Yeast can be expensive. Liquid yeast from Wyeast and White Labs costs between $6 – $20 (depending on your location). As frugal people, we like to reuse our yeast. There are two ways to do this: pitching on the yeast cake or yeast washing. We often prefer pitching on the yeast cake. The yeast cake is all the gunk that is left at the bottom of your primary fermenter. Don’t throw that yeast out; it can be reused.

How to Pitch on a Yeast Cake

The beauty of pitching on a yeast cake is it’s simplicity.

Step 1: Remove (rack) beer from the primary fermenter

Maybe you are bottling the beer, maybe you are transferring it to a secondary. It doesn’t matter. Just remove the beer. Ensure that nothing unsanitized touches the beer, the inside of the fermenter or the yeast cake. Once the beer is out the primary, the yeast cake is exposed. Put the lid/airlock back into the primary fermenter to seal it. Try to make sure the yeast cake is exposed to the open air as little as possible. This reduces the chance of infection.

(A French Witbier waiting to be racked out of the fermenter for bottling)

Optional Step: Remove the remaining the beer

If there is beer still remaining in your fermenter, you can dump it out.

(Yeast cake at the bottom of the primary fermenter. Don’t take pictures of your yeast cake; you have more important things to do)

Step 2: Add new beer to the fermenter

Just pour/siphon the new beer onto the yeast cake. The yeast in the yeast cake should be more than happy to start fermenting another beer (unless you’ve abused the yeast in some way, then the yeast will get back at you by making your beer taste awful)

Note: The more air and foreign material the yeast cake is exposed to, the greater the risk of infection. This does not mean an infection is bound to happen, but be aware there is a risk.

Pros:

Save money You can use one yeast packet to ferment multiple beers, saving you the cost of buying new yeast.

No need for a starter There is a very high yeast cell count in a yeast cake, which allows it to ferment nearly any beer without stressing the yeast. We often put our high gravity beers (Original Gravity higher than 1.070) on yeast cakes.

Less esters/fusels This is related to the previous comment. Since the yeast cell count is so high, the yeast does not have to reproduce as much, producing less yeasty flavours that may be undesirable depending on beer style

Simpler than yeast washing Yeast washing is another technique for reusing yeast. It requires you to remove the dead yeast cells from the living yeast cells. It is a very useful techinque. However, it is more time consuming and requires more caution to prevent potential infections. We wash our yeast if we are concerned the previous beer (if it is very dark or hoppy) will impart unwanted flavours t0 our new beer.

Ferments very quickly Since there is so much yeast in a yeast cake, there is nearly no lag time. We’ve had beers finish the fermentation stage within 24 hours. It might be necessary to use a blow-off tube because the fermentation is so vigorous



Cons:

Increased risk of infection Anytime you open up your fermenter, you are at risk of infection. However, this is a risk that can be managed. If you have good sanitation practices, this should not be a concern for you. If you have poor sanitation practices, you should probably not pitch on a yeast cake; you probably should not make beer either.



Less esters (over-pitching) Repitching on the yeast cake is not recommended if you want flavour from the yeast. It is difficult to know how many yeast cells are in your yeast cake, making over-pitching very likely. Generally, we are not concerned with over-pitching because we rarely want yeasty flavours (we brew so few Belgian/German styles).



Off flavours from dead yeast cells The yeast cake contains a lot of dead yeast cells. These cells can start to affect the flavour of your beer if you reuse the yeast too many times or you let the yeast cells sit in the fermenter too long. However, this should not be an issue if you reuse the yeast just a couple times. We’ve never had an issue with this, but we generally do not use a yeast cake more than 2 times. We recommend not keeping the same yeast cake for more than 5 weeks. Also, do not pitch onto a yeast cake that was used to ferment a high alcohol beer. High alcohol stresses the yeast, and can result in off flavours.

Time constraints A yeast cake with no beer on top of it should be used as soon as possible. We don’t like to let it sit for more than a day before pouring another beer on to it (some people say it can be kept for several days if it has a thin layer of beer to covering it, but we have never tried this).

Flavour from the previous beer There are two factors to keep in mind to prevent this problem: colour and hoppiness Do not ferment a light coloured beer on the yeast cake of a dark beer, or you will have dark flavours in your light coloured beer. Do not ferment a un-hoppy beer on the yeast cake of a hoppy beer, or you will have hoppy flavours in your un-hoppy beer.



Our position is the pros outweigh the cons (as most of the cons can be mitigated). But that is for you to judge. We don’t think this technique necessarily makes better beer (unless you really don’t want esters), but it does make more efficient beer: spending less money on yeast, and fermenting the beer more quickly. These issues may not be of primary concern to you as a homebrewer, but they are things to consider nonetheless.