What Happens If You Ferment Beer Too Warm?

Temperature control during fermentation is one of the most critical factors in home brewing and significantly impacts what your beer tastes like.

Yeast has a temperature range where it works best. At the higher end of the temperature range fermentation is more vigorous. If the yeast gets too cold, it may stop fermenting all together.

When beer ferments at too high a temperature, the yeast creates more esters leading to fruity off flavours, similar to overripe bananas. Higher temperatures also produce more fusel alcohols which can make your beer taste of paint thinners.

In this article, we take a look at what happens if you ferment beer too warm and how to prevent it from happening in the first place.

Photo of an electronic thermometer used for measuring the temperature of wort when homebrewing
Electronic thermometer

What temperature should beer ferment at?

The correct temperature for fermenting beer depends on the type of yeast you are using, which in turn depends on the style of beer you are brewing. Each strain of yeast has an ideal temperature range, and you should keep the fermenter within this range throughout fermentation.

Fermentation temperature is especially critical during the first forty-eight hours since this is when fermentation is most vigorous, and the yeast is most active. You should also bear in mind that fermentation is an exothermic reaction, which means that the wort will usually be a few degrees warmer than its surroundings.

What is the correct temperature for yeast?

The ideal temperature for your yeast should be printed on the packet, if not you will be able to find it on the manufacturer’s website. To save time, we’ve compiled a list of the most popular yeasts used by homebrewers. See the table at the end of this post.

What happens if you ferment beer too warm?

At higher temperatures, yeast grows more quickly, and fermentation is more vigorous. This causes the yeast to create by-products which spoil the flavour of the beer. When yeast grows too quickly, it also depletes the number of nutrients present in the wort, which also stresses the yeast, causing it to generate even more unwanted by-products.

The most common by-products generated when you ferment beer too warm are esters and fusel alcohols.

Esters give the beer a fruity taste, usually described as being similar to bananas, although apples, pears and raisins are also frequently mentioned. Although some of these fruity flavours are expected in French Saisons and some Belgian beers, they are inappropriate for most ales and lagers.

Fusel alcohols are alcohols which contain two or more carbon atoms and are sometimes referred to by homebrewers as heavy alcohols because of their higher molecular mass.

Although fusel alcohols are always present in beer and wine, they impart a very harsh alcohol taste in larger quantities. Most homebrewers compare the smell of fusel alcohols to nail polish remover or paint thinners. Fusel alcohols are also believed to be responsible for hangovers, although there is some debate about this.

Can you fix beer if it’s fermented at too high a temperature?

Accidents happen, and letting the fermenter get too warm is a pretty common mistake. Many homebrewers leave the fermenter in a spare room or cupboard without temperature control, and if there’s a sudden heatwave, the beer can soon get too hot.

If this happens, don’t panic, there’s no need to dump the batch at this stage. If you notice that the temperature is rising, you might have time to rig up one of the solutions described below.

If not, just let things run their course and bottle or keg the beer as usual. It may not be the best beer that you’ve ever brewed and it probably won’t taste as you expected, but it might not be too bad either.

After fermentation has completed, if you leave the beer on the yeast for a week or two, it will generally clean up some of the esters and fusels. Dry hopping will also help cover up unwanted fruity flavours.

Bottle conditioning also reduces the harsh alcohol smell considerably. I still remember the first time I home-brewed mead. When I racked the mead out of the fermenter, it smelt more like cheap tequila than wine. A month after bottling, the harshness had disappeared, and it smelt and tasted like mead.

How to control fermentation temperature

As discussed, you must keep the fermenter at the correct temperature throughout the fermenting process.

Even if you haven’t invested in one of the cooling systems described below, you should at least try to keep the temperature as stable as possible. The easiest way to do this is by placing your fermenter in a closed dark space, typically in a cupboard or pantry if you are fortunate enough to have a basement, all the better.

It’s also a good idea to brew according to the season. For example, I live in Spain and brew ales from October through to April and then brew Mead and Belgian Tripels or French Farmhouse Ales in the summer. Speciality yeasts such as Kveik, a Norwegian yeast strain, can even be used to brew ales in the height of summer.

How to cool a fermenter down?

Here are four ways that you can prevent your fermenter from getting too warm:

1. Water Bath (thermal inertia)

Liquids warm up more slowly than air does, and larger volumes of liquids warm up more slowly than smaller volumes. This effect is called thermal inertia.

If the temperature in your house fluctuates a lot during the day, is much warmer during the day than at night, you can keep the fermenter at a more constant temperature by standing it in a bath full of water.

The larger volume of liquid in the bath takes longer to heat up and cool down and helps keep the temperature of your beer more stable.

2. Fermentation chamber (insulation)

Another way to help stabilise the temperature of your fermenter is to build a fermentation chamber. In its simplest form, this is no more than a large insulated box made of plywood or MDF that you put the fermentation vessel in.

