← Soul of Beer
Open the protocol in the app →

Same lesson, with the figures in motion.

Glycerol Freezer Bank

A jar of slurry keeps a couple of weeks in the fridge. A slant keeps longer — but it should be re-cultured after about three months: the yeast goes on living slowly in the cold, and the longer it sits the less certain you can be that the strain is still the same one (genetic drift).

The freezer is the answer when you want to keep a strain for years. The cold stops metabolism — a frozen culture doesn't drift, it stands still. But it needs cryoprotection: plain yeast that freezes is torn apart by ice crystals, whereas glycerol creeps into the cells and makes the ice harmless. The result is a small rack of vials where each one is a ready starting point for a new culture.

Treat the bank as an investment: you spend one evening freezing, then draw one strain at a time from it for years.

Materials

  • Glycerol 99.5 % (pharmacy or food grade) — 50 ml lasts a very long time
  • Screw-cap freezer vials (cryovials 1.5–2 ml), 8–10 per strain
  • Glass bottle with a lid for the glycerol solution (100–250 ml)
  • Pressure cooker
  • Precision scale or measuring cylinder
  • A healthy, young yeast culture (finished starter, slant or colony)
  • Sanitiser
  • Labels + a marker that survives the cold
  • A freezer holding −18 to −20 °C, preferably without auto-defrost

Before you freeze — which culture?

  1. Freeze a young and healthy culture: a fresh starter that has just finished fermenting, or a culture grown up from a slant or colony. It's yeast in peak condition you want to save.
  2. Do not freeze harvested yeast cake from a finished batch. It's stressed and full of trub — and you'd be stuck with that bank for years.
  3. Cold crash the starter, pour off the wort and keep the thick slurry at the bottom — that's what gets mixed with glycerol.
  4. Plan the number of vials: one vial = one wake-up. A thawed vial is never refrozen. 8–10 vials per strain lasts many years.
This is the job where sterile technique pays off most. A contamination that gets frozen in follows every future culture grown from that vial.

Step 1 — Make the glycerol solution (40 %)

  1. Mix 40 ml glycerol (50 g) + 60 ml water in the glass bottle = a 40 % glycerol solution. Lid on loosely.
  2. Pressure cook for 15 min (121 °C). Let it cool to room temperature with the lid still on.
  3. No pressure cooker? Boil the bottle in a water bath for 20 min with the lid loose. It's a weaker guarantee — use the pressure cooker if you have one.
  4. The solution keeps for months in the fridge, so make the whole batch at once.
Glycerol is viscous and clings to the measuring glass. Weigh rather than measure by volume: 1 ml glycerol ≈ 1.26 g.
Let the pressure cooker cool down on its own before you open it — never quench it with cold water.

Step 2 — Mix and fill the vials

  1. Sterilise the vials (pressure cook them empty, caps loosely on) or use ready-made sterile cryovials. Let them cool.
  2. Mix equal parts thick slurry and 40 % solution — that puts you at roughly 20 % glycerol in the vial, the standard range for a freezer bank.
  3. Fill the vials two-thirds at most. Liquid expands as it freezes and can burst a full vial.
  4. Invert the vials a few times and let them stand 15–30 min at room temperature. The glycerol needs that time to work its way into the cells — skip the rest and you're freezing unprotected cells.
  5. Label every vial with strain, date, generation and source (e.g. "US-05, slant, gen 2"). Write small but legibly — the text has to survive frost.
Keep a log of the bank: strain, date, vials left. A vial taken out gets struck off the list straight away.

Step 3 — Freeze and store

  1. Put the vials in the freezer at −18 to −20 °C. Leave them undisturbed — no special cooling technique is needed at home scale.
  2. Avoid frost-free freezers. Their defrost cycle lets the temperature swing up and down, and every swing builds fresh ice crystals in the vials. A manual chest freezer is far better. If a frost-free freezer is all you have: put the vials in an insulated box, a block of polystyrene or a small cool bag, together with a few ice packs, at the back of the freezer. The mass around the vials takes the edge off the swings.
  3. Store the vials at the back and the bottom, where the temperature is most stable — not in the door.
  4. Expect several years of shelf life at −18/−20 °C. The lab's −80 °C is another league (decades), but a home freezer is plenty for a house bank. Still, don't take it on trust: thaw a test vial once a year and count the cells. Then you know the bank is alive instead of hoping. Pro Cell Counting — read more →
Glycerol vials look like food but aren't. Label the box clearly so nobody else in the household grabs them.

Step 4 — Wake a strain

  1. Take out one vial and close the freezer immediately — the others mustn't start thawing at the edges.
  2. Thaw the vial quickly in your hand or in lukewarm water, just until the contents turn liquid. Don't leave it out to get warm.
  3. Pour the whole contents into a small sterile starter, ~100 ml of weak wort (about 1.020–1.030), at room temperature.
  4. Be patient: a frozen culture may need 24–72 hours before anything shows. Don't write the vial off too early.
  5. Once it's going: step it up to a pitchable amount following the starter protocol.
  6. Building a Yeast Culture — step up to pitchable →
  7. Work out the step-up in the pitch calculator →
Top the bank up while you have a healthy culture going anyway: freeze fresh vials of the same strain and write a new generation on the label.
A thawed vial is never refrozen. Freeze–thaw cycles kill yeast faster than anything else in this protocol.
Next: Main­tain­ing a Yeast Li­brary →
See also: Sterile Tech­nique & Sani­ta­tion →
The Kitchen Lab