Summer is coming, and in NYC that means heat, lots of it. Especially in our tiny apartments. Here at the store I am often asked what is the right temperature for fermentation and how do I maintain it? As the saying goes, brewers make wort, yeast makes beer. I like to joke with my classes that the yeast does not care about us, but boy do we care about them. How we take care of them makes all the difference in beer flavor and temperature control is one of the most important factors in that process.
Our single celled friends are basically little enzyme bags. Enzymes are a type of protein that act as a catalyst for chemical reactions. In the case of fermentation the enzymes the yeast use are mainly for the metabolizing of sugars that are in the wort. Enzymes work best at certain temperatures and this is where our fermentation temperature comes to play (among other things). I would love to go deep into yeast metabolic properties but I'd rather just give some practical advice. Certain yeast strains offer different depending on the temperature they ferment at. One example is Wyeast 3068 Weihenstephan Weizen ale. Lower fermentation temperatures (64˚ to 70˚F) will produce less banana esters and more clove flavors whereas higher temperatures (70˚ to 75˚F) will increase that banana flavor. So by manipulating the temperature you can greatly change the flavor profile of your beer. This will vary from strain to strain.
Pitching temperature is important too. People go back and forth on this but I personally like to pitch a few degrees cooler than my target fermentation temp, especially for lagers. This keeps ester production down and produces a smoother, cleaner beer. I will let my wort and yeast sit in the same area until they are at the same temperature, oxygenate and then pitch. Every yeast strain will tell you the range of temperatures that best fit the profile of the strain. Going cooler than suggested will make the yeast sluggish and in some cases make them dormant, resulting in under attenuated (fermented) beer. Too warm of a fermentation temperature will speed up the metabolism of the yeast and can cause many off flavors. This is the biggest beer flavor problem I run into. It is hard to describe but beer fermented too warm has a chalky, solventy flavor caused by higher molecular weight alcohol created called fusel alcohol.
Proper fermentation temperature of lagers (as opposed to ales) is important (45˚ to 55˚F) to reduce ester production. As lager fermentation temperatures are much cooler than ales, other compounds created during fermentation (sulfur, diacetyl or buttery flavors) don't get to dissipate or get metabolized. That is why after primary fermentation of a lager is near complete you need to warm lagers up to around 70˚F in order to mellow out these flavors. And then rack off the yeast and lager (cold store) at around 40˚F.
So, now we know that temperature control during fermentation is important. But have you seen the sweat box I live in? How to I maintain temperature especially during the dog days of summer? The first thing most home brewers do is make Saisons during the summer. Saisons are a delicious French farmhouse style ale that can ferment up to 90˚F depending on the yeast strain. But what if you don't want to make Saisons all summer? This is where home brewers get creative. The first and best thing to do is get a freezer that your carboy or bucket will fit in. Put a temperature thermostat on the freezer that has a separate probe, and then you can adjust ambient temperature to any degree that you'd like. If you really want precision you can use a thermowell. This is a hollow metal tube that your temperature probe will fit into so that the thermostat adjusts to the temperature of your beer, not just ambient temperature. During fermentation yeast create heat, by using a thermowell you will have the most constant temperature control. While this is the optimal solution, it isn't the cheapest and requires a decent amount of space.
What other alternatives are there? What I do at home is make 2.5 gallon batches that fit perfectly into an igloo cube cooler. I fill a bottle with water and put a liquid crystal thermometer on it in order to be able to see the temperature. Then I put plastic bottles that I freeze with water into the cooler to keep the right temperature. Remember to change the ice bottles regularly. Another option is a swamp cooler. All you do is put your fermenter into a large bucket with ice (or frozen water bottles) and water. Put a t-shirt on your fermenter, put one end of the shirt in the cool water. It will act as a wick and keep your fermenter cool. Low tech but effective. You can make it even more effective by aiming a small fan at it to make the water evaporate more quickly.
These solutions are not as important during the winter. If you have a closet or somewhere away from light and direct heat, room temperature usually works. What you want to look out for is temperature fluctuation. When the temperature of fermentation fluctuates wildly the yeast create heat shock proteins which inhibit enzymatic reaction. You don't want these in your beer. The other temperature problem I get asked about, although less often, is how to keep the fermenter warm. This is mainly asked by Saison brewers during the winter time. There is an electric brew belt available that will keep your fermenter around 75˚F. Not really warm enough for saisons, but consistent. I have heard that people will use electric blankets to ramp temp up but I've never tried it. How about you? How do you maintain your fermentation temperature at home?
Keep cool. John