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.
VIEW PRESENTATION 235