Liquid yeast that arrived warm: what to do before you pitch

What warmth does to a yeast pack

A liquid yeast pack or vial is a suspension of living cells, kept cold so they stay dormant and use up as little of their reserves as possible. When a pack travels warm, the cells wake up and burn through glycogen and other reserves. Some die. The survivors are weaker and slower. They carry more risk of a long lag phase, which is the quiet period before fermentation starts.

How much damage depends on how hot, how long and how old the pack was to begin with. A few hours at 25 to 30 °C (77 to 86 °F) in a ventilated box is a very different event from three days in a metal van. We cannot give you a single viability figure for a warm pack. Anyone who does is guessing. What you can do is measure the result yourself.

Read the pack before you open it

Start with the printed date. Many packs are best used within a few months of packaging even when kept cold, so a warm and old pack is a double risk. Then look at the pack itself.

• A pack that has swollen hard has been fermenting or producing gas. That is not always a disaster, but it tells you the cells have been active.
• A vial that has leaked or smells sour or sulphurous should go in the bin.
• A pack that feels cool and looks like a normal creamy slurry is promising, whatever the courier tracking said.

Open it with sanitised scissors, over a sanitised container and smell it. Healthy yeast smells bready, a little fruity or neutral.

The starter is your viability test

A starter is a small batch of low-gravity wort that lets a small or weakened culture multiply before you pitch. It doubles as a test. For a 19 L (5 US gal) batch, make about 1 L of wort at roughly 1.036 to 1.040 specific gravity. Dissolving about 100 g of dry malt extract in 1 L of water gets you there. Boil for 10 minutes, cool to 20 to 25 °C (68 to 77 °F) in a sanitised jar and add the yeast.

Aerate by shaking hard for a minute. Cover loosely with sanitised foil or an airlock so gas can leave. Keep it at room temperature, out of direct sun. A stir plate helps if you own one, but regular shaking works.

Watch for signs of life in 12 to 24 hours: a foam cap, a thin layer of bubbles on the surface or a pale layer of fresh yeast settling on the bottom. Strong activity means you can step up or pitch. Slight activity means you may need a second, larger step. No activity after 24 to 36 hours means the culture is dead or close to it.

How many cells you actually need

The commonly used pitch rate is about 0.75 million cells per millilitre per degree Plato for ales and about 1.5 million for lagers. A 19 L batch of a 12 °P (1.048) ale therefore needs roughly 170 billion cells. A fresh pack holds fewer than that, which is why brewers make starters even with healthy yeast. A pack that has had a rough trip holds fewer still.

Under-pitching shows up as a long lag and as stress flavours, such as esters you did not plan for. It also gives bacteria a better chance of getting established first. Pitching a good starter costs you a day. Pitching a weak pack straight into wort can cost you the batch. If you want to be exact, a cell count under a microscope with a stain is the proper method. Most homebrewers will not have the equipment. A starter is a fair stand-in.

When to pitch, when to stop and use dry yeast

Pitch the starter at high krausen, when it is most active, or crash it in the fridge for a day and decant the liquid so you pitch a thick slurry rather than a sour-tasting jar of old beer. Either works. Match the temperature of the starter to the wort within a few degrees, so you do not shock the cells.

If the starter does nothing, do not pitch it. Keep a packet of dry yeast on the shelf for this exact situation. Dry yeast tolerates heat and storage far better and rehydrates in warm boiled water in about 15 minutes. It will not give you the exact strain you ordered, but it will give you beer. A clean, simple ale made with a reliable dry strain beats a half-fermented batch made with the strain you wanted.

If you brew in warm weather generally, our guide to homebrewing in a hot climate covers how to keep fermentation in range once the yeast is going.

Stop it happening again

Ask the seller how they ship in summer. Many suppliers use insulated boxes with ice packs. Some will not ship liquid yeast in hot weather at all. Choose a delivery day when someone is home to take the box in, rather than letting it sit at a door. Open it the moment it arrives and refrigerate. If you live where parcels often travel for days in hot conditions, buy dry yeast as your standard and treat liquid yeast as an occasional order for a special beer.

Dry yeast, a good starter habit and a clean sanitation routine remove most of the risk. Our guide to importing ingredients covers the wider question of how long supply chains affect what you buy.

Common questions

Can I still use liquid yeast that was warm for two days?

Possibly. Make a starter and watch it for 12 to 24 hours. If it shows clear signs of fermentation, it is alive enough to use. If it shows none, it is not worth the risk.

Should I smell or taste the pack?

Smell it. A bready, fruity or neutral smell is normal. A strong sulphur, vinegar or rotten smell means bacteria or dead yeast. You should discard it.

Is dry yeast a safe backup?

Yes. Dry yeast survives heat and storage much better than liquid and rehydrating it in warm boiled water takes about 15 minutes. Keep a packet in the fridge in case a pack fails.

Can I wash and reuse yeast from a warm pack?

Not from a stressed pack. Weak yeast gets weaker with every generation. Start with healthy, tested yeast before you harvest anything.