Yeast and Brewing
Yeast, Saccharomyces cerevisiae, and
Saccharomyces uvarum are the genus and species of ale, and lager yeast
respectively. These are the primary types of yeast cultures which
produce most of the worlds beers. Other types of yeast and bacteria
are also utilized in various styles of beer and brewing beer like
beverages. Many of these organisms were discovered more by chance,
than by design. Beverages including wine, fermented milk products, and
mead from honey are some examples of what developed from spontaneous
fermentation, which is now understood and managed in a scientific
manner.
Yeast cells are round, or ovate and
reproduce by multipolar budding. They are approximately 6-8 microns in
size. The characteristics which are beneficial to brewing are flavor
production, the ability to attenuate the wort, and their flocculating
nature, or how well they settle out and clear the beer after
fermentation is complete.
As described by Gay-Lussac at the
beginning of the nineteenth century, the chemical reaction of
fermentation is as follows;
C6H12O6
+ Saccharomyces cerevisiae = 2C2H5OH + 2CO2
(sugar plus yeast yields
alcohol and carbon dioxide )
Yeast and Their Flavor
Affects in Beer
Around the world throughout the
centuries, yeast cells, like most living things, have evolved and have
adapted to their environment in order to survive.
Due to this mutation and adaptation,
subspecies, or variations of yeast have evolved due to the specific
climate, and food sources. For this reason we are fortunate to have a
wide variety of yeast strains today. These strains can be identified
by their fermentation characteristics and selected to produce certain
beer styles.
Ale yeast ferment at warmer
temperatures than will lager yeast. In turn, Ales typically become
fruitier, softer and more robust than lager beers. Lager beers tend to
be dry, crisp and steely, the classical example of this is the Pilsner
Lager Beer. By selecting specific yeast, one can emphasize maltiness,
certain fruity esters, hop character, and a number of other
fermentation characteristics desirable for a given beer.
In addition to the Ales and lagers
of the world, wheat beers, and lambic style beers utilize other
relative yeast strains and bacteria which impart unusual flavors and
aromas such as apple, banana, plum, apricot, bubble gum, and even
sweaty horse hair. These flavor and aroma compounds are an essential
aspect of certain beer styles. Without these rare types of cultures
which have evolved through time, the dimension and diversity of beer
styles would be greatly limited.
By selecting specific yeast strains
and providing a certain environment, the brewer creates the beer style
of choice. These parameters include the type of water, whether hard or
soft, the variety of malts, whether malted lightly or dark, and the
choice of hops, whether bitter or aromatic. Taking these components
and controlling the environment, the brewer puts their signature on
the beer with the silent regimen of the hard working yeast.
Cultivation and
Maintenance of Yeast
Yeast cells are maintained with two
basic parameters; feed them so they don't die, and keep them clean, or
free of contamination by other organisms. By doing this, along with
selective culturing, the identity and character of each specific
strain can be maintained. This process requires dedicated
microbiologists in an environment free of contamination. There is an
ongoing process which involves the storage, reviving, testing, and
cataloging data which is collected as the identifying characteristics.
Yeast can be stored or maintained on
agar slants of food nutrient to allow continued survival of the
organism. Yeast can be dried or freeze dried for prolonged shelf
storage, or they can be frozen in certain mediums or held under
refrigeration in liquid suspension. All methods have benefits and
drawbacks. It is the scientists and brewers job to control the
parameters which effect the quality of the yeast and their survival.
Three key instruments in this
process, are the autoclave, incubator and microscope. The use of these
tools and culture medium conducive to yeast growth, allow's one to
take care of their yeast. In addition basic laboratory supplies and
glassware are used.
The autoclave is a pressure cooker
or sterilizer. This equipment is used to sterilize the growth medium
and instruments which come in contact with the yeast. This is how the
yeast is kept clean. The sterilization of the medium kills any of the
many potentially contaminating bacteria and organisms which abound.
The autoclave produces steam heat to 250 degrees fahrenheit at 15
pounds of pressure to produce the sterilizing effect, or to kill
anything in it. The growth medium and equipment having been processed
in the autoclave, are ready to be introduced into the yeast
environment aseptically. Note, that the yeast cannot be autoclaved, or
they too would die.
When the proper medium is prepared,
and yeast aseptically transferred to the medium, the suitable
temperature which is conducive to rapid or exponential yeast growth is
necessary. This is achieved in an incubator. An incubator is a
controlled environment, which provides the optimum temperature. It
minimizes temperature fluctuations and extremes. Temperatures too cold
or too warm will inhibit growth or even kill the yeast.
The microscope is beneficial to
confirm the presence of the desired organism. Single yeast cells
cannot be seen by the naked eye. Magnification by several hundred
times, to 10,000 times, by the microscope, can bring you into the
microscopic world of the unknown. It can also be used to identify
potential contaminating intruders. Depending on the magnifying
strength, the microscope can bring to life, vivid morphologies, which
help identify one organism from another.
