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Building a Yeast Culture — Slant to Pitch-Ready Culture (20 L)

Builds yeast up step by step from a slant to ~450–500 billion cells — enough to pitch a strong 20-litre (5 US gal) wort (>1.075 SG). We do it in three steps at ~10–50× growth per step: growing from so few cells in one big leap stresses the yeast and gives an unreliable cell count. The quantities are a proven starting point to adapt to your own equipment.

Materials

  • Erlenmeyer flask, 1000 ml
  • Erlenmeyer flask, 5000 ml
  • Sanitised container with lid (e.g. glass jar) — to hold the step-1 culture while you clean the 1000 ml flask
  • Hot plate
  • Magnetic stirrer + stir bar
  • Light dry malt extract (DME / spray malt)
  • Yeast nutrient
  • Aluminium foil
  • Tap water
  • Kitchen scale (ideally 0.1 g resolution for the nutrient)
  • Thermometer
  • Sanitiser (e.g. StarSan) for surfaces, flask necks and foil
  • Tube with a well-grown, fresh streaked strain (slant)

Before you start — rules throughout

  1. Sanitise all surfaces, flask necks and foil before every step.
  2. Drop the stir bar into the flask before boiling, so it's boiled clean at the same time.
  3. Always cool the wort to ~20 °C (check with the thermometer) before adding yeast — wort that's too hot damages it.
  4. Run each step on the stirrer until the krausen has risen and fallen (~24–48 h) before moving on.
Boiling wort causes burns. Never cover airtight — steam must escape, or the pressure can shatter the glass.

Step 1 — Wake the culture (~100 ml)

  1. Weigh 15 g DME into the 1000 ml flask and top up with water to the 150 ml mark (≈ 1.036 SG). A little boils off, and that's fine. Drop in the stir bar and cover loosely with sanitised foil.
  2. Bring to a boil and boil for 15–20 min.
  3. Cool in a cold water bath to ~20 °C and set on the stirrer.
  4. Aseptic transfer: flame the mouth of the tube, pour in a little of the cooled wort, swirl the cells loose and pour back into the flask. Work quickly in still air.
  5. Let it run 24–48 h until activity tapers off. (~0.1 → ~3–5 billion cells.)
The whole point of a slant is a PURE culture — a sloppy transfer contaminates the strain. Flame the tube mouth, work fast and near a flame or in still air.

Step 2 — Build up (~500 ml)

  1. Pour the step-1 culture into the sanitised container (lid on) and clean the 1000 ml flask.
  2. Weigh 50 g DME into the flask and top up with water to 500 ml (≈ 1.036 SG). Drop in the stir bar and cover with foil.
  3. Boil 15–20 min, cool to ~20 °C and set on the stirrer.
  4. Pour the whole step-1 culture into the flask and let it run 24–48 h. (~40–50 billion cells.)
In a hurry? You can skip this step and go straight from 100 ml to the final culture — but you won't reach the cell target. 100 ml holds ~3–5 billion cells, and the final step would then have to multiply them tens of times, where the other steps go ~10×.

Step 3 — Final culture (~3 L / 3 qt)

  1. Weigh 300 g DME into the clean 5000 ml flask, top up with water to 3000 ml and add 0.3 g yeast nutrient (≈ 1.036 SG). Drop in the stir bar and cover with foil.
  2. Boil 15–20 min, cool to ~20 °C and set on the stirrer.
  3. Pour the whole step-2 culture (from the 1000 ml flask) straight into the 5000 ml flask and let it ferment out 24–48 h. (~450–500 billion cells.)
No holding container needed here — the 5000 ml flask is free, so you make the wort straight in it and pour the step-2 culture right over. One contamination step fewer.

Before pitching

  1. Cold-crash the final culture in the fridge for 12–24 h so the yeast settles to the bottom.
  2. Decant most of the spent wort and pitch the slurry.
  3. Allow ~4–6 days total (~2 days per step + cold crash).
Pitching the whole culture means ~3 L (3 qt) of old starter beer in your 20 L (5 US gal) batch (~15 %) — decant and you avoid it, keeping the beer clean.

Assumptions & cell counts

  1. Starting point: rough estimate ~100 million viable cells on a well-grown slant (can vary a lot).
  2. Wort strength ~1.036 SG in every step (100 g DME/L).
  3. ~10–50× growth per step, final density ~100–150 billion/L (realistic for a stir plate).
  4. The pitch calculator reckons a starter from a pack at a cautious 100 billion/L – a pack's viability is uncertain. This protocol's higher figure comes from a fresh culture built up step by step.
  5. After step 1: ~3–5 bn · after step 2: ~40–50 bn · after step 3: ~450–500 bn cells.
  6. The ~450–500 bn target suits a lager or strong ale (>1.075 SG); a normal ale needs less.
Next: Cell Counting (mi­cro­scope) →
See also: Sterile Tech­nique & Sani­ta­tion →
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