The properties and genetics of barley malt starch degrading enzymes

Abstract

The properties and quality of barley malt starch degrading enzymes are of primary importance to the efficiency and profitability of brewing (beer and whiskey), and the bio-fuel (bio-ethanol) industries. The barley starch degrading enzymes hydrolyse starch into fermentable sugars that yeast converts into alcohol. This process is key for the alcohol producing industries as the starch substrate makes up approximately 60% of grain weight (Holtekjolen et al., 2006). Malted barley is the main source of the diastase or diastatic power (DP) enzymes that hydrolyse starch. The DP enzymes comprise the combined activity of α-amylase, β-amylase, α-glucosidase and limit dextrinase whose concerted action hydrolyse the α-(1,4) and a-(1,6) glucosyl linkages in starch (Fig. 6.1) into fermentable sugars (i.e., glucose, maltose, etc.), dextrins and limit dextrins. The actions of the DP enzymes are summarised as follows: (1)α-amylase cleaves α-(1,4)-linkages internally (endo-acting) to primarily produce oligosaccharides, limit dextrins and some fermentable sugars;(2)β-amylase cleaves α-(1,4)-linkages from the non-reducing ends (exo-acting) to produce maltose;(3)Limit dextrinase hydrolyses internal a-(1,6)-linkages (endo), to remove branch-points in amylopectin or α-limit dextrins;(4)α-Glucosidase primarily cleaves α-(1,4)-linkages from the non-reducing ends to produce glucose.Fig. 6.1.