Reusing a Yeast Cake

Professional brewers reuse yeast all the time.  This reuse allows them to save a substantial amount of money, rather than growing up or purchasing a new yeast pitch each time.  Additionally, many people claim that yeast cultures take a couple generations to hit their stride (a generation is fermenting a single batch of beer) and that the yeast performs best after the third batch.  Eventually, they yeast colony begins to mutate, alter its performance or resulting flavor profile and needs to be removed from the brewery.  The number of generations a brewery uses its yeast varies by brewer beliefs, yeast strain, equipment and other factors.  Many breweries maintain labs and run chemical and biological analyses of the yeast to determine its maximum generation number, while other breweries rely on experience and work processes.  All things considered, yeast reuse is almost mandatory in the professional beer world.

The same is not true in homebrewing.  Yeast costs are much smaller for homebrewers, so there is less incentive to try to save money by reusing yeast (if you are making your own beer solely to save money, you may be in the wrong hobby).  Homebrewers also do not have much experience with saving yeast and performing proper procedures to limit bacterial load and optimize yeast health.  But, the largest reason for not reusing yeast, in my opinion, is scheduling.  Liquid yeast does not have a long storage life, even in optimal conditions.  If a homebrewer saves a batch of yeast and does not use it within two weeks, the yeast may need a starter to revitalize it before repitching.  Most homebrewers I know do not brew more than once or twice a month, so storage becomes a problem.  Combine this with the fact that we rarely brew the same beer style back-to-back, often using different yeasts, so scheduling becomes quite an issue.

All that being said, I have intended to reuse a batch of yeast for more than one beer for quite some time.  The easiest way to do this is to ferment a batch of beer, siphon the new beer off the cake, and have fresh wort ready to go on the yeast immediately afterwards.  This method does not require rinsing the yeast; nor does it require lengthy yeast storage.  It is a simple process, but requires planning to time the two batches properly and ensure they can use the same yeast (I did an Irish Red and an Oatmeal Stout with White Lab's Irish Ale Yeast).

If you consider reusing a yeast cake, keep the following in mind:

• Do not use the same cake more than two or three times.  Because you are not rinsing the yeast, each batch leaves trub, hop matter, and dead yeast behind.  This can slowly degrade and produce off-flavors in your beer.
• Do not use a high-gravity wort or very hoppy wort until the terminal batch of beer.  High-gravity wort stresses the yeast and may make it less likely to fully ferment the following beer.  Highly-hopped worts leave bittering compounds in the yeast cake that can over-bitter the subsequent beer.
• Target a yeast that settles out well, so that it is easy to rack off the yeast cake and not have it be overly disturbed before receiving the new wort
• Avoid using fining agents, like gelatin, that will be left in the yeast cake and can inhibit the subsequent fermentation.
• Be prepared for VERY quick yeast activity and use a blow-off tube because of the violence of subsequent fermentations (see the picture for what my Oatmeal Stout left in the fermentation chamber)

Reusing the yeast cake has been a fun experiment for me.  The stout is not done yet, so I cannot compare the yeast's performance to other batches of the recipe I have completed.  But I am confident that it will have performed at least as well as previous batches.  However, the difficulty and limitation of reusing the cake makes the process likely to only happen infrequently in my homebrewery.

If you have ever tried reusing yeast, leave us a comment and let us know how it went.

Cheers,

TW