Besides yeast health, controlling fermentation temperatures is one rule that homebrewers (and professional brewers) should always follow. When you buy a packet of yeast, there is a suggested range of temperatures that fermentation should occur.  The yeast company determines this range so the customer has the best chance of making a beer of good quality. This range varies considerably between strains. For example, Wyeast 1028 (London ale) has a recommended range of 60°F to 72ºF  while Wyeast 2308 (Munich Lager) has a range of 48°F to 56°F. Some strains can even go as high as 95°F (WYeast 3724 – the dupont strain).

What are the Effects of Temperature on Beer?

Fermenting at higher temperatures produces unwanted flavors in beer due to the increased metabolic activity. Off flavors such as fruity esters and solventy higher order alcohols, are generated. Acetaldehyde (green apple), in particular, is produced at a much higher rate during increased fermentation temperatures and is one of the most common faults in homebrewed beer. Fermenting below a yeast’s range will decrease these metabolites, but you also risk a sluggish fermentation, poor start to fermentation, and possible attenuation issues. The first generation of yeast is also susceptible to high fermentation temperatures as further pitches will suffer in quality.

How Does Temperature Affect Yeast?

As eluded, metabolic activity increases along with yeast growth at higher temperatures. Metabolism will proceed at such a high rate that the cell cannot keep up with byproducts produced from elevated temperatures. Also, heat (even a swing of 4°F) is a stress factor and causes proteins which are important for wort fermentation to unfold within the yeast cell. To counter this, the cell expresses a class of proteins called heat shock proteins. These are basically molecular chaperones that guide proper protein folding after translation and expressed to a high degree under heat stress. At higher temperatures there is also some DNA damage that occurs and is dealt with by cellular DNA repair machinery. The question then becomes, would you rather have the cell repair damage or make great beer for you to drink?

What does this mean for the Homebrewer?

Control your fermentation temperature within the given range as consistently and stably as you can. Try avoid any swings in temperature, especially during the first three days of fermentation. There are a couple of ways to do this:

1. Find a cool spot that stays at a constant temperature. This is the easiest method if you happen to have a cool basement.
2. Plastic tub/wet t-shirt/fan method. This technique is a bit more complex. The benefit is this is approachable to homebrewers on a budget. The downside is getting a consistent temperature is very hard. Basically you put the primary fermentation in a tub filled with 5 inches of cold water. Wrap the fermentor in a t-shirt with a fan blowing on it. The t-shirt acts as wick, pulling up water and the fan serves to evaporate any heat coming from the fermentation. You can add frozen water bottles to regulate the temperature even more.
3. Fermentation chamber. The best method, but most expensive. It involves using either a chest freezer or refrigerator set at constant temperature with a digital temperature controller.

You can measure the temperature of the fermentation by using a heat sensitive sticker. However, the most accurate method is using a thermowell to place a thermometer directly in the fermenting wort. The latter is obviously more expensive than the former. Whatever method you choose from above, keep in mind that fermentation is an exothermic reaction and will generate heat. Fermentation temperatures could rise 5-10°F above ambient. This article in Brew Your Own is great for controlling fermentation temperature.

The Experiment

I will be teaching a class on yeast and homebrewing at the end of August at Brooklyn Homebrew, and since I touch on fermentation temperatures in the lecture I thought this would be a good experiment to share with the class. The owners of Brooklyn Homebrew, Benjamin and Danielle, donated the supplies and are simply awesome. Data collection will be similar to my experiment on yeast pitching rates by taking a flavor profile poll on each beer that is brewed. The goal is not necessarily to prove that high fermentation temperatures can affect beer (this is well-known), but rather to highlight differences between samples fermented at different temperatures. I chose WYeast 1388 (Duvel strain) primarily for its wide range of suggested fermentation temperature.

Experiment conditions:

• Same wort produced for each sample.
• Same yeast and cell number (50 billion cells) used for each sample.
• Style of beer: Belgian Blonde Ale
• Yeast strain: Wyeast 1388
• Each sample gets aerated with pure oxygen for 30 seconds.
• Pitching temperature will be different for each, but with my setup I have no way of controlling this. Pitching temperature is also important and may need to be questioned in another experiment.
• I will not be pitching at the same time. I will need to bring my high temp sample into work and pitch then (1 hour lag time).
• Fermentations will occur for one week and bottle conditioned for another three.

Samples:

1)  Fermentation temperature of 60°F. This fermentation will occur in my chest freezer that is controlled by a digital temperature controller.

2)  Fermentation temperature of 72-75°F. This sample is subject to most variability in temperature as I will be using a water bath and ice bottles.

3)  Fermentation temperature of 86°F. This sample will be brought into my lab and placed into a constant 30°C (86°F) yeast incubator.

Beer (4.5 gallon batch):

• 6 pounds of golden light DME
• 2.0 ounces of Styrian Goldings at 60 minutes (3.8% AA for 27 IBUs)
• 1.0 ounce of Czech Saaz at 0 minutes (flameout)
• 1 whirlfloc tablet
• 1/2 tsp of yeast nutrient

Victuals:

• OG: 1.059
• IBUs: 27 (Tinseth)

For more scientific reading on the effects of temperature on beer fermentation I have two papers that are worth looking at. Olaniran et al., published an interesting article studying fermentation temperature effects on beer color, foam stability, flavor profiles, and aroma profiles. While there was no effect on the color of the beer, there were differences in volatile compounds in the headspace of the beer. Yeast viability decreased at higher temperatures and tasters reported that the high fermentation samples tasted more medicinal. The second paper is a bit older but describes the formation of esters during fermentation. Quoting Peddie et al.:

“Temperature: When temperature is elevated, the concentration of esters produced during the fermentation is also increased. For example, 15°C temperature increase from 1O°C to 25°C can result in 75% increase in ester concentration. This increase in temperature may make the membrane more fluid and so affect the activity of the AAT enzyme, if it is membrane bound. An increase in membrane fluidity may allow more ester to diffuse into the medium. AAT is also known to be unstable at higher temperatures. However, the higher temperature may simply increase enzyme activity. Engan and Aubert have shown that the effect of temperature differs with both ester type and temperature range.”