Part Three in the discussion of the different
elements involved in brewing your favourite pint. Belgian beer
expert Steve Mitchell writes about another important component
of your pint
YEAST
A single cell micro-organism, the function of the yeast fungus
is to transform malt-sugars in the 'wort' (the fermentable infusion
of malt, hops and water) into ethyl-alcohol and carbon dioxide.
This is achieved by the yeast secreting an enzyme which feeds
upon the sugars (the fermentables) and excreting equal volumes
of alcohol and C02 by weight. The alcoholic content of beer can
not usually exceed approx. 12% as beyond that concentration beer
yeast dies in its own excreta. There are exceptions, as wine yeasts
are more tolerant to alchohol and are sometimes employed to make
very strong 'barley wine' type beers.
Three types of beer yeast can be distinguished according to the
three modes of fermentation. In the case of bottom-fermentation
(such as in lager production) the yeast is of the type Saccharomyces
Carisbergensis. It works at temperatures of between 60 and 100C,
for 8 to 10 days at the bottom of the fermenting vessels. For
top fermentation, (ale production) the yeast is of the type Saccharomyces
Cerevisiae. It works at temperatures between 180 and 250 C, for
4 to 6 days at the top of the 'wort'.
Spontaneous fermentation, (as in the Belgian 'Lambic' beers)uses
wild yeasts which are contained in the ambient brewery atmosphere.
During fermentation yeast reproduces in abundance. Its quantities
triple and quadruple. When the main fermentation has subsided,
it is recovered, checked and preserved in cold surroundings, and
is used in later brews. Each re-use is called a "generation".
Top fermenting yeasts are skimmed off the surface, and may be
re-used a dozen times. Bottom fermenting yeast is harvested from
the bottom of the fermenting vessel, but degenerate very rapidly.
The same stock can not be used more than six to eight times. Excess
yeast is used by the pharmaceuteal industry as it is rich in 'B'
vitamins. It is also used in the manufacture of canned and packet
soups, and is added to copper and brass cleaning products.
Some brewers are very proud of their own yeast, which may originally
have been cultivated from a single cell. Yeasts may be decades
old, but often it is the yeast which makes a particular brewer's
product distinctive and unique.
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