Fermentation Temperature Control

Fermentation temperature control is the single most important thing you can do that will make the most dramatic improvements in your beer. And it can be a big problem, especially when brewing in the Deep South. Winter brewing is great but summer brewing can be brutal in the 100°+ days. Without temperature control, it's simply impossible to brew most beers correctly. Many homebrewers underestimate its importance and therefore are doomed to brew mediocre beers. Some of the major problems associated with fermenting too warm are:

Fermenting inside fridge

  • The biggest problem is the off flavors from esters and fusel alcohols that the yeast produce. Sometimes the flavors are not so much "off" as they are inappropriate for the style.
  • Your yeast can blast out of the starting gate, consuming everything in sight, then run out of nutrients before finishing the sugar. This usually ends in an incomplete fermentation.
  • Poor temperature control often results in fermentations that are too hot, causing the yeast to become too sensitive to alcohol toxicity (meaning that they will die off from the alcohol before their usual tolerance is met).
  • Yeast begin to die off from heat stress, leaving the remaining yeast to do all the work. In effect you ended up under-pitching the yeast and will get off flavors as a result.
  • Since yeast metabolism generates a lot of heat, starting fermentation at too high of a temperature will quickly lead to problems as the temperature will climb in the 80°F+ range and yeast die off.

When brewing in colder climates or during the winter without temperature control, yeast will exhibit stress problems too. Winter is the traditional time to brew because there is less spoilage bacteria and wild yeast to contend with. For a homebrewer, too cold of a fermentation temperature can cause the following problems:

  • Your fermentation may never get started.
  • Your fermentation may be sluggish and drag on for weeks before finally getting stuck.
  • For ales that require a degree of fruitiness from esters, fermenting too cold can lead to a beer that is too clean or bland for the style. Judges will be looking for flavors and aromas that just didn't make it into the beer due to the cold fermentation.
  • If there is any contamination in your beer, it is going to be a race to see which can dominate the fermentation. A sluggish start from poor temperature control can give these bacteria a chance to take over and ruin a batch of beer.
  • When fermentation does commence in an environment that is too cold, CO2 becomes entrained in the cold beer. Flavors that are normally "gassed off" stay in solution and may make it through the entire fermentation process to the final product. This is especially true for the "sulphur" aromas and flavors produced in lager fermentations.

So what are the solutions? What can I do to control the fermentation temperature? It goes without saying that before you can use temperature control in your fermentation, you must know what that temperature is. There are a few options here.

  • A separate thermowell with temperature probe installed will give you the exact temperature of your fermentation. This is the most accurate way to measure and control the temperature of the fermenting wort.
  • Or you could rely on the setting on the dial of your temperature controller. You want to control the actual temperature of the wort and not the ambient temperature inside the fridge (which can vary by quite a bit). To do this, tape the temperature probe to the side of your fermenter and then tape a piece of bubble-wrap on top of the probe for insulation.
  • For measurement only, an indoor/outdoor thermometer works well for monitoring the ambient temperature inside your refrigerator. Place the control unit on the outside of your fridge, and the temperature for the outside (which is now inside your fridge) will be the inside ambient temperature of your fridge.
  • Of course, the better the quality of the thermometer, the more accurate the temperature will be. I place my traceable thermometer inside and compare it to the cheaper indoor/outdoor temperature. In my case there is a two degree difference. I just keep that in mind when I monitor the temperatures.
  • Your local homebrew supply store will probably have the stick-on thermometers you can use for your fermenter. It's not the most accurate instrument but is better than nothing.
  • A good option is a liquid-filled thermometer that you place inside the fridge. The liquid reacts much slower to changes in ambient temperature than does the air inside the fermentation fridge. If you have the door open for a short period of time, while measuring the SG of your fermenting wort for example, the liquid filled thermometer will still measure the same temperature. It is also a little more accurate measuring the fermenting beer than just a typical thermometer, since it is filled with liquid just like the fermenter is.

Once you can monitor temperatures, here are a few solutions for controlling the actual temperature of your fermentation:

  • Refrigerator or Freezer The most common solution is the spare refrigerator/freezer. Because fermentation temperatures need to be more precise than the refrigerator can provide, you will need to bypass the thermostat. The simplest way is to use either an analog or digital electronic thermostat controller.
  • The analog temperature controller (pictured is the Johnson Controls model most often seen in homebrew catalogs or websites). You can use this controller to regulate the temperature in your freezer between 20º – 80ºF (-7° to 27°C) by bypassing the unit's internal thermostat. You can also use the controller for turning a chest freezer into a keg refrigerator. It is strictly a mechanical thermostat for cooling only. It operates with a gas filled probe on a 6-foot capillary tube.

    Analog Temperature Controller

    When the gas contracts or expands in the probe, it triggers a mechanical device inside the control which turns the power on or off. There is no probe-temperature readout on the dial, so keeping a dedicated thermometer inside the refrigerator is highly recommended to keep track of the actual ambient temperature. On most refrigerators, the Controller will allow a usable range of 37° to 80°F (3° to 27°C)., while on freezers, the controller will be able to reach its full 20º to 80º (-7° to 27°C) range. Keep in mind that if you use it to control a refrigerator, you will lose the use of the freezer compartment for food storage, as it will not stay cold enough for safe frozen food storage. It is made of stainless steel and is adjustable with the turn of the know. To install, plug the unit into the wall, then run the sensor into your refrigerator/freezer. Then you plug the freezer or refrigerator into the control unit. It will cycle the power on the freezer based on the dial's temperature setting. NOTE: Freezers do not have a means of draining and evaporating condensation like a refrigerator does. Check out this simple solution at the bottom of my DIY page.

