Reusing yeast is certainly one of those things that makes sense from
a cost-savings perspective. Indeed, brewers have been harvesting yeast
crops at the end of fermentation, temporarily storing the yeast and
re-pitching it into subsequent batches for a very, very long time.
One thing that has been demonstrated through research is that brewing
yeast is not something found in nature; mutations over thousands of
years of brewing have resulted in yeast strains that are only found in
breweries. This is really not too different from domesticated animals.
Another thing that is known about brewing yeast is that some of the
properties strains are selected for by brewers change with subsequent
generations. One example is a yeast strain’s flocculation
characteristic; lager yeasts, in particular, tend to become less
flocculent if the yeast is used for too many generations. Some strains
change faster than others, but as a general rule most breweries do not
continue to reuse yeast indefinitely and new yeast is brought into the
brewery by growing cells from some sort of storage form (liquid
nitrogen, freeze dried samples or cultures stored on a growth medium).
The best yeast to reuse comes from a healthy and normal fermentation.
Batches that do not ferment normally, or those that have unusual aromas,
are not top candidates for cropping. Assuming that you have a batch
that is a good candidate for yeast cropping, you need to determine how
to harvest the yeast. Most yeast strains, including most ale yeast, will
eventually settle to the bottom of the fermenter within a few days
following the end of fermentation. I think the easiest way to harvest
yeast at home is to rack the beer out of the primary, swirl the yeast up
from the bottom of the fermenter and pour it into a clean and sanitized
storage container. Most thick slurries harvested in this manner contain
about 750 million cells per milliliter.
It is best to store yeast in a vented container because there is a real
possibility that the cropped culture will produce carbon dioxide. Do not
store yeast in sealed glass containers as this may result in exploding
glass containers. I am a proponent of using plugs of cotton batting to
close the mouths of Erlenmeyer and Fernbach flasks.
Yeast cells have a finite storage life, and as energy reserves, such as
glycogen, are consumed during storage, cells begin to die. The most
common method used to extend the storage life of yeast cultures is to
rapidly cool the yeast culture and store it somewhere around 32–38 °F
(0–3 °C). Some brewers wash yeast with cold water after harvesting to
dilute the beer content of the slurry since the alcohol content of beer
is detrimental to cells during storage. Yeast can easily be stored for
up to a week in this temperature range without losing too much viability
in the culture. Anything greater than a week is too long for most
commercial brewers because the economic risk of using old and tired
yeast cultures is simply too great.
Things are not too different with homebrewing, except the risk of
failure. If I am running a brewery that brews 100-barrel batches and
ferments these batches in 400 barrel fermenters, the cost of a failed
batch is equal to about 20 man hours of work plus about 20,000 pounds of
malt and 200 pounds of hops; roughly $8,300 in labor and raw material
At home the cost of failure is perhaps even greater. You spend your
hard-earned free time and you pour your heart and soul into crafting
that perfect batch of wort. If you want to re-use yeast at home, do not
push the limits of storage time and expect anything miraculous to occur.
In fact, you should actually expect poor results because that is what
you could see.