Yeast and Fermentation Session
Jessica Herrera, Instituto de Biotecnología, Facultad de Ciencias Biológicas, Universidad Autónoma de Nuevo León, Monterrey N.L., México
Co-author(s): Luis Damas, Cuauhtémoc Moctezuma, Monterrey, México; Clara Leal, Instituto de Biotecnología, Facultad de Ciencias Biológicas, Universidad Autónoma de Nuevo León, Monterrey México; Juan Cabada, Cuauhtémoc Moctezuma, Monterrey, México; Luis Ga
ABSTRACT: The accumulation of sulfur volatile compounds (VSC), such as hydrogen sulfide, sulfur dioxide, mercaptans, and methyl thioacetate, negatively affects the aroma and flavor of beer. Most VSC are synthesized by brewing yeasts, and their production depends on wort composition and type of yeast strain. In this sense, the accumulation of some VSC has been related to levels of FAN and to specific amino acids present in wort. Moreover, using DNA microarray analysis, we identified different genes in two lager yeast strains whose expression impacted the biosynthesis of VSC. In this work we studied the effect of wort amino acid composition on the synthesis of VSC using two lager yeasts (C790 and C820). Fermentations were carried out with two worts: wort 1 had a FAN of 150 ppm, and wort 2 had a FAN of 200 ppm. Additionally, wort 1 was enriched with either 10 ppm Ser, 20 ppm Met, or a combination of both (10 ppm Ser plus 20 ppm Met). Genetic expression of genes implicated in the VSC production was evaluated by quantitative-PCR (qPCR) after 2 days of fermentation, and concentration of the VSC was measured by gas chromatography at the end of fermentation. Our results showed significant differences in the content of several amino acids between the worts. In particular, wort 2 had a 1.4 times higher concentration of Met, 12.0 times higher concentration of Ser, and 1.9 times higher concentration of Thr compared with wort 1. Irrespective of yeast strain used, the VSC concentration was higher in wort 1 than in wort 2; however, when wort 1 was supplemented with Ser plus Met, the VSC concentration was lower even than in high FAN content wort 2. Furthermore, the metabolic response of the two yeast strains was significantly different since the strain C790 produced less VSC than C820 when fermented in any of these worts. These results were correlated to those of qPCR, since the analysis of genetic expression of 18 VSC related genes showed that whereas strain C820 overexpressed CYS4, SER2, and MHT1 when fermented in wort 2 strain C790 kept all genes at the same expression level in the two worts. The contribution of these genes to sulfur compound production is discussed. Our results demonstrate that the accumulation of VSC in beer is mainly the consequence of the specific interaction of two factors: wort amino acid composition and yeast strain genetic background. Also, this work indicates that modification of the amino acid profile of wort can help to produce beers with desirable sensory properties.
Jessica Herrera received an M.S. degree in biochemical engineering from Instituto Tecnológico de Durango in Durango, México. Currently, she is a Ph.D. student in the biotechnology program of Universidad Autónoma de Nuevo León, México. Her thesis focuses on the study of the interaction between raw materials and the genetic response of yeast in the lager brewing process.