Introduction to Fungi, Third Edition

274 HEMIASCOMYCETES
in habitats which contain abundant nutrients,
such as the surface of ripe fruits (Starmer
et al., 1987). Not surprisingly, contaminations
by killer yeasts can be a problem in long-term
fermentation processes, e.g. wine production
(van Vuuren & Jacobs, 1992), and attempts have
been made to incorporate the killer virus into
yeast strains used for biotechnological purposes
(Javadekar et al., 1995). Killer toxins
cannot themselves be used against clinically
relevant yeasts because the molecules are so
large that they would elicit an immune response
in the patient. However, it is possible to create
antibodies which mimic the membrane-disrupting
action of killer toxins (Polonelli et al., 1991).
Whether these will become useful in medicine,
e.g. against Candida infections, remains to be
seen, but the existence of natural human
antibodies with a killer toxin effect on Candida
points to potential applications of this strategy
(Magliani et al., 1997, 2005).
10.2.8 Bread-making
The principle behind the leavening of bread
dough by baker’s yeast is the same as in brewing,
i.e. the anaerobic metabolism of glucose and
other reducing sugars via pyruvic acid into
ethanol and CO 2 . The difference is that the
released CO 2 is the important product in breadmaking
because it is responsible for the texture
of the bread. Ethanol may, however, contribute
to the flavour of fresh bread. Originally, a portion
of the risen dough medium was retained as
a starter for the next baking session, or surplus
yeast from brewing processes was used (Jenson,
1998). Specific yeasts for baking were first
produced in Vienna in 1846, and baker’s yeast
is now produced commercially under aerobic
conditions because the yield of biomass can be
maximized (Caron, 1995). In the bread dough the
yeast cells are subjected to anoxic or anaerobic
conditions and must be able to release CO 2
quickly. The carbon sources available to yeast
cells in bread dough are hexoses, especially glucose,
and the disaccharides maltose and sucrose,
all of which are present at fairly low concentrations.
Starch is not utilized by S. cerevisiae but can
be hydrolysed by amylases present in the flour,
and the glucose thereby released may be
available to the yeast (Oliver, 1991; Jakobsen et
al., 2002). A very thorough account of the
microbiology and processes of baking is that
by Spicher and Brümmer (1995).
10.2.9 Beer brewing
Several good accounts of the process of beer
brewing have been given (e.g. Oliver, 1991;
Russell & Stewart, 1995; Hartmeier & Reiss,
2002), and there are numerous popular books
exploring the diversity of beers worldwide.
In his masterful history of beer, Hornsey (2003)
has summarized evidence of the first known
records and recipes of beer which date back
6000 years or more and originate from Mesopotamia
and ancient Egypt, where beer was more
widely consumed than wine. The art of brewing
may be almost as ancient as the cultivation
of cereals, and indeed some historians believe
that brewing was a major incentive for the
development of agriculture around 6000 BC
(Hornsey, 2003). Brewing has remained the
most important area of biotechnology to this
day. Astonishing quantities of beer are being
consumed, with several sources agreeing on the
Czech Republic as the top beer-drinking nation
at around 160 l per person per annum, followed
by the Republic of Ireland (155 l) and Germany
(128 l). These values appear frugal when put
into the historical context, e.g. of one gallon
(3.8 l) as the daily personal allowance of ale for
monks in medieval England (Hornsey, 2003).
Two fundamentally different types of fermentation
exist, and these differ in the strains of
yeast used. In bottom-fermenting beers, especially
lager beers, the yeast settles as a sludge at
the bottom of the brewing vessel at the end
of the fermentation, whereas it floats at the top
in top-fermenting beers, especially the English
ales, porters, stouts, and the German Altbier. In
Germany, a purity law was passed in 1516 which
banned the use of ingredients other than water,
yeast, malted barley and hops. Although now
formally abolished by the European Union, most
brewers still abide by it. In contrast, ale and
lager brewers outside Germany often add other
ingredients to their beer, e.g. cereals other than
barley, other fermentable sugar sources such