Yeast Starters 101

Same lesson, with the figures in motion.

Why make a starter?

A pack or vial of liquid yeast holds roughly 100 billion cells. A normal 20 litre (5 US gal) ale wants around 200 billion, a lager about twice that — and yeast loses cells for every month it has sat. Pitch too little and you get a sluggish start, more esters and fusels than you intended, and a fermentation that can stall outright.

A starter is a small batch of wort where the yeast multiplies before it meets your beer. It takes a day or two and costs a handful of dry malt extract.

1. Clean the Flask

Scrub the flask clean with hot water and a bottle brush — dried yeast and protein residue must go. Rinse until the water runs clear. No sanitiser needed: the wort is boiled in this same flask for 15–20 minutes, and that boils it clean along the way.

2. Weigh the Wort

Weigh out 100 g of light malt extract per litre of water. That gives a wort of about 1.036 SG — strong enough to feed the yeast, weak enough not to stress it. After the boil you will have a little under a litre, which is fine. Never fill the flask beyond about three fifths (3 L in a 5 L flask); the krausen needs room above the liquid.

3. Boil & Chill

Drop the stir bar into the wort before you turn on the heat, and it gets boiled clean along with everything else. Cover the mouth with a generous piece of aluminium foil and press it down around the sides of the neck — it stays on through the whole boil and chill, keeping dust and spores out without sealing the steam in.

Boil gently for 15–20 minutes — that kills bacteria and wild yeast (not every spore, so the boil isn't sterile, but it's clean enough for a starter). Keep an eye on it, DME boils over fast. A drop of Fermcap-S or cooking oil keeps the foam down. Then chill to room temperature, ideally in an ice bath, before you pitch.

4. Pitch & Spin

Lift off the foil, pitch the yeast and lay it back loosely over the mouth. Put the flask on the stir plate — if you boiled without the stir bar in the wort, sanitise it with Star San before dropping it in now.

The vortex pulls air in continuously throughout the run, and that is oxygenation enough — no extra whisking before pitching. Without a stir plate, swirling the flask by hand a few times a day is enough.

5. Read the Culture

Expect 24–48 hours. A thick, pale krausen and a clear smell of yeast and bread mean it is fermenting hard. Let it run one more day after the krausen falls — the yeast then eats most of the remaining sugar and builds up energy reserves for the pitch.

Chill the flask for a day before brew day and decant the clear wort off the top — you pitch only the compact yeast bed at the bottom, not the whole starter.

YeastMaster