Harvesting and repitching your yeast

A finished fermenter holds several times more yeast than you pitched into it, and it is generally healthier and better adapted than a fresh sachet. Reusing it costs nothing and frequently ferments faster and cleaner. It also introduces contamination risk, so it is worth doing properly.

Top-cropping and bottom-cropping

Top-cropping collects yeast from the krausen during active fermentation. It gives the cleanest yeast — you are skimming the most vigorous cells, above the trub entirely. It only works with strains that form a decent head and needs an open fermenter.

Bottom-cropping collects from the cake after the beer is racked off. It works with any strain and any vessel, which is why most home brewers use it, but the cake contains trub, hop matter and dead cells alongside the good yeast.

Bottom-cropping, step by step

Rack the beer off, leaving the cake behind.
Add roughly 500 ml of cooled, boiled water to the fermenter and swirl to make a slurry.
Pour into a sanitised jar and let it settle in the fridge for a few hours. It separates into layers: dark trub at the bottom, creamy yeast in the middle, liquid on top.
Pour the creamy middle layer into a second sanitised jar, leaving the dark sediment behind.
Seal loosely — yeast keeps producing CO2 — and refrigerate.

That is yeast washing. What it really does is separate yeast from trub, not sterilise anything.

A note on acid washing: it is sometimes recommended to kill bacteria in stored slurry. It is hard on the yeast and easy to get wrong, and for home-scale reuse over a handful of generations, it is not worth it. If a slurry is contaminated, discard it.

Storage and viability

Store at 2–4 °C. Viability drops steadily:

Within one week — near enough to fully viable, pitch as-is.
Two to four weeks — noticeably reduced, pitch more, or make a starter.
Beyond two months — assume it needs a starter to be usable.
Beyond six months — discard it.

Label every jar with the strain and the date. You will not remember, and pitching an unidentified slurry into twenty-three litres is an expensive way to find out what it was.

How much to pitch

Thick slurry contains roughly 1 to 2 billion cells per millilitre. For a standard 23 litre ale needing around 200 billion cells, that is roughly 100–200 ml of thick slurry — about half a cup.

People routinely pitch an entire cake, which is several times too much. Massive overpitching produces its own problems: very little yeast growth, thin and underdeveloped flavour, and sometimes an oddly bland beer. More is not better.

Generations, and when to stop

A generation is one reuse. Commercial breweries often run ten to fifteen with careful handling. At home, five is a sensible ceiling, and three is comfortable.

Watch for these signs of drift:

Flocculation changes. Usually the yeast stops dropping clear and the beer stays hazy. Sometimes it drops early, leaving a beer that finishes sweet.
Attenuation falls. The same recipe finishes two or three points higher than it used to.
Lag time grows. Healthy pitched slurry should show activity within twelve hours. Twenty-four or more suggests declining viability.
Flavour drifts. New esters, phenols, or a sourness that was not there. That is contamination, and it is the point to stop.

Pitch order matters

Reuse works best going from lighter beers to heavier ones. Yeast from a pale ale into an IPA is fine. Yeast from an imperial stout into a pilsner is not — you carry across colour, hop matter and stressed cells that have already been through high alcohol.

Never reuse yeast from a soured or Brett beer for a clean one. Those organisms live in scratches and gaskets, and you will not get them out.

When to just buy a sachet

Reusing yeast makes sense when you brew regularly — every two or three weeks — so slurry gets used while fresh. If you brew every couple of months, viability will have fallen far enough that you need a starter to make it work, and at that point a fresh sachet is cheaper than the time and the risk.

It is a technique that rewards frequency. If you are brewing often, it is free yeast that performs better than what you bought. If you are not, it is a fiddly way to save a few dollars.