Hot and Bothered About Temperature Differences
Dear Mr. Wizard,
I was pouring a homebrewed Belgian wit today and I was wondering if I was about to enjoy the fruit of my labor at the proper temperature. I measured the temperature with a recently calibrated thermometer at approximately 50 ºF (10 °C). Typically, I keep my converted chest freezer at approximately 38 ºF (3.3 °C) using a refrigerator thermostat and I monitor the temperature with an accurate commercial grade thermometer. I ferment in another converted chest freezer using the same method of temperature control. The 12 ºF (6.7 °C) difference in temperature raises several questions. Are there variations between fermenting wort/beer temperature vs. ambient temperature? What, if any, affects will these temperature variations have on my finished product? Are the recommended temperatures by yeast labs suggested for wort/beer temperature or ambient temperature? Please enlighten me.
Mr. Wizard replies:
I want to clarify my understanding of your question. Your question is about fermentation temperature and this question came to you when you were pouring your wit. I will address this question, but first want to comment on what may have happened with the wit you poured. Let’s assume that both of your thermometers were reading correctly and the refrigerator temperature was indeed 38 ºF (3.3 °C) and the beer temperature after pouring was 50 ºF (10 °C). Obviously, there are only two things that may have been responsible for this difference in temperature. The first is that the wit was not refrigerated for long, had not equilibrated with the refrigerator and was warmer than 38 ºF (3.3 °C). The second possibility, which most likely occurred to some extent, was that the glass you poured the wit into was warmer than 38 ºF (3.3 °C) and it warmed the wit.
Based on an assumption about the specific heat of glass, I calculate that a glass beer mug weighing 32 oz. (900 g) could warm beer from 38 ºF (3.3 °C) to 50 ºF (10 °C) if the glass was originally at 72 ºF (22 °C). This is not a very unusual scenario and explains why some bars used those awful frosted mugs for beer. Tossing a mug in the refrigerator before use prevents this heating affect from occurring and does not turn your cold beer into a beer slushy like a frosted mug.
The real question you have is about differences between the fermenting beer in your carboy and the air temperature of the refrigerator. Heat is produced by yeast during fermentation and is removed by the surrounding air. In all cooled systems, the difference in temperature between the thing being cooled and the cooling medium drives the rate of cooling. (The same is true of heated systems.) As the temperature of the two components of the cooling system approach each other, the rate of cooling slows. When thickness is added to this argument, a temperature gradient between the core of the body being cooled and the surface of the body is seen. If the body is solid, the mode of heat transfer is called conduction because the heat is conducted through the solid. In liquids things get a bit more involved since the liquid moves and this movement sets up convection currents.
Applying this rule to a fermenter of beer, you can see that the center of the fermenter will be warmer than the surface and that stirring the fermenter will increase the rate of heat transfer through convection. Although beer fermenters are not usually stirred using a mixer, there is considerable movement caused by the release of carbon dioxide from fermenting beer. In any case, there is a temperature gradient in a beer fermenter and the temperature at the surface is typically cooler than the temperature within the fermenting beer.
At home this difference is small because the volume of liquid is small and the surface to volume ratio is large. In larger fermenters, the surface-to-volume ratio decreases and the temperature gradient within the fermenter can become significant. When yeast companies suggest a certain fermentation temperature for a certain yeast strain, they are referring to the temperature of the fermenting beer, not the air temperature of the surrounding environment. However, in a small fermenter such as a 5-gallon (19-L) carboy the difference between the air temperature and the beer temperature is usually within about 5 ºF (3 °C). So if you have a yeast strain that produces the best beer when fermentation is held at 70 ºF (21 °C) the surrounding air temperature should be around 65 ºF (18 °C). You can periodically monitor this by inserting a thermometer into the fermenting beer.
In larger fermenters, a cooling jacket is used because air cooling is ineffective and the fermenter becomes way too warm. A cooling medium such as propylene glycol (food-grade anti-freeze) is pumped through the cooling jacket and the heat added to the glycol coolant is then removed using a refrigeration system. (Even though the anti-freeze does not touch the beer, the jackets can develop leaks and anything in a food plant used as a coolant must be food-grade in the event of a leak.) These larger fermenters are typically equipped with a valve that opens and closes in response to the temperature of the beer inside of the tank. Simple systems use “on/off” control and the beer temperature fluctuates around the target temperature. The difference between the target, 70 ºF (21 °C) for example, and the temperature where the valve opens or closes is called a dead-band. Most simple controllers are set up to control the beer within a 2 ºF (1 °C) dead-band around the set-point and the beer temperature is constantly moving within this 2 ºF (1 °C) dead-band around the set-point.
More sophisticated control systems employing proportional control valves and PID controllers (proportional, integral and derivative control is a mathematical-based control scheme to achieve much tighter process control) greatly reduce temperature fluctuation around the set-point value and in many cases can match the actual temperature to the set-point value over long time periods.
Where I work, we have on/off control and operate on a 2 ºF (1 °C) dead-band. The key, in my opinion, is having some target and being consistent in controlling around that target. When it comes to fermentation temperature, it is important to have a target fermentation temperature and have some method to achieve the goal. Absolute accuracy is less critical than having a target and a plan of action. If you allow the temperature to get too far off course, you will most likely see the effect of temperature of the fermenting beer. If it is warmer than planned, expect accelerated fermentation rate and the production of more esters. An overly cool fermentation may be very sluggish and it may fail to properly attenuate.
The key with most brewing is to keep it simple. There is absolutely nothing wrong with relying on the ambient temperature of your chest cooler to control fermentation temperature