Continuous immobilized yeast reactor system for complete beer fermentation using spent grains and corncobs as carrier materials

Despite extensive research carried out in the last few decades, continuous beer fermentation has not yet managed to outperform the traditional batch technology. An industrial breakthrough in favour of continuous brewing using immobilized yeast could be expected only on achievement of the following process characteristics: simple design, low investment costs, flexible operation, effective process control and good product quality. The application of cheap carrier materials of by-product origin could significantly lower the investment costs of continuous fermentation systems. This work deals with a complete continuous beer fermentation system consisting of a main fermentation reactor (gas-lift) and a maturation reactor (packedbed) containing yeast immobilized on spent grains and corncobs, respectively. The suitability of cheap carrier materials for long-term continuous brewing was proved. It was found that by fine tuning of process parameters (residence time, aeration) it was possible to adjust the flavour profile of the final product. Consumers considered the continuously fermented beer to be of a regular quality. Analytical and sensorial profiles of both continuously and batch fermented beers were compared.(Fundação de Amparo a Pesquisa do Estado de São Paulo, Brazil (FAPESPFundação para a Ciência e a Tecnologia (FC

Genome-wide transcription survey on flavour production in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is widely used as aroma producer in the preparation of fermented foods and beverages. During food fermentations, secondary metabolites like 3-methyl-1-butanol, 4-methyl-2-oxopentanoate, 3-methyl-2-oxobutanoate and 3-methylbutyrate emerge. These four compounds have a major influence on the final taste of fermented foods. Their presence is influenced by the availability of free branched chained amino acids (BCAA). To study the underlying molecular mechanism of the formation of these compounds, we performed genome-wide transcription analyses with cDNA microarrays. The expression profile of yeast during flavour formation, when cultivated on L-leucine, was compared to the expression profile of cells cultivated on ammonia. In addition, the expression profiles of cells cultivated in a batch culture were compared to cells cultivated under continuous growth conditions. Genome-wide gene analysis of these samples revealed a group of 117 genes, which w! ere more than two-fold up- or down-regulated and significantly altered in gene expression (P < 0.001) under both cultivation conditions. This group included genes encoding enzymes of different amino acid metabolism pathways. The group of the BCAA metabolism was not significantly altered in gene expression. Genes identified with altered expression levels, in only batch or continuous culture fermentions, represented functional groups concerning energy, protein fate, cell cycle and DNA processing. Furthermore, clustering of genome-wide data revealed that the type of cultivation overruled the differences in N-source in the gene-expression profiles. This observation emphasizes the importance of sample history in gene expression analysis.