Introduction to Fungi, Third Edition

SACCHAROMYCES (SACCHAROMYCETACEAE)
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as syrups or enzymatic starch digests, fruits or
interesting spices.
Since S. cerevisiae cannot utilize starch, this
has to be hydrolysed into fermentable sugars
first. The starch reserves in the endosperm of
barley are hydrolysed naturally by endogenous
amylases when the grains germinate. At a
suitable time, the germination process is terminated
by drying and heating (kilning), and the
degree to which the malt is roasted determines
the colour of the beer. For instance, heavily
roasted malts are used for the dark milds, porter
ales and stouts. During mashing, the ground
malt is heated in water to about 65°C; surviving
endogenous enzymes or added enzymes continue
with starch hydrolysis, and with the
degradation of proteins to amino acids. Hops
are added to the liquid (wort) which is then
boiled, traditionally in a copper vessel, and after
filtration and cooling the yeast is added. Top
fermentation of ales takes a few days at 15 22°C
whereas lager beers are fermented for up to
2 weeks at 8 15°C. In addition to ethanol, yeast
metabolites which impart flavour to the beer are
esters of higher alcohols (e.g. isobutanol), diketones,
diacetyl, isobutyraldehyde and methylglyoxal.
Sulphur-containing metabolites may also
be important. There are several ways in which
the flavour profile can be modified to give a
desirable taste. For example, a yeast strain
with the appropriate ester profile can be used,
or new strains expressing the required enzymes
can be engineered. A re-fermentation, often
with Brettanomyces spp., may be performed in
order to alter the flavour profile (Vanderhaegen
et al., 2003).
Towards the end of the fermentation, the
yeast cells should flocculate, i.e. form aggregates.
Flocculation is dependent on the expression of
a number of surface proteins (flocculins) which
recognize and bind to the mannose residues on
mannoproteins located in the outermost wall
layer of other yeast cells (Verstrepen et al., 2003).
These flocculins are probably located in fimbriae,
short hairs (0.5 mm long) on the cell surface
which have been observed by ultrastructural
studies (Day et al., 1975). Numerous environmental
factors such as carbon and nitrogen deficiency,
ethanol levels and cell age contribute to
efficient flocculation, the control of which is one
of the most difficult tasks in brewing. Whether
flocculated yeast accumulates at the top or
bottom of the vessel seems to depend on the
flocculins as well as other wall surface properties
(Dengis & Rouxhet, 1997). Attempts are being
made to engineer improved brewer’s yeast
strains with respect to their flocculation behaviour
(Verstrepen et al., 2003) and also their ability
to utilize other carbon sources, including starch
(Hammond, 1995).
Most beer in Europe is brewed in batch fermentations,
but in other countries continuous
cultures following the chemostat principle are
performed. Either way, the ale or lager must be
stored for a while before it is sterilized and
filled into barrels or bottles. In the case of
cask-conditioned Real Ales, the beer is filled
directly into the casks and stored until it is ready
to be sold to public houses; cask-conditioned ale
is therefore in direct contact with the yeast until
it is served to the customer. It requires skilled
brewers and publicans to keep cask-conditioned
Real Ale, but it does, in the opinion of many,
result in a superior pint.
10.2.10 Wine production
Wine is the fermentation product of starting
materials which already contain high levels of
monosaccharides, i.e. typically fruit juices and
especially the must of grapes. Wine is at least
as ancient as beer and seems to have originated
in Transcaucasia and the Near East in the early
Neolithic, around or before 6000 BC (McGovern,
2003). One of the pre-requisites for wine-making
was the invention of pottery, since the fermentation
process requires anaerobic conditions. Wine
has been given a special place in many civilizations
by its association with religious ceremonies,
e.g. in ancient Egypt, Greece, Rome and in
Christianity. In France, Portugal, Luxemburg
and Italy, more than 50 l wine are consumed
per person per annum.
Red wine takes its colour (and high tannin
content) from the skin of the grapes which
are macerated, and the juice (‘must’) is left in
contact with the solid parts for some time. In
contrast, in white wine and rosé wine, the must