The Advantages of Using Natural Substrate-Based Methods in Assessing the Roles and Synergistic and Competitive Interactions of Barley Malt Starch-Degrading Enzymes

Abstract

Existing methods of assay of malt starch-degrading enzymes were critically appraised. New methods based on natural substrates, namely starch and its natural intermediate-derivative, were developed for all the enzymes, except limit dextrinase for which pullulan was used. Thermostability, optimal temperatures and pHs were established. α-Amylase and limit dextrinase were the most thermostable and β-amylase, α-glucosidase and maltase were the least stable while diastase occupied an intermediate position. The optimal temperatures were congruent with thermostability, β- amylase having the lowest (50°C) and α-amylase the highest (65°C) with the remaining enzymes, including diastase, falling in between. In contrast, α-amylase has the lowest optimal pH (pH 4.5) and β amylase the highest (pH 5.5) while the others have pHs in between the two values. The roles of the enzymes were evaluated taking into account the level of activity, thermostability, optimum pH, the nature of the product(s), and the relevance to brewing. β-Amylase production of maltose was synergistically enhanced, mostly by α-amylase but also limit dextrinase. α-Glucosidase and maltase are unimportant for brewing, because of their low activity and the negative impact on β-amylase activity and the negative effect of glucose on maltose uptake by yeast. The starch-degrading enzymes (diastase) in a gram of malt were able to degrade more than 8 g boiled starch into reducing sugars in 10 min at 65°C. The latter, suggests that it will be possible to gelatinise most of the malt starch at a higher temperature and ensure its hydrolysis to fermentable sugars by mixing with smaller portions of malt and mashing at lower temperatures e.g. 50–60°C.