Gamma-Aminobutyric acid (GABA)—A practical indicator for the detection of heterogeneities during malting?

Malt and Grains Session
Philip Wietstock, Technische Universität Berlin, Germany
Co-author(s): Christian Mueller, Technical University, Berlin, Germany; Maik Kleinwaechter and Dirk Selmar, Technical University, Braunschweig, Germany; Frank-Jürgen Methner, Technical University, Berlin, Germany

ABSTRACT: Nowadays, very large malt batches are processed, which frequently leads to heterogeneities within the grain beds. As a result beta-glucanase activities, among others, vary within the batches, thus high beta-glucan concentrations remain unhydrolyzed in parts of the batches and can lead to lautering and filtration problems during the brewing process. The kernels’ enzyme activities mainly depend on their physiological status. Accordingly, the metabolic status of the seeds corresponds to a reliable marker for detecting heterogeneities in the grain beds and for predicting potential processing problems. Up to now, the Calcofluor method according to Carlsberg has been the only standardized method to determine the kernels’ homogeneity; however, its results are not very precise. In this study, malting trials under differing conditions were carried out to assess if gamma-aminobutyric acid (GABA) can be used as a reliable marker for determining the physiological status of the kernels. Malting trials from different barley varieties were conducted under differing conditions (steeping degree, steeping, germination temperature, and anoxia) and using different scales (pilot and industrial trials). The malts produced were evaluated by standard malt analyses. Additionally, the content of GABA was analyzed in samples taken during malting using HPLC and compared with germination control parameters (chitted kernels, acrospire length). The study demonstrates that the accumulation of GABA in malt kernels is influenced by many factors such as anoxia and long wet periods; however, relevant parameters such as higher steeping degree and raised temperatures during steeping and germination were shown to increase GABA formation to a higher extend. Further outcomes of this study suggest that short periods of anoxia during malting do not harm the kernel’s physiological status; the kernels survived, and germination resumed. The heterogeneities in large malting batches were examined in trials using an industrial malting plant. Varying temperatures and O2/CO2 ratios could be detected in the high grain beds of three different steeping systems. Without aeration of the grain, oxygen consumption was very fast, especially in the later steeping phases (dry and wet). Anoxia again did not lead to a dying of the kernels but to a delayed growth of rootlets and acrospires. Nevertheless, the formation of the most important enzymes alpha- and beta-amylase and beta-glucanase was slower at the beginning of the germination, but no markable lack of enzymatic activities could be found at the end of germination. During kilning the formed GABA was decomposed faster in the lower layers of the grain bed. The outcome of this research provides the brewing and malting industry with new results concerning the use of the stress metabolite GABA as an indicator for evaluating the physiological status of germinating barley.

Philip Wietstock is a scientific assistant at the Technische Universität Berlin, Germany. After graduating from his biotechnology studies with a diploma in engineering from the Technische Universität Berlin (2009), he worked for one year as an intern at the Department of Food Science and Technology at Oregon State University, Corvallis, OR. In 2011, he transferred to his present position, where he is working on his dissertation which focuses on the investigation of the influence of hops on oxidative beer stability.