What fermentation actually does, and how to keep yeast happy

Fermentation is the part of brewing that happens without you, which is exactly why it is the part worth understanding. You do about an hour of work at the start, then a few days later you either have beer or you have a problem, and the difference is almost always something that was decided in the first 48 hours.

Quick note before we get into it, because the two get lumped together and they are not the same. Fermenting beer, cider and mead at home is perfectly legal and needs nothing from anybody. Distilling spirits at home needs an excise manufacturer licence from the ATO, so everything below is about fermentation and nothing below is about running a still.

What the yeast is actually doing

Yeast eats sugar and gives you back alcohol and carbon dioxide, and that much everybody knows. The part that matters more is that it also produces a long list of flavour compounds on the way through, and those are what make a beer taste of banana or clove or apple or nothing much at all. Esters give you the fruity end of it, phenols give you the spicy clove end, and how much of each you get depends almost entirely on how warm the yeast was while it worked.

So the yeast is not a neutral ingredient that turns sugar into alcohol. It is closer to the main ingredient, and the same tin of extract fermented at 18 degrees and at 28 degrees will give you two noticeably different beers.

Temperature is most of the job

Most ale yeasts want somewhere around 18 to 22 degrees, and the temperature that matters is the temperature of the beer rather than the temperature of the room. Fermentation gives off its own heat, so a fermenter can easily sit a few degrees above the air around it while it is going hard.

Too warm and you get fusel alcohols, which taste hot and solventy and give you the sort of headache people blame on the homebrew rather than on the shed it fermented in. Too cold and the yeast slows right down or stops, and a stalled ferment that gets bottled anyway is how you end up with bottles that keep fermenting on the shelf.

A stick on thermometer on the side of the fermenter costs almost nothing and tells you more than anything else you can buy. If summer is the problem, a tub of water with a wet towel over the fermenter will hold it several degrees cooler without any equipment at all.

The airlock is not telling you what you think

This one catches nearly everybody. Bubbles coming through the airlock mean gas is leaving the fermenter, and that is all they mean. A lid that is not quite sealed will let the gas out somewhere else and the airlock will sit there doing nothing while the beer ferments perfectly well underneath it. A fermenter warming up through the afternoon will push gas out and bubble away even when the yeast has finished days ago.

So don't judge anything by the airlock. The only thing that tells you fermentation has finished is a hydrometer reading that hasn't moved in two or three days.

Cleaning and sanitising are two separate jobs

Cleaning takes the residue off. Sanitising kills what is left on a surface that is already clean. You cannot sanitise a dirty fermenter, because the sanitiser never reaches the thing you are worried about, and the scratches in an old plastic fermenter are exactly where that hides. That is the one real argument against buying a cheap secondhand one.

No rinse sanitiser is the easy version. Mix it, wet everything that will touch the beer after the boil, tip it out and don't rinse it off.

Three more things worth knowing

• Pitch enough yeast. A stressed, under pitched ferment throws off more of the flavours you didn't want and takes longer to finish.
• Take a reading at the start as well as the end. Without the first one the second one tells you nothing about how much alcohol you actually made.
• Write it down. Temperature, tin, sugar, dates. When a batch comes out well, that page is the only way you get it twice.

Shop related products

• Coopers Original Draught 1.7kg. A forgiving first tin with a yeast that behaves itself.
• Coopers Brew Enhancer 2 1kg. Gives you body and head retention rather than only alcohol.
• Coopers Light Malt Extract 1.5kg. For when you start building your own recipe.
• Beer Essentials No Rinse Sanitiser 25g. One sachet, no rinsing, and the cheapest insurance in brewing.

The tins and malts are in the homebrew collection, and the Coopers range is here.

Did you know we also stock the gear that goes with it in store, including 30 litre and 60 litre fermenter systems, a 2 scale hydrometer with the test jar, glass thermometers, airlocks, bench and hammer cappers and PET bottle caps. A fair bit of the homebrew range carries our supplier's branding so it doesn't go up on the website. Give us a ring on (08) 8536 4333 to check what is on the shelf.

If you want to work out a first brew before you come in, Brewhilder is our homebrew helper and it will talk you through it.

More from the brewing bench

• Homebrewing on a budget, what you actually need to start
• Brewing beer with liquid malt extract, the basics
• A brewer's guide to using hops

Still learning this one myself, and I am reliably told the hard part is leaving it alone.

Cheers,
Candeece