Beer yeast, what it does and how to choose one

February 25, 2025

Ask a homebrewer what actually makes the beer and most of them will point at the yeast rather than the malt or the hops, because the yeast is the bit doing the work while you're off doing something else, and it's the bit that decides what the finished beer tastes like.

What beer yeast actually does

Yeast is a single celled fungus and its whole job is to eat the sugars in your wort and turn them into alcohol and carbon dioxide, so without it you've just got sweet unfermented liquid. It doesn't stop at the alcohol either, because the by-products it throws off along the way are what give you the fruity notes in a pale ale or that clove and banana thing in a wheat beer.

Ale yeast or lager yeast

There are two families and the difference between them matters more than anything else you'll pick.

Ale yeast, Saccharomyces cerevisiae, works warm and ferments up the top, and it tends to give you fruitier, rounder beers, which is why it sits under most pale ales and stouts.
Lager yeast, Saccharomyces pastorianus, works cool and ferments down the bottom, and it gives you the cleaner, crisper finish you want in a pilsner or a lager.

Every strain has its own temperature range and it's printed on the sachet, so read the pack rather than a number you found on the internet, and then hold it steady, because a fermenter swinging around with the weather will do you more damage than being slightly off the mark.

Dry yeast or liquid yeast

Dry yeast keeps for ages and it's cheaper, and you either pitch it straight in or rehydrate it first depending on what the pack tells you, so it's the sensible place to start. Liquid yeast comes in a much wider spread of strains and it's what you move to when you're chasing one particular character, but it's perishable and it wants looking after. I'd start dry and stay dry until you've got a real reason not to.

Where the flavour comes from

Brewers talk about esters and phenols and the words sound far more technical than they are. Esters are the fruity ones, so pear, apple, banana, bubblegum, whatever the strain leans toward. Phenols are the spicy, clove-like ones you find in a lot of Belgian and wheat beers. Fermentation temperature is the main dial on both, because a warmer ferment pushes the esters up and a cooler one keeps things clean, and how much yeast you pitch and how well you aerate the wort beforehand both feed into it as well.

The mistakes that catch people out

Not pitching enough yeast, which gives you a sluggish start and off flavours, and the fix is just to follow what's on the packet instead of guessing.
Letting the fermenter swing around with the weather, which is honestly the commonest reason a beer tastes wrong for no obvious reason. A thermometer is worth having, because it tells you what's actually going on in there.
Not giving the yeast any oxygen, because it needs some early on to reproduce, so give the wort a decent splash and a stir before the yeast goes in.
Pitching a sachet that's been sitting in a hot shed for a year. Yeast is alive, the date on the pack means something, and a tired sachet is a slow start you'll end up blaming on everything else.

Reusing yeast

Once you're brewing regularly you'll notice the yeast bill adding up, and you can harvest it off the bottom of the fermenter, keep it in a sterilised jar in the fridge under a bit of fresh beer, and wake it up in a starter before it goes into the next batch. It's fiddly and it isn't for everyone, but it gives you more control over how the next one turns out.

Shop related products

S-04 Safale ale yeast 11.5g. A dry ale yeast that drops clear and suits English style ales.
US-05 Safale ale yeast 11.5g. The clean neutral ale yeast, good when you want the hops out front.
S23 Saflager lager yeast. A dry lager yeast for when you're brewing cool.
Coopers English Bitter 1.7kg. A kit with its own yeast under the lid, which is the easiest way to start.

Did you know we also stock cider, wine and mead yeasts in the shop, including M02 cider yeast, Safcider AB-1, R56 red wine yeast, M05 mead yeast and EC-1118. They're not on the website, so give us a ring on (08) 8536 4333 to check what's on the shelf.

There's plenty more brewing gear in our homebrew range, and if you're not sure which yeast suits the beer you've got in mind, have a yak with Brewhilder, our homebrew adviser.