Temperature Control

Key points

  • Yeast perform best at a set of specific temperatures 
  • You can control these temperatures at a home brew level

                One of the realisations that beginning brewers have before turning into intermediate brewers is the necessity of controlling the temperature of your fermentation. Yeast do different things during their life cycle and to avoid revisiting high school biology, we’re just going to divide them into three phases: Lag, Log and Stationary. Being able to adjust the temperature for these stages means that you can control off flavours produced by the yeast and, once you get good at it, predict and control the flavour profile that will be produced by your yeast.

                Lag Phase

                The Lag phase is where the yeast, once pitched into their new environment, start looking for two things: oxygen and sugar. If the temperature is too cool during this phase, yeast will be sluggish, slow to reproduce and you will likely not have enough to finish fermentation. The beer will be insipid and prone to infection. Contrary to popular belief, the temperature during this phase can be as high as 42°C before negative flavour effects are seen.* Please note, I don’t actually recommend pitching at that temperature – it’s going to make the next step rather difficult. However, it does mean that you can relax a bit if your temperatures aren’t exactly on point at pitching.

                Log Phase

                At this phase, the yeast grows exponentially, budding new daughter cells. This is where vast quantities of nutrients are consumed. If the yeast didn’t get enough oxygen during the lag phase, daughter cells will leave holes that esters can pass through leading to off flavours (or desireable flavours if that’s what you’re after). In addition, the temperature plays a very big role during this phase. Higher temperatures will mean faster reproduction. When some nutrients are exhausted, yeast may keep reproducing in sub-optimal conditions, leading to more esters, phenols and diacetyl. Ideal temperatures for ales are going to be between 18-20°C and lagers are going to be between 6-10°C

                Stationary Phase

                Once most of the nutrients are consumed, yeast prepare for some belt-tightening. They’ll store glycogen and trehalose, which is similar to maltose being composed of 2 glucose units, but the bonds are slightly different, allowing it to act as an antifreeze. Diacetyl and acetaldehyde will also be converted into the flavourless 2-3-butandiol and ethanol respectively. Temperatures should increase during this phase in order to give the yeast the energy they need to finish absorbing and converting these fermentation byproducts. For most ales, 22-24°C is sufficient and for lagers, not higher than 15°C. Some POF+ yeast strains may need additional temperature bumps, such as wheat, saison and Belgian strains to finish.

                Controlling Temperature

If you’re like the majority of home brewers, you probably just stick your carboy of wort in a warmish or coolish place, and RDWAIHAHB. However, since you’re reading this, you’re probably a bit beyond average and are looking for a more consistent fermentation.

                Measuring temperature

                During the fermentation process, yeast will generate a significant amount of heat, so those temperature stickers on the outside of your carboy or bucket aren’t going to be particularly useful, aside from giving you a ballpark of the ambient room temperature. You’re going to want some sort of probe or thermometer that is in contact with, or inside a thermowell. Stainless steel is obviously the best choice for sanitation. You can find a stainless, weldless thermowell for almost nothing. Mind you, if you’re using a plastic bucket, the extra piece of stainless isn’t going to up your sanitation game, you might as well just plop the probe directly in there. In the case of a glass carboy (much better, sanitation wise), a straight wall thermowell can be used to house your temperature probe and fitted to your stopper.

                Controlling temperature

                For those trying to do this on the cheap, I’m going to recommend everyone’s favorite, the STC 1000. It’s basically an aquarium controller which sends a signal to various terminals based on whether it’s too hot or too cold. They’re fairly easy to set up, though the instructions on how to wire them can be hit or miss. The clearest set of instructions that I’ve found are here, and there are several YouTube videos that help you set them up if you get lost. For those who can’t be bothered to risk mild (or severe) electric shock, blown fuses and angry spouses, the InkBird temperature controllers are fairly straightforward, though they can get a bit pricey. You simply plug in your desired heating element or cooling mechanism and off you go.


                Heating is thankfully fairly simple. A space heater or electric jacket, such as a brew belt are pretty effective, though if you’re finding those to be a struggle, you might want to consider moving your brew to a warmer environment. If the garage is only 10°C at night, your brew belt might struggle and burn out.


                Cooling is a much more difficult process, so if you’re doing lagers, start by placing your fermenter in the coldest part of the house. Several options exist, from the wildly expensive (SS Brewtech’s glycol chillers) to the simple, but not particularly elegant, such as a swamp cooler. In between would be water/ice chilled coils inside your fermentor and plugged into your temperature controller. These also tend to be out of most brewers’ price ranges at between $400-$500.

                Other solutions that won’t destroy your wallet have to do with housing your fermenter in a cold room. Build a box, insulated and place an old air conditioner in it (I did this for years) or find an old fridge and hook that up to the temperature controller. Of course, these both require evaluating the space available to you in your brew room.

                Whatever you decide to do, it’s always best to consult the community as I guarantee that someone has come up with a hacked piece of equipment already. I’ve even seen one home brewer’s home-made glycol chiller. I’m not going to recommend you build your own glycol chiller, but home brewers seem full of bad ideas, which is part of what makes it such a wonderful community.