from the President
By Chris White
Brewers carry out lots of fermentations, and each
fermentation needs yeast, of course. They reuse that yeast many times.
We will look at some of the best ways to reuse yeast, as healthy yeast
results in better fermentation profiles and better flavors.
Yeast is a living organism, and is most happy and
healthy when feeding on wort sugars. When fermentation is complete, they
flocculate to the bottom of the fermentor. They then go into a resting
state. Yeast under beer is fairly stable, and most brewers agree that
the best place to store yeast is under beer. But two crucial factors are
temperature and time.
The yeast cake at the bottom of a conical fermentor
can rise in temperature. Yeast is an excellent insulator, and heat can
build up in the middle of the slurry, 10-15 degrees F above the beer
temperature, for very flocculent strains. When yeast heats up, its life
span plummets. If the cone is not chilled, effects are even more
significant. For this reason, brewers try to remove yeast slurry shortly
after fermentation is complete, and the beer is chilled.
Once yeast is removed, you ideally want to use the
yeast as soon as possible (within 8 hours). This allows little time for
yeast to deteriorate and die. But this is not often possible, as you may
not brew another beer until the next week. The most common way to store
yeast for brew pubs is to put it into 5-gallon, stainless steel soda
kegs. These work well, and additionally the lid can be modified to your
desire. But the two problems with these kegs are the many small parts
and gaskets that can harbor bacteria, and the fact that they do not vent
pressure unless modified in some fashion. Carbon dioxide can build up
quickly in yeast slurry, and if kept under pressure, will cross the cell
walls and kill yeast cells. Pressures over 35 PSI can be toxic to yeast,
and soda kegs are rated over 100 PSI. So if you use these kegs, shake
and vent pressure on a regular basis, at least once per day.
Other vessels can be used for yeast storage. Brewers
often shun plastic, because it scratches easy and scratches can harbor
bacteria and wild yeast. But it can actually be a good choice. Be sure
to use a high grade (and food grade) plastic (polyethylene,
polypropylene), and be sure the buckets are used exclusively for yeast
storage. The advantage of plastic is the fact that the yeast slurry is
visible, so you can evaluate the condition and quantity of yeast by
sight. For example, if you pull off yeast slurry and it is very runny,
without counting with a microscope you will be unsure of how much yeast
to use in the next batch. By using a plastic bucket to store yeast, you
can see how much yeast settles out, and pitch accordingly. Plastic
buckets also need to be vented occasionally.
How long can yeast be stored? The best-case scenario
is to use the yeast within 1-3 days. Again, this is often not possible,
especially if multiple strains are being used in the brewery. The magic
number seems to be two weeks. If less than two weeks, brewers will
usually have no problem reusing yeast. Over two weeks, you may have
problems. After four weeks, the viability of yeast slurry is usually 50%
or lower. Also remember that lager yeast does not store as well. The
same applies to unhealthy yeast.
As yeast sit in storage, they consume their glycogen
reserves. Glycogen deprivation weakens their cell walls, and makes them
more susceptible to rupture. Cold temperatures retard this process, but
you want to avoid freezing yeast, as ice crystals will also rupture
cells. The ideal storage temperatures range is between 33-38oF. When
yeast rupture, they release their contents into the liquid phase.
Bacteria can feed off the nitrogen released, and multiply rapidly. So
the yeast slurry needs to be as contamination free as possible when
stored. Cold temperatures will also help retard bacterial growth.
To be confident, brewers should test yeast after
storage, and before use. Check it for viability and for possible
contamination. Ideally you want to use yeast that is over 95% viable,
but most brewers just compensate for lower viability by using more
slurry. This can be successful, but can also lead to problem
fermentations. The overall health of the yeast may be low, so the slurry
may not produce the expected range of flavor and aroma compounds, and
may not attenuate correctly. To check for viability, a brewer needs a
microscope. Always keep extra, unused yeast on hand in case a problem is
encountered with the yeast you intend to use.
To test for contamination, the slurry needs to be
plated out on to specialized media 3-5 days before use. You should check
the yeast slurry for aerobic bacteria, anaerobic bacteria, and wild
yeast. Of the three, anaerobic bacteria is the most common bacteria
found in brewers yeast slurry, and is also the hardest for a brewer to
irradicate. The most common anaerobic bacteria are the lactic acid
bacteria, Lactobacillus and Pediococcus. A 10 ml sample of
yeast slurry should be removed, diluted 1:100 with sterile water, and
0.1ml to 1.0 ml plated on suitable media. The types and procedures for
this would take up an entire article, but if bacteria counts are over 1
per ml, and wild yeast is over 1 per 0.1ml, the yeast slurry should not
The best thing to do for yeast after it has been
stored for two weeks – if it tests clean — is to add some fresh wort
before using. This helps to restore yeast strength, and ensures a
successful fermentation. Simply pour off beer that has separated from
flocculated yeast, add fresh wort at 9-12 Plato, and let it sit at room
temperature for 10-20 hours. Assuming yeast activity was evident in this
"starter" or "activator," pitch into fresh wort as
Brewers have always reused yeast in brewing, long
before they knew yeast was responsible for beer production. In fact, the
continual reuse of yeast has led to the impressive genetic variety of
brewing strains, and to their suitability for brewing. For most of
history, yeast has been skimmed from the top of fermentations, and
reused. Today we usually use conical bottom fermentors that aid in
cleaning and yeast collection. While these vessels help in yeast
collection, the quality of yeast that is collected is not as good as
from top cropping. Top cropped yeast rises at a particular time in the
fermentation, has a high viability, and is relatively free from trub.
When yeast is forced to the bottom of a conical fermentor, it mixes with
dead yeast, trub, and bacteria. This means we have to be careful when
collecting yeast, store it for short times, and test it before reusing.
With careful attention to these parameters, a brewer should get 5 to 10
generations of high quality yeast.
Chris White is President of White Labs Inc. and is a chemistry and
biochemistry lecturer at the University of California, San Diego. He has
a Ph.D in biochemistry. To comment about this article, write him at