JULIEN BILLARD (1), Huu Vang Nguyen (2), Philippe Cario (3), Mustapha Nedjma (4)
(1) R&D Department, Spindal AEB Group, Gretz-Armainvilliers, France;
(2) INRA, AgroParisTech, Thiverval-Grignon, France; (3) AEB Group,
Brescia, Italy; (4) nedjman@aol.com
Glutathione is an important antioxidant against the toxic effects of O2
and other oxidative compounds; hence, it would be helpful to keep beer
flavor when increasing GSH content of brewer’s yeast. GSH is synthesized
by two sequential reactions in Saccharomyces cerevisiae
catalyzed by γ-glutamylcysteine synthetase (EC 6.3.2.2) and glutathione
synthetase (EC 6.3.2.3), and the GSH1 gene is responsible for coding the
former enzyme, which is crucial to GSH synthesis. As a smaller peptide,
GSH can be excreted to the outside of the yeast cells. Glutathione can
be produced by an enzymatic method and a direct fermentative method. In
the latter method, S. cerevisiae and Candida utilis are
currently used to produce glutathione on an industrial scale. Factors
for increasing glutathione production have been investigated. The rates
of GSH production of the wild-type strains usually vary from 0.1 to 1%,
dry matter. Medium culture conditions, selected yeast strains, and yeast
breeding are key factors for increasing the GSH concentration. A
nucleophilic center of cysteine is responsible for the high reductive
potential of GSH. The role of GSH in redox regulation of gene expression
has been described in many studies, highlighting the couple properties
of GSH/GSSG and the reduced SH-Group of GSH. It can participate in the
regulation of the cell cycle and is an essential reductant during normal
metabolism in yeast strains. We investigated the influence of feedstock
amino acids, salt, carbon, and nitrogen sources on glutathione
production by S. cerevisiae. Glucose, yeast extract, oligoelements, and amino acids were found to be suitable feedstock. Highest glutathione production was obtained after cultivation with shaking for 72 h in selective medium and growing conditions. Using this medium, the glutathione
concentration increased 3- to 5-fold to 95–110 mg/g of dry matter
compared to YM basal medium. The increase of glutathione during the
propagation resulted in the protection of the yeast cells against
hydrogen peroxide production (H2O2). In the mean
time, the addition of catalase activity during propagation increased the
protection of the yeast cells from osmotic and oxidative stresses,
demonstrating that these are the major causes of the stress response
throughout the process of beer biomass production. In fact, the synergy
of glutathione and catalase during the yeast propagation leads to an
increase the number of yeast cells produced and causes a positive impact
on yeast metabolism.
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.
