JULIEN BILLARD (1), Huu Vang Nguyen (2), Mustapha Nedjma (3)
(1) R&D Department, Spindal AEB Group, Gretz-Armainvilliers, France;
(2) INRA, AgroParisTech, Thiverval-Grignon, France; (3) nedjman@aol.com
The development and production of selected beer yeast for fast and
complete metabolization of maltose, maltotriose, and glucose, the three
main fermentable sugars in wort, has been considered. Maltose,
maltotriose, and glucose are the most abundant fermentable sugars in
wort; in case of incomplete fermentation, maltotriose can cause a range
of qualitative problems in beer and ethanol loss. Fermentation
performance was followed through the optimization of the culture medium,
reproducing accurately the wort composition by monitoring yeast growth,
ethanol synthesis, original gravity and attenuation, and sugar
consumption during the fermentation process. Beer flavor was evaluated
through the content of higher alcohols, volatile esters, and aroma
compounds. Here, we investigated the influence the medium composition on
the expression of different MAL genes, especially those encoding the
maltose and maltotriose transporters. We tried to correlate the impact
of yeast metabolism with beer aroma profile. The equilibrium and
reproducibility of the aromatic profiles were also analyzed and compared
with traditional yeasts after successive inoculation: mutation,
membrane permeability. Flavor expression and stability is generally
affected by oxidation during beer processing and storage. In order to
improve aroma quality and stability, the GSH1 gene, which encodes
g-glutamylcysteine synthetase and glutathione synthetase, an enzyme
essential for glutathione (g-glutamyl-L-cysteinyl-glycine) synthesis, is
needed. The glutathione is one small peptide that can be excreted from
the yeast cells during fermentation. The over-expression of GSH1 in
brewer’s yeast may improve the GSH content in beer. The glutathione
“GSH” is an important antioxidant against the effects of oxygen and
other oxidative compounds. This is particularly beneficial to beer
flavor protection and stability during beer storage. Accordingly, GSH
plays an important role in balancing the redox potentials in different
subcellular compartments and maintaining the redox balance in Saccharomyces cerevisiae.
The effect of the medium containing precursor amino-acids, salts,
carbon, and nitrogen sources upon the yield and the productivity of
glutathione from the isolated strain of S. cerevisiae and the
corresponding hybrid were investigated in detail. Another way to exploit
the potential of the selected yeast, an interesting solution, is its
hybridization with S. cerevisiae or S. uvarum, by crossing
of spores or cells from selected strains. Hybrids constructed in this
way, using strains from our collection, showed increasing enzymatic
expression thanks to the union of the two parental genomes.
Julien Billard is currently a microbiologist in the Research and
Development laboratory of AEB Group. He is currently working on the
selection of yeast strains for fermentation of beer and specific
propagation for expression of MAL and GSH.
