Summer Fermentation: 10 Tips for Keeping Your Brew Cool

Australian legal note: Brewing beer, cider and wine at home for non-commercial use needs no licence in Australia. Distilling alcohol is different: using a still of any size to distil alcohol requires an excise manufacturer licence, and the ATO does not grant licences for personal consumption. These tips apply to any fermentation, from home brew to washes in licensed operations. See the ATO's guidance on stills for the current rules.

Pure Distilling, one of the brands we stock, has a good post called "Beating The Freeze: 5 Winter Fermentation Tips" about fermenters sulking in a cold garage. In most of Australia the bigger problem arrives in December. A 38°C day in Melbourne's west, a hot northerly, a fermenter left in the shed, and by evening your brew is bubbling like a pot on the stove. It will still make alcohol, but not the clean, well-behaved batch you were after.

Here is how we keep fermentations cool through an Aussie summer, using things most of us already have at home.

Why heat is a problem

Yeast is a living thing with a comfortable range. Push it above that range and it changes behaviour:

• Off flavours. Hot, fast ferments produce more fusel alcohols and esters. In beer that means solvent or banana notes, in a wash it means harsher flavours that are hard to remove later.
• Stressed or stalled yeast. Very high temperatures can exhaust yeast before the sugar is finished.
• Infection risk. Wild yeast and bacteria love warm, sugary liquid.
• Lost aroma. In wine and beer, a vigorous hot ferment blows delicate aromas out through the airlock.

There is a catch most people miss: fermentation makes its own heat. During the most active days, the liquid inside the fermenter can sit a few degrees warmer than the room it is in. So a room at the top of your yeast's range means a brew that is already over it.

1. Pick the coolest spot in the house

Location does most of the work. Garages and sheds can get much hotter than the house on a summer afternoon. Look for:

• An inside room on the south or east side, away from afternoon sun.
• A concrete slab floor, which often stays cooler than timber.
• A laundry, bathroom or under-stair cupboard with little temperature swing.

Keep the fermenter off a sunny windowsill. Sunlight warms the liquid and, in clear vessels, can harm beer flavour too.

2. Use a water bath

Stand the fermenter in a large tub, esky or laundry trough of water. Water has far more thermal mass than air, so it soaks up the heat of fermentation and smooths out the daily swing. Fill it to around the level of the liquid in the fermenter.

3. Add frozen bottles

Freeze a few PET bottles of water and drop two into the bath morning and evening, rotating them back into the freezer. It is a cheap, surprisingly effective way to knock a few degrees off on hot days. Do not overdo it: a sudden drop can shock the yeast. Aim for steady, not cold.

4. Try the wet towel and fan trick

Wrap a damp towel or old t-shirt around the fermenter with the bottom edge sitting in the water bath so it wicks up moisture, then point a fan at it. Evaporation pulls heat away. It works best in dry heat. On muggy, humid days it is less effective, so lean on the frozen bottles instead.

5. Choose a yeast that suits the season

Yeast strains vary a lot in how much heat they tolerate. Read the recommended temperature range on the packet before you buy, and choose a strain whose range matches your conditions. For beer, browse our Brewing Yeast range and ask in store which strains cope with warmer weather. For washes in licensed operations, see our Distilling Yeasts and the comparison in Turbo Yeast vs Distillers Yeast.

6. Start cool and start early

• Pitch cool. Bring wort, must or wash down to the lower end of the yeast's range before adding yeast. Starting hot means you will fight heat for days.
• Pitch when it is cool. Pitching on a cool evening or early morning, rather than in the afternoon heat, gives the yeast a calmer start.
• Watch the first few days. Heat output peaks in the most active phase. That is when the water bath and frozen bottles matter most.

7. Measure, do not guess

The temperature that matters is the liquid, not the room. A stick-on strip thermometer on the fermenter is a good everyday check. A stainless stem thermometer such as the Mad Millie Boxed Thermometer gives you a closer reading when you take a sample, and an infrared thermometer gun is handy for checking surface temperatures around the shed. Log your readings alongside gravity, as we suggest in Record Keeping for Distilling Runs.

8. Watch the forecast

Melbourne summers swing wildly, with a cool change one day and 40°C the next. If a heatwave is coming, it is often better to wait a few days than to pitch into it. Spring and early autumn are kinder brewing seasons, and many brewers plan their batches around them.

9. Size and shape help

Smaller batches are easier to keep cool, and fermenters that fit neatly in a tub or esky make the water bath practical. Options include the Fermzilla 30 L Fermenter Kit and the 30 Lt Brewing Fermenter with Screw Lid. Measure your tub or esky first and check the fermenter will fit with room for water around it. See our Fermenters for Stills range.

10. Clean harder in summer

Warm weather is good news for bacteria and fruit flies. Summer is not the time to skip a step.

• Clean and sanitise everything that touches the brew, including the airlock, spoon and hydrometer.
• Keep the lid and airlock on, and top up the airlock water, which evaporates faster in heat.
• Clean up spills straight away so they do not attract insects.
• Store sanitiser where it stays effective, away from direct sun.

Our Cleaners and Sanitising range has what you need.

A word on wine

Red wine ferments are often run warmer by design, to extract colour and tannin from the skins. Even so, a red sitting in a hot shed can run away and lose its fruit. Whites and rosés prefer it cooler still. Keep an eye on the must temperature as well as the gravity, and see Understanding Brix, Specific Gravity and Baumé in Winemaking to track progress.

After fermentation: storage counts too

Once your beer, wine or cider is bottled, heat keeps working against it. Store bottles somewhere cool and dark, not in the garage. Heat ages beer quickly and can push wine past its best.

Frequently asked questions

How hot is too hot for fermenting?

It depends on the yeast. Check the recommended range on the packet and keep the liquid, not the room, inside it. Remember the brew can run a few degrees warmer than the air during active fermentation.

Will frozen bottles shock my yeast?

Not if they go in the water bath rather than the brew, and you use a couple at a time. You want a gentle, steady temperature, not a sudden chill.

My brew got too hot. Is it ruined?

Usually not. It may have more fruity or harsh flavours than you wanted. Cool it down, let fermentation finish, and give it time to condition. Make a note for next batch.

Is the garage OK in summer?

Often not. Garages can get much hotter than the house on a summer afternoon. If it is your only option, a water bath with frozen bottles becomes essential.