In the malting process, barley (Hordeum vulgare L.) is germinated under semi-controlled conditions to provide soluble extract and hydrolytic enzymes for the brewing and distilling industries. In barley seed germination, respiratory processes convert oxygen (O2) to carbon dioxide (CO2) to produce energy for the growing meristematic tissues. The availability of these gaseous compounds influences germination and is of particular concern in industrial-scale malting since grain batches up to 600 tons are malted. Despite the industrial relevance, limited knowledge is currently available concerning the industrial germination conditions and the effects of unfavourable gaseous conditions on malt quality parameters including embryo growth, enzymatic activity, and grain modification. The present project aimed to investigate how the gaseous levels of O2 and CO2 develop and impact malt quality throughout the germination phase of industrial-scale malting and broaden the understanding of the inhibitory effects of unfavourable levels of O2 and CO2.
Following the development of experimental equipment capable of monitoring gas and temperature development, the germination conditions present during industrial-scale malting were investigated. The temperature, levels of O2, CO2 and gene transcripts differed over time and between grain layers throughout the germination phase of industrial-scale malting. Consistently higher temperature and CO2 levels were measured in the top grain layer compared to the bottom, while the highest O2 concentrations were observed in the bottom grain layer. This pattern was observed due to upward-directed aeration through the germination vessel. The observed germination conditions did not result in increased malt heterogeneity nor reduced malt quality due to regular malt turning events ensuring a high degree of malt homogenization.
The identification of industrial germination conditions enabled the development of a nano malting system able to mimic these conditions and test adverse gas compositions to further elucidate the effects of high CO2 and low O2 on barley malt quality. Overall, all assessed barley lines were inhibited by 10% CO2 and 5% O2 exposure during germination, although the hull-less lines were less affected than hulled lines in terms of differences in germination rate, embryo growth, and biochemical analyses. The germination rate was most inhibited by the low O2 level, while embryo growth and α-amylase activity were most impaired by the high CO2 treatment. Gene transcription patterns involved in germination processes were affected by the investigated gaseous conditions and were generally most impaired in seeds exposed to high CO2 levels during germination. On a transcript level, the regulation of the germination was similar for barley germinated in high CO2 and low O2 concentrations indicating a tightly controlled regulation during germination. Whether the inhibitory effect of elevated CO2 is caused by specific responses or related to a more general metabolic downregulation remains to be elucidated.
The results from this project increased the understanding of the present germination conditions at industrial scale and how unfavourable levels of CO2 and O2 affects germination processes and malt quality. However, further studies with additional barley lines and gas compositions and associated assessment of interdependencies between temperature, CO2 and O2 would provide valuable insights for the understanding of the mechanisms behind CO2 inhibition.