Like the water bath method, this doesn’t actively cool the fermenter down; it just helps keep it at a more stable temperature.

3. Swamp Cooler (evaporation)

When water is in contact with a warm surface, it evaporates, cooling the surface down. This is why our body sweats to keep cool, as the sweat evaporates it removes heat from our skin.

Before refrigeration, people used to keep things cool using what’s known as a swamp cooler. You can quickly build a swamp cooler for your fermenter using a large pot and an old T-shirt or towel.

Stand the fermenter in a pot full of cold water, wet the T-shirt and then place it over the fermenter so that the shirt’s bottom is in the water. The shirt acts as a wick, drawing water up from the pot and keeping the surface of the fermenter damp. As the water evaporates, it cools the fermenter down.

A swamp cooler can lower the temperature of the wort by about five degrees. You can increase the cooling effect by adding an electric fan.

4. Fermentation chamber MK2 (refrigeration)

Another popular way of keeping a fermenter cool is by placing it in a converted refrigerator or chest freezer.

Clearly, this slightly more expensive than the other methods. If you already have an old refrigerator, you can convert it into a fermentation chamber by connecting an external temperature converter such as the Inkbird ITC 308.

The setup is straightforward; you simply plug the refrigerator into the Inkbird. You then turn the temperature controller of the refrigerator to its minimum setting and then tape the Inkbird’s temperature sensor to the side of the fermenter.

If the temperature of the wort is higher than the value set in the Inkbird, it switches the refrigerator on. To complete the setup, if you also need heating, you can plug a suitable heater into the second output of the Inkbird.

Ideal temperature ranges for brewers yeast

This table lists the ideal temperature ranges for popular brewers yeasts. All temperatures are from the yeast manufacturers websites.

Yeastideal temp (ºC)Ideal temp (ºF)
Bulldog Mead Yeast20 – 3068 – 86
Lalbrew Abbaye17 – 2563 – 77
Lalbrew Belle Saison15 – 3559 – 95
Lalbrew Bry-9715 – 2259 – 72
Lalbrew Diamond10 – 1550 – 59
Lalbrew Köln12 – 2054 – 68
Lalbrew London18 – 2265 – 72
Lalbrew Munich Classic17 – 2263 – 72
Lalbrew New England15 – 2259 – 72
Lalbrew Nottingham10 – 2250 – 72
Lalbrew Verdant IPA18 – 2364 – 73
Lalbrew Voss35 – 4095 – 104
Lalbrew Windsor15 – 2259 – 72
Lalbrew Wit17 – 2263 – 72
Mangrove Jack’s M05 Mead Yeast15 – 3059 – 86
Mangrove Jack’s M12 Kveik Yeast30 – 4086 – 104
Mangrove Jack’s M15 Empire Ale Yeast18 – 2264 – 72
Mangrove Jack’s M20 Baravrian Wheat Yeast18 – 3059 – 86
Mangrove Jack’s M21 Belgian Wit Yeast18 – 2564 – 77
Mangrove Jack’s M29 French Saison Yeast26 – 3279 – 90
Mangrove Jack’s M31 Belgian Tripel Yeast18 – 2864 – 82
Mangrove Jack’s M36 Liberty Bell Yeast18 – 2364 – 74
Mangrove Jack’s M41 Belgian Ale Yeast18 – 2864 – 82
Mangrove Jack’s M42 New World Strong Ale Yeast16 – 2261 – 72
Mangrove Jack’s M44 U.S. West Coast Yeast18 – 2359 – 74
Mangrove Jack’s M47 Belgian Abbey Yeast18 – 2564 – 77
Mangrove Jack’s M54 Californian Lager Yeast18 – 2064 – 68
Mangrove Jack’s M66 Hophead Ale Yeast18 – 2264 – 72
Mangrove Jack’s M76 Bavarian Lager Yeast8 – 1445 – 57
Mangrove Jack’s M84 Bohemian Lager10 – 1550-59
SafAle BE-13418 – 2864 – 82
SafAle BE-25615 – 2059 – 68
SafAle K-9715 – 2059 – 68
SafAle S-0415 – 2059 – 68
SafAle S-3315 – 2059 – 68
SafAle T-5815 – 2059 – 68
SafAle US-0518 – 2864 – 82
SafAle WB-0618 – 2464 – 75
SafBrew DA-1620 – 3268 – 89
SafBrew HA-1825 – 3577 – 95
SafBrew LA-0110 – 2550 – 77
SafLager S-18912 – 1553 – 59
SafLager S-2312 – 1553 – 59
SafLager W-34/7012 – 1553 – 59
Sourvisiae15 – 2259 – 72
Wildbrew Philly Sour20 – 2568 – 77