Slides
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Brewing Yeast and Brewery Fermentation
Karl J.
Siebert
Introduction
Of the main ingredients of beer (malt, hops, yeast and water), yeast has
the most profound effect on flavor. It produces alcohol, CO2,
and a range of esters, higher alcohols and fatty acids, many of which
contribute to beer flavor. By varying the fermentation conditions, the
same yeast can be used to produce very different products such as
regular and light beers and malt liquors.
Yeast
In brewing, in contrast to the practice in wine making, yeast is
virtually always collected after each fermentation and reused in
subsequent fermentations. Brewers call the practice of adding yeast to
wort 'pitching', although it most often is done by pumping a yeast
slurry into the cooled, aerated wort stream as it flows to the
fermenter.
Spontaneous fermentation with endogenous organisms is done rarely, and
only with a few specialty products like some of the Belgian Lambics and
Geuezes.
Lager yeasts throughout the world are remarkably similar and difficult
to distinguish other than at the genetic level. Ale yeasts vary in cell
or colony morphology (cell size or shape, colony color or appearance,
etc.). Most commercial brewing yeasts are polyploid. This means they
have multiple copies of each chromosome (typically estimated between 3
and 4 rather than 2); as a consequence they sporulate very poorly. That
in turn essentially prevents sexual recombination (they reproduce by
budding) and results in considerable genetic stability.
Fermentation
Brewery fermentations can be carried out in open or closed tanks made of
a variety of materials with a range of geometries (rectangular,
barrel-shaped, horizontal cylinder, etc.). The most common vessels in
new large breweries are huge (as large as 5,000 bbl = 155,000 gal)
stainless steel tanks of vertical cylindroconical design.
Classically, lager fermentations are carried out at cooler temperatures
(8oC-15oC) than ale fermentations
(15oC-18oC) and take longer. Lager fermentations:
6-12 days; ale fermentations: as fast as 3 days. At the end of a
fermentation yeast flocculates and sinks to the bottom in a lager
fermentation or rises to the top in an ale fermentation. Malt liquors
are made with lager yeasts using fermentation temperatures that are
warmer than a typical lager process; this results in a beer with more
higher alcohols and esters. Secondary fermentations, either with
(kraeusening) or without addition of fresh wort, are used for some
products.
Beer is always safe to drink, as no pathogens can grow in it. However,
brewers are very careful to avoid contamination of their culture yeast
with non-culture ('wild') yeasts or bacteria, which can lead to
off-flavors and hazes.
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FS430:
Understanding Wine and Beer