hand peeled roots was separated from the inner tissue, and each portion was ground and analyzed raw and after cooking (1 hr, 70 ºc ). Analysis was perfomed by high perfomance liquid chromatography (HPLC) for sugars and spectrophotometry (alpha-amylase). The cambium of 'Beauregard' had significantly (p< 0.05) more AIS and less sugars than the inner tissue. Conversely, 'Jewel' root cambium contained significantly less AIS and sugars than the inner tissue. Alpha-amylase activity was higher in 'Jewel' cambium but was not significantly different between the two zones of 'Beauregard'. Between cultivars, 'Beauregard' contained more alpha amylase, sugars (except sucrose) and AIS than 'Jewel'. Overall, large roots contained less alpha-amylase than small roots but more maltose when heated. Cooking significantly reduced AIS and sucrose and increased maltose content. There was no significant variation in glucose and fructose content as a result of cooking. Four months storage yielded significantly higher alpha-amylase activity, AIS, and sucrose but less glucose and frutose than three months. These observations demonstrate that alpha-amylase activity and the content of sugars are influenced by cultivar, storage, size, zone and cooking. It can also be inferred that the content of alpha-amylase in raw roots is not linearly related to maltose production during cooking.