Malt limit dextrinase and its importance in brewing: Dissertation

The roles of a-amylase and ß-amylase in starch hydrolysis during mashing are well documented, whereas very little information on limit dextrinase is available. Limit dextrinase (EC 3.1.2.41) is a debranching enzyme catalyzing the hydrolysis of a-1,6-glucosidic linkages in starch. In the present study the importance of malt limit dextrinase in the brewing process was assessed by monitoring limit dextrinase activities during malting and mashing and by relating the measured activities to observed changes in branched dextrins and fermentable sugars during mashing. Limit dextrinase exists in free (soluble-active), latent (soluble-inactive) and bound forms. During germination the total limit dextrinase activity and the proportion of free limit dextrinase increased. In kilning, 40-45 % of the free as well as of the total limit dextrinase activity was lost. On average only 40 % of the total limit dextrinase activity of the malts was available in an active form during mashing. Under high gravity mashing conditions limit dextrinase was more heat stable than b-amylase. Limit dextrinase retained most of its activity in the temperature range 60-63°C, in which the major part of starch hydrolysis occurs. After one hour at 65°C, 35-45 % of the limit dextrinase activity but only 5-10 % of the ß-amylase activity measured in the beginning of the mashing remained. Adjusting the mash pH from 5.8 to about 5.4 increased the limit dextrinase activity by 40-70%, resulting in a higher fermentability of the wort. The amount of branched dextrins left unhydrolysed in the wort strongly depended on the free limit dextrinase activity. By addition of pullulanase, a microbial debranching enzyme, the major part of the branched dextrins could be hydrolysed into fermentable sugars during mashing. The addition of purified malt limit dextrinase was less feasible due to the presence of a limit dextrinase inhibitor complexing part of the added enzyme. Addition of limit dextrinase and pullulanase increased the concentrations of maltose and maltotriose, whereas the glucose concentration was unaffected. High gravity mashing conditions promote the activity of limit dextrinase. The main reason for the incomplete hydrolysis of branched dextrins into fermentable sugars in mashing does not lie in the characteristics of limit dextrinase, but in its low activity in normal brewer's malts. Only by increasing the limit dextrinase activity in mashing can the branched wort dextrins be converted into fermentable sugars.