Food Biochemistry and Food Processing

29 Biochemistry and Fermentation of Beer 673

• Adjustment of the bitterness of beer (if
  necessary) by adding hop products.

In the presence of yeast, the principal changes that
occur are the elimination of undesirable flavor com-
pounds—such as vicinal diketones, hydrogen sul-
phide, and acetaldehyde—and the excretion of com-
pounds enhancing the flavor fullness (body) of the
beer.

VICINALDIKETONES

In traditional fermentation lagering processes, the
elimination of vicinal diketones required several
weeks and determined the length of the maturation
process. Currently, the maturation phase is much
shorter since strategies are used to accelerate the
vicinal diketone removal (see above). Diacetyl is
used as a marker molecule. The objective during
lagering is to reduce the diacetyl concentration
below its taste threshold (0.10 mg/mL).

HYDROGENSULPHIDE

Hydrogen sulphide plays an important role during
maturation. Hydrogen sulphide, which is not incor-
porated into S-containing amino acids, is excreted
by the yeast cell during the growth phase (see Fig.
29.4). The excreted amount depends on the used
yeast strain, the sulphate content of the wort, and the
growth conditions (Romano and Suzzi 1992). At the
end of the primary fermentation and during the mat-
uration, the excess H 2 S is reutilized by the yeast. A
warm conditioning period at 10–12°C may be used
to remove excessive levels of H 2 S.
Brewing yeasts produce H 2 S when they are defi-
cient in the vitamin pantothenate (Walker 1998).
This vitamin is a precursor of coenzyme A, which is
required for metabolism of sulphate into methion-
ine. Panthothenate deficiency may therefore result in
an imbalance in sulphur–amino acid biosynthesis,
leading to excess sulphate uptake and excretion of
H 2 S (Slaughter and Jordan 1986).
Sulphite is a versatile food additive used to pre-
serve a large range of beverages and foodstuffs. In
beer, sulphite has a dual purpose, acting both as an
antioxidant and as an agent for masking certain off-
flavors. Some of the flavor-stabilizing properties of
sulphite are suggested to be due to complex form-
ation of bisulphate with varying carbonyl com-

pounds, of which some would give rise to off-flavors

in bottled beer (Dufour 1991). Especially, the un-

wanted carbonyl trans-2-nonenal has received par-

ticular attention since it is responsible for the “card-

board” flavor of some types of stale beer. It has been

suggested that it would be better to use a yeast strain

with reduced sulphite excretion during fermentation

and to add sulphite at the point of bottling to ensure

good flavor stability (Francke Johannesen et al.

1999). Therefore, a brewer’s yeast disabled in the

production of sulphite has been constructed by inac-

tivating both copies of the two alleles of the MET14

gene (which encodes for adenylylsulphate kinase).

Fermentation experiments showed that there was no

qualitative difference between yeast-derived and

artificially added sulphite, with respect to trans-2-

nonenal content and flavor stability of the final beer.

The elimination of the gene encoding sulphite

reductase (MET10)in brewing strains of Saccha-

romycesresults in increased accumulation of SO 2 in

beer (Hansen and Kielbrandt 1996a). The inactiva-

tion of MET2resulted in elevated sulphite concen-

trations in beer (Hansen and Kielbrandt 1996b).

Beers produced with increased levels of sulphite

showed improved flavor stability.

The production of H 2 S could be reduced by the

expression of cystathione synthase genes from S.

cerevisiaein a brewing yeast strain (Tezuka et al.

1992).

ACETALDEHYDE

Aldehydes—in particular acetaldehyde (green apple–

like flavor)—have an impact on the flavor of green

beer. Acetaldehyde synthesis is linked to yeast

growth. Its concentration is maximal at the end of

the growth phase, and is reduced at the end of the

primary fermentation and during maturation by the

yeast cells. As with diacetyl, levels may be enhanced

if yeast metabolism is stimulated during transfer,

especially by oxygen ingress. Removal also requires

the presence of enough active yeast. Fermentations

with early flocculating yeast cells can result in too

high acetaldehyde concentrations at the end.

DEVELOPMENT OFFLAVORFULLNESS

During maturation, the residual yeast will excrete

compounds (i.e., amino acids, phosphates, peptides,

nucleic acids, etc.) into the beer. The amount and