Since yeast is a living organism and is not used up in the process of fermentation, it can be collected and reused in multiple brews.
Over the next two weeks we will talk about how the home brewer can harvest yeast by top cropping for ale yeasts and bottom cropping for lager yeast. Commercial brewers have used these techniques for hundreds of years to save costs of a commercial pitch for every brew and for the benefits of using already active yeast in the next fermentation. However there are a few drawbacks to harvesting yeast:
- Sanitation practices must by very high as to not introduce wild yeast and bacterial into the next fermentation.
- Depending on the method of harvesting, selective pressures may come into effect which can affect attenuation, flavour profile and clarity of the final beer.
- Fermentation is hard on the yeast, as a result it is recommended that yeast not be reused for more than 5-10 generations, however there are some exceptions.
Ale strains are also known as top-fermenting yeast due to the hydrophobic surface of the yeast adhering to the CO2 being produced rising to the top of the fermenter. By day 2-3 of fermentation, top cropping yeast will have risen to the top of the fermenter and is ready for collecting. To top crop yeast from an open top fermenter you will need:
- Large stainless steel spoon
- Plastic or glass storage bottle around 250 ml (8 US fl oz)
- Filtered water 100 ml (3 US fl oz) at fermentation temperature
If you have a fermenter with restricted openings like glass or plastic carboy you will need to device a method of vacuuming the yeast from the surface of the fermenting beer.
- On the second or third day of fermentation sanitise all equipment.
- Partially open the lid of the fermenter as to shield the beer as much as possible from the environment.
- Using the stainless steel spoon take a single scoop off the top of the beer and discard this into another area of the surface of the beer. This scoop will be mainly composed of the protein.
- From the same location taking a second and third scoop from just below the surface is usually the best yeast to collect and transfer to your glass jar.
- Add your water so that you have about 50-150 ml (1.5-4.5 US fl oz) of ‘yeast slurry’.
- Loosely seal the jar with the lid or use sanitised aluminium foil store in the fridge.
- Ensure your sample is labelled as clearly as possible and that the jar is vented but not opened.