  • A better choice, although more expensive, is the digital electronic thermostat control. With this controller you can precisely control the ambient temperature inside your refrigerator or freezer, or you can control your fermentation. You can control fermentation temperatures three ways. First you can place the probe inside the space that is being cooled or heated to control the ambient temperature around the fermenter.

    Digital Temperature Control

    I put a pint jar filled with distilled water inside the freezer and punched a hole in the top. Then I place the probe inside the water. Because the 6' coil will want to tip the jar over, I use tie wraps to hold the jar in place. The temperature inside the jar of water will change more slowly than the ambient temperature inside the freezer. Doing this enables you to open the door for a few minutes without the controller cycling the compressor. A better choice is to place the probe directly into the fermenter using a stopper thermowell. If you don't have a thermowell, here's what I do. Tape the probe directly to the side of the fermenter. Then tape a piece of bubble wrap over the probe to insulate it from the air inside the fridge. The probe will read the temperature of your fermenting beer and make adjustments to the refrigerator to keep it within the controller's range. You can change from cooling to heating mode directly from the digital control panel. When in cooling mode, and hooked up to a refrigerator or other cooling device, the controller turns the compressor on when the ambient temperature rises above your set point. When in heating mode and hooked up to a FermWrap heater for example, the controller turns on the heater when the temperature drops below your set point.

    Fermwrap Carboy Heater

  • A FermWrap around a fermenter or inside an unplugged refrigerator can be the ideal solution for fermenting in cold climates when your fridge or freezer is in your garage. The digital temperature controller features an adjustable differential down to within one degree in both heating and cooling modes. This means you can precisely control an environment to within one degree of your set point. The included sensor probe monitors the temperature while the digital readout displays it in either Fahrenheit or Celsius. The sensor probe is 1/4" in diameter and works well with the stopper thermowells. The temperature range of these controllers is -30° to 220°F (-34° to 104°C). Another great feature of this device is that you can ferment two completely different beers at the same time in the same fridge. Set the temperature controller for the colder fermentation (a lager for example) at around 48°F (8.88°C), this will keep the ambient temperature low enough to ferment your lager. Then use the FermWrap on an ale by wrapping it around the carboy and setting the controller to your ale fermentation temp, say 65°F (18.33°C). The FermWrap will heat the carboy and maintain the ale wort at the proper fermentation temperature for that style as well. NOTE: The heating tape you buy (called a "brew belt") is not the same as this device. It should not be used on glass carboys, only plastic fermenters. The heat applied is too localized and the glass could crack or shatter. The FermWrap distributes the heat over a much broader area and is therefore safe to use on glass carboys.
  • Electronic controller units designed for air conditioners or commercial refrigerators are available everywhere and can be used for temperature control. Or you can build thermostats from designs that you can find online. I find the temperature controller to be much easier and simpler to use.
  • The only downside to using a spare fridge for fermentation temperature control is when you also use it to store your bottled beer. The temperature inside is always fluctuating based on whether you are fermenting an ale or a lager. Aging beer benefits from stable temperatures. I'm not sure what effect this has on the beer's quality. It may be minimal for beers you will consume within a few months. But for big beers meant for long aging, like barleywines, lambics, and old ales, the consequences of constant temperature fluctuations may be more dramatic. More research is in order.
  • Evaporation You can place your fermenter in a spare bathtub or large tub. By placing an old t-shirt over the fermenter, or wrapping a towel around it, you can wick the water into the fabric as it evaporates. This provides a fairly constant cooling effect. To increase the cooling effect, add a fan. You can drop the fermentation several degrees using this method. One drawback is that you tie up your spare bathtub for a few weeks. I'm sure many great ales have been made using this cooling method.
  • Ice Bath Instead of adding water to the tub, try adding ice instead. Or if you are handy, try building an insulated container to place your fermenter in along with the ice. You have no way of controlling the temperature, and it will vary with the amount of ice left, but it will definitely get your fermenter near the lower ranges for most ale yeasts (55°- 60°F or 13°- 16°C).

Fermetation temperature control is probably the most important thing you can do to improve your beer. If you have your sanitation control and brewing processes down pat, then most of the off flavors in your beers can be attributed to improper fermentation temperatures. Ales fermented too warm will exhibit fusel alcohol and fruity esters that may or may not be appropriate for the style you are brewing. Although the yeast love the warmer temperatures and ferment faster, the flavors and aromas of your beer will suffer. The same goes with lager fermentations. You can't even try making many of the world's finest beers if you lack temperature control. I'd suggest that if you don't have a fermentation fridge, that you purchase or find one before you buy many of the other luxury homebrew gadgets on the market. You will make better beer, and isn't that the whole point?

Information for this article on brewing temperature control was adapted from Randy Mosher's books Radical Brewing and The Brewer's Companion as well as articles on fermentation control in the wine industry.


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