By Greig McGill
Fermentation temperature control is, in my opinion, one of the most critical yet most commonly neglected keys to brewing great beer. Having just come out of an unusually warm summer that just kept going, it seems like a good time to talk about what it is, what it isn’t, and what it means for you in your quest to make great beer.
It’s commonly said that brewers don’t make beer - yeast makes beer. This is definitely something worth remembering. Just like us, yeast will do things differently (or not at all) depending on how warm or cold it is. We’re far more likely to hop in the car and drive to the beach on a hot day than a cold one. Likewise, yeast are far more likely to produce certain esters and other flavour compounds at warmer temperatures than cooler ones. As we struggle to get out of bed and get work done when it’s freezing outside, yeast will be just the same, either opting to stay in bed and pull the yeast equivalent of a sicky, or turning up to work, but spending more time shivering and making hot cups of coffee than doing any work! OK, that analogy might have gotten away on me a little. The point stands that yeast has a band of temperatures that it will work optimally within, and it’s in our interest to utilise that knowledge to help the yeast to make our beer taste great.
The basic rule to live by, and the one thing to take away from this article should you not be interested in the science or the possibilities for tweaking and just want to make great quality beer is this: Control your temperature! Always go from cooler to warmer, avoid sudden drops or rises in temperature, and never cool your beer until fermentation is 100% complete.
So, now that’s out of the way… why? Well, yeast, like most living things, prefers a particular temperature range. Different strains have different ranges of temperatures they will ferment well in, but generally they won’t ferment at all and will go dormant if there’s no ambient energy (heat) for them to use, and will ferment poorly, and even die, if they get too hot. Within the zone where they will ferment, they will perform differently, producing different compounds at different rates as the temperature rises or cools. In general, the middle range of a yeast strain’s stated fermentation temperature range is where you will aim if you want to produce a fairly neutral or classic example of the beer style that the strain is suited for.
So why always go from cooler to warmer? As yeast perform their magical beer creation work, they change, divide, and struggle under the conditions - they are literally producing waste products to them (CO2 and ethanol) and nobody likes working hard in their own waste! Well, with the standard exception for Rule 34. Anyway… as yeast become accustomed to working at a certain temperature, they are more likely to adjust to it and cease working before vital cleanup of waste can occur - we’ll talk about this more shortly. This is even further exacerbated if temperatures go backwards, with cooling causing the yeast to assume their work is complete and they can rest. Follow this logically, and we understand why extreme temperature shifts are bad - too hot too fast, and the overexcited yeast might produce all kinds of bad flavours before quitting on you. Suddenly getting cold tells them it’s sleep time, even if they’ve not finished yet, and lots of the unpleasant flavours they would normally clean up are still left in your beer.
Time to get really nerdy now. You may be aware of the different yeast phases often ascribed to fermentation - lag, growth, fermentation, and flocculation/sedimentation. Lag is the perceived waiting time while the yeast takes up available oxygen in preparation for the growth phase. Growth is what it sounds like - the yeast spending that new energy on dividing to the point where there is enough biomass for stage three. Fermentation - the main event! What we and the yeast are here for - the yeast converts the available sugars in the wort into ethanol, CO2, and various other precursors and compounds. Finally, when it’s done, it sinks out of the new beer, clumping together (flocculating), and forming a sediment at the base of the fermentation vessel. The problem with this model is that it’s not so cleanly divided in reality. During the lag phase, fermentation is still occurring, and new cells are still budding right up until (and even during) flocculation and sedimentation. This is particularly evident with STA1-positive (diastaticus) yeasts, which are able to secrete an enzyme which breaks down previously unfermentable dextrins and starches due to possessing an STA1 gene. So, that’s a geeky way of saying “everything is happening, all of the time, in differing amounts, and at different rates”.
Most of the “things” that are happening are chemical reactions. Yeast tolerance aside, all chemical reactions proceed more swiftly as the temperature rises. This is because a reaction involves reactants (molecules) hitting each other, and as molecules move faster when they are heated, more “hits” occur in a given time. So, back to our fermentation, the hotter it is, the faster each yeast cell will go through its individual cycle, and the faster your fermentation will proceed. That’s variable number one you can tweak, and in a commercial brewery, it’s the base for tradeoffs - how fast can I get this beer done and out of the tank so I can sell it and get another beer in there? As a homebrewer, it’s much nicer to put the optimum fermentation profile we want from our beer first!
Of course, some things we do want to happen more quickly. The warmer we start our fermentation, the more healthy yeast cells will grow, and faster, reducing our risk of contamination and ensuring a good vigorous fermentation. Of course, the brewing gods giveth and the brewing gods taketh away, and the tradeoff here is the increased risk of cell mutation leading to unexpected fermentation results, and more obviously, higher production of some of those precursors I mentioned earlier. In this case, alpha acetolactate - a precursor for diacetyl. This isn’t too much of a problem assuming you followed the important rules above and increased temperature during fermentation. A good warm rest for at least 24 hours after terminal gravity is achieved should ensure the yeast will metabolise and break down diacetyl precursors and other negative flavour compounds produced. How do you know how long to rest your beer? Taste it! Every yeast strain is different, and some may barely need any time for cleanup, while others might take days. You can perform a VDK test to specifically test for the most common off flavour at this stage - diacetyl and its brothers.
At the end of fermentation, and post VDK rest/testing, there is the consideration of crash cooling. Rapid temperature change is very stressful on yeast, and can cause them to partly break down, releasing unwanted compounds back into your beer. It also has the potential to damage the cells, reducing the ability to repitch your yeast. While this latter problem is not too much of an issue for most homebrewers, and can be solved by just buying more yeast, those on a tight budget might benefit from looking after their yeasty buddies as well as they can! While some strains cope a lot better with crash cooling than others, and may have no issues at all, if you can afford to take the time to cool slowly to your final 0-2C temperature, that’s the best thing for your beer, and your yeast.
Finally, here are some broad examples of off flavours produced by failures in temperature control. If you smell or taste these in your beer, your most likely cause is temperature related, though some could be related to pitch rate or lack of oxygenation.
Fusel alcohols - hot, rough smelling alcohol content is often caused by fermenting at too warm a temperature.
Haze in a clear, non-dry hopped beer - this can be a result of rapid temperature change - yeast will produce proteins when they are temperature-shocked which are designed to help shield the cells from damage. These remain in the beer as unstable haze-forming proteins, and also mean that the cell may well be producing off flavours, or simply not fermenting at all while creating these proteins.
Overly fruity (often banana/pear) or phenolic (plastic/cloves/band aids) - Yeast will naturally produce more esters - the fruity aromas and flavours - at higher temperatures, and Phenolic-Off-Flavour Positive (POF+) strains of yeast will also produce higher levels of phenols at warmer temperatures.
Buttery - there’s that VDK we talked about! It’s most likely that your yeast didn’t get a long enough warm rest at the end of fermentation, or was too warm for too long at initial pitch and produced too much for proper cleanup.
Green Apple notes - Acetaldehyde is commonly attributed to underpitched yeast, but is also produced in much higher quantities at warm temperatures. If you’re pitching plenty of yeast, and giving it enough oxygen, it might pay to check your temperature control.