Yeast harvesting

What is actually happening

A fermentation produces several times the yeast it started with, so a brewery that discards it is throwing away its next pitch. Most breweries crop and repitch, and a house strain maintained this way is one of the few genuinely proprietary things a brewery owns.

Where the yeast is taken from matters. Top-cropping skims the healthy, flocculent population from the surface of an open or lightly covered fermenter; bottom-cropping takes it from the cone, where the first material out is trub and dead cells rather than yeast worth keeping.

Each generation drifts. Selection pressure favours whatever flocculates and survives cropping, mutations accumulate, and attenuation and ester production shift with them — slowly enough to be invisible batch to batch and unmistakable over twenty.

So the practice is bounded rather than indefinite: breweries set a generation limit and return to a fresh propagation, and the limit is a judgement about how much drift the beer can carry.

What goes wrong here

Risks rather than certainties. This step does not cause these faults; getting it wrong can.

• Yeast autolysis — Dead yeast breaking down and releasing rubbery, meaty compounds — the result of leaving beer too long on a yeast bed.
• Stalled fermentation — Fermentation stopping well short of terminal gravity — leaving a sweet, underdeveloped beer and a package-pressure risk.

Related steps and concepts

• Pitching — What the harvest supplies.
• Primary fermentation — Where the crop comes from.
• Open fermentation — The vessel that makes top cropping straightforward.