Temperature-moisture gradients during kilning introduce heterogeneity in malt starch properties and α-amylase activity

Abstract

In industrial malting, kilning inevitably results in temperature-moisture gradients over the grain bed. These gradients can introduce heterogeneity in malt quality parameters across the bed, adversely affecting brewing. The impact of such gradients on starch properties and α-amylase activity is not understood, despite being essential for malt quality. In this work, the temperature-moisture gradient during the first 13 h of withering was quantified, and its impact on the starch gelatinisation characteristics, starch content and α-amylase activity of the green malt undergoing kilning at different locations and depths in the kiln bed was assessed. During the first 13 h of kilning, the upper layer retained a high moisture content (41.2 ± 0.1 %) and low temperature (26.4 ± 0.1 °C). No significant changes were observed in starch properties and α-amylase activity. The middle layer started to heat up to 41.5 ± 3.1 °C and started to dry slightly to a moisture content of 39.4 ± 2.1 %. In contrast, the bottom layer heated up to 58.0 ± 1.3 °C, and the moisture content was reduced to 25.5 ± 0.8 %. This resulted in an accelerated germination process in the bottom layer, leading to a decrease from 48.6 ± 0.5 to 46.1 ± 0.8 dw% in starch content and a simultaneous increase in α-amylase activity from 121.1 ± 4.7 to 176.0 ± 0.4 CU/g dw. An increase of 1.5 °C in the onset temperature of starch gelatinisation was observed after malting, of which 0.6 °C could possibly be attributed to kilning in the bottom layer.