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Same lesson, with the figures in motion.

Harvesting & Washing Yeast

The yeast cake at the bottom of the fermenter holds several times more yeast than you pitched — free raw material for your next brew. This protocol shows three routes: direct harvest (simplest and gentlest), rinsing when the cake is loaded with trub and hop debris, and top cropping mid-fermentation for British top-fermenting yeast. All three end in a jar of slurry you measure into the pitch calculator.

A word on words first: what homebrewers call "washing" yeast is really rinsing it with water. The acid wash breweries use is something else entirely — see the last section.

Materials

  • 2–4 glass jars with lids (about 0.4–0.7 L, pint or 1½-pint), e.g. mason or jam jars
  • Sanitiser (Star San or similar)
  • Funnel
  • Ladle or spoon
  • Pre-boiled, cooled water, ~1 L / 1 qt (Route B only)
  • Labels + permanent marker
  • Fridge space (1–4 °C)
  • Optional: microscope + counting chamber to verify the cell count

Before you harvest — is this yeast worth keeping?

  1. Only harvest from a healthy fermentation: it reached final gravity in reasonable time and smells and tastes clean. Sour, vinegary or nail-polish aromas = bin the cake.
  2. Avoid yeast from beers above roughly 8 % ABV — it is badly stressed and low in viability. For a strong beer, harvest from a lighter one instead: crop your blond and pitch it into the tripel.
  3. Harvest within a couple of days of fermentation finishing. Cold crashing first is fine (it compacts the cake); it's the weeks after that, sitting in finished beer, that wear the yeast down.
  4. Heavily dry-hopped beers leave a cake full of hop debris — use Route B for those, or harvest from a cleaner batch instead.
  5. Keep count of the generation. 5–10 reuses is a common homebrew limit; retire the yeast when fermentations start dragging or the character shifts.
Everything that touches the yeast must be sanitised — but this is clean work, not sterile. Work quickly, keep lids on the jars and never touch the inside of a jar, lid or ladle.

Route A — direct harvest (recommended)

  1. Sanitise jars, lids, funnel and ladle. Let them drip dry — don't rinse the sanitiser off with tap water.
  2. Rack the beer off but leave 100–200 ml (about ½–¾ cup) behind on top of the cake. The liquid protects the yeast from air and makes the cake pourable.
  3. Swirl the fermenter until the cake is an even slurry. For the cleanest yeast: leave the coarsest bottom layer behind and take the creamy layer above it.
  4. Pour into the jars through the funnel. Fill them two-thirds at most — the yeast keeps giving off CO₂.
  5. Label every jar with strain, harvest date, generation and the beer's OG. The date isn't decoration — the pitch calculator derives viability from it.
  6. Put the jars straight into the fridge (1–4 °C). After a day the yeast has packed down and you can see how much slurry you actually have.
Don't screw the lid down tight for the first few days — the yeast gives off CO₂ and the pressure can burst the jar. Keep the lid loose, or burp the jar daily until the bubbling stops.
One jar per planned brew is handier than one big one: you avoid opening the same jar over and over.

Route B — rinse off the trub (when the cake is dirty)

  1. Boil ~1 litre (1 qt) of water for 15 minutes with the lid on. Cool it to roughly the same temperature as the beer — hot water kills the yeast, ice-cold water shocks it.
  2. Rack the beer off, pour the water onto the cake, put the lid back on and swirl until everything is an even, muddy soup.
  3. Let it stand 15–30 min. Heavy trub and hop debris settle first; the yeast stays suspended in the milky liquid above.
  4. Carefully decant the milky liquid into your jars — stop the instant the dark layer from the bottom tries to follow.
  5. Chill 12–24 h. The yeast packs into a creamy layer; then pour off the water that has cleared on top.
  6. Label and refrigerate as in Route A.
Rinsing costs some viability (osmotic stress + oxygen pickup) and adds a quarter of an hour. If the cake is already clean, skip it and use Route A.
Want it cleaner still? Repeat steps 3–4 once more into a fresh jar. Each round gives cleaner yeast but a smaller yield.

Route C — top cropping (for yeast that rises)

  1. This only works with strains that ride up in the krausen — the classic British ones such as Wyeast 1469 West Yorkshire and 1968 London ESB. Dry yeast and most modern strains drop out and leave too little at the top. That includes Kölsch yeast: it is top-fermenting but builds no thick yeast head, so harvest it from the bottom instead (Route A). You also need a fermenter you can get into from above: a bucket, not a carboy.
  2. Sanitise a ladle or stainless spoon and a jar. Top cropping happens mid-fermentation, so the beer is open while you work — have everything ready before the lid comes off.
  3. Day 1–2: when the first krausen has formed, skim it off and throw it away. It is brown and full of hop resin, proteins and dead yeast.
  4. Day 2–3, at high krausen: the new, pale, creamy layer is yeast. Skim it gently into the jar. Take only the top layer and leave plenty of yeast behind so the fermentation can finish.
  5. Put the lid back on straight away. Label the jar as in Route A and put it in the fridge. After a day the foam has settled, and you judge the thickness as usual.
Why bother? A top crop is cleaner than the bottom cake: free of trub and hop debris, and it is the yeast that is healthiest and most active right then. It also selects itself — generation after generation you harvest the cells that float, and the strain keeps its top-fermenting character.
Every time the lid is off, something can fall in. Work quickly with clean hands, and put the lid back as soon as you are done. Only top crop from a fermentation that is properly under way, so the yeast has the upper hand.

Storage, shelf life & pitching

  1. Fridge, dark, 1–4 °C. Use within 1–2 weeks if you can; up to about a month works, but with noticeably lower viability.
  2. The app's rule of thumb is that about 20 % of the living cells are lost per month, counted from the harvest date (≈ 80 % after 30 days, about half after three months). It's an estimate — count the cells when the brew matters.
  3. Judge the thickness once the yeast has packed: thin ≈ 1.0 billion cells/mL (dirty dishwater), medium ≈ 2.0 (pancake batter), thick ≈ 3.0 (clay). Not sure? Pick thinner — slightly too much slurry beats too little.
  4. Bin the jar if it smells sour or vinegary, has a film on the surface, or if the yeast doesn't get going within ~36 h of pitching.
  5. Work out how many mL to pitch →
  6. ▦ Cell Counting protocol — verify count & viability →
Take the jar out 1–3 hours before pitching and pour off the liquid on top. Yeast at the right temperature going into wort at the right temperature means a shorter lag phase.

About real acid washing (advanced)

  1. What breweries call acid washing is not this: the yeast is taken down to pH 2.0–2.2 with dilute phosphoric acid, held cold (0–4 °C) for no more than 60–90 min and pitched straight afterwards. The point is to knock back bacteria.
  2. It's not a beginner's step. It needs a pH meter, only works on healthy yeast (stressed yeast is damaged by it) and does nothing about wild yeast such as Brettanomyces.
  3. If you suspect the cake is infected, the right move is to bin it and pick up a clean colony on a plate instead.
  4. From a Single Colony (CFU) →
Phosphoric acid is corrosive. Goggles on, always add acid to water (never the other way round) and mix where there's ventilation.
Next: Main­tain­ing a Yeast Li­brary →
See also: Sterile Tech­nique & Sani­ta­tion →
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