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If you have moved beyond brewing basics and you are now using or considering using better quality yeast, whether it be dry yeast like SAFALE (http://www.fermentis.com) or liquid yeasts such as Wyeast ("http://www.wyeastlab.com"), you'll probably start to consider the cost of your yeast amongst the other costs of brewing but using the leftover yeast cake has another benefit in that you are putting your new beer on to yeast that is already fully active, so fermentation will start much sooner, and in my experience often is finished much earlier.

There are several methods of re-using your yeast even propagating it to be able to re-use indefinitely, but for this article I'll start with the simplest method.

Providing you take all of the normal precautions with sanitisation when brewing and a brew has not sat too long in the fermentor (such as for a barley wine, or similar heavy brew which takes a long time to ferment fully), you can put a new brew straight onto the yeast cake left in the bottom of the fermentor.

While there is a lot of dead yeast in this residue, there is a huge amount of active live yeast there, and rather than going to the effort of yeast washing (I'll go over this one in another article) you can make up your next brew, and after bottling or kegging your beer and providing the temperature is low enough you can 'drop' your next brew straight into the fermentor.

A couple of things you need to consider

You must be prepared

I strongly recommend you do not leave a yeast cake in a fermentor for an extended period (1-4 hours is OK), as when it is exposed to the air with no or only a limited protective CO2 layer, the risk of infection if quite high.

Do not put hot wort or hot water (over 35degrees C or 95degrees F) onto the yeast

To get around this, do not to make up the entire wort if you are doing a boil, or dissolve you fermetables in 6-7litres (1.5 gallons) of hot water, then fill you fermentor with the extra water required for the brew (even helps if you can have this water a little cooler if possible) then you can just pour the hot wort into this and it will save you yeast for being destroyed by the high temperatures. I often try cool my wort before pouring into the fermentor as well, as I'll often add aroma hops at the end of the boil and I'll leave them in the brew as it cools a little.

Watch out if you have a very heavy style of beer

There is a lot more active yeast in there than you normally have in dry yeast packets or liquid yeast packs, the beer will start fermenting actively very quickly and if it is on the heavy side it may well foam excessively and try to 'crawl out' of the fermentor!!

I have heard of brewers doing this more than once, but I've never tried to re-use the yeast cake more than once, due to concerns about the health of the yeast, the dead yeast breaking down introducing off flavors, and potential infections getting into the yeast.

I hope this has been of some interest of help to you, I'll be writing more on re-using yeast sometime soon

Cheers