Food Biochemistry and Food Processing

666 Part VI: Fermented Foods

explained as an inhibition of the oxidative system by
high concentrations of glucose. Nowadays, it is gen-
erally accepted that the formation of ethanol at aero-
bic conditions is a consequence of a bottleneck in
the oxidation of pyruvate (e.g., in the respiratory
system; Petrik et al. 1983, Rieger et al. 1983, Käp-
peli et al. 1985, Fraleigh et al. 1989, Alexander and
Jeffries 1990).
A reduction of ethanol production can be
achieved by metabolic engineering of the carbon
flux in yeast, resulting in an increased formation of
other fermentation products. A shift of the carbon
flux towards glycerol at the expense of ethanol for-
mation in yeast was achieved by simply increasing
the level of glycerol-3-phosphate dehydrogenase
(Michnick et al. 1997, Nevoigt and Stahl 1997,
Remize et al. 1999, Dequin 2001). The GDP1gene,
which encodes glycerol-3-phosphate dehydroge-
nase, has been overexpressed in an industrial lager
brewing yeast to reduce the ethanol content in beer
(Nevoigt et al. 2002). The amount of glycerol pro-
duced by the GDP1-overexpressing yeast in fer-
mentation experiments—simulating brewing condi-
tions—was increased 5.6 times, and ethanol was
decreased by 18% compared with the wild-type
strain. Overexpression did not affect the consump-
tion of wort sugars, and only minor changes in the

concentration of higher alcohols, esters, and fatty

acids could be observed. However, the concentra-

tions of several other by-products, particularly ace-

toin, diacetyl, and acetaldehyde, were considerably

increased.

METABOLISM OF

BIOFLAVORING BY-PRODUCTS

Yeast is an important contributor to flavor develop-

ment in fermented beverages. The compounds that

are produced during fermentation are many and var-

ied, depending on both the raw materials and the

microorganisms used. The interrelation between

yeast metabolism and the production of bioflavoring

by-products is illustrated in Figure 29.2.

BIOSYNTHESIS OFHIGHERALCOHOLS

During beer fermentation, higher alcohols (also

called “fusel alcohols”) are produced by yeast cells

as by-products and represent the major fraction of the

volatile compounds. More than 35 higher alcohols in

beer have been described. Table 29.3 gives the most

important compounds, which can be classified into

aliphatic [n-propanol, isobutanol, 2-methylbutanol

(or active amyl alcohol), and 3-methylbutanol (or

Figure 29.2.Interrelationships between yeast metabolism and the production of flavor-active compounds.
(Hammond 1993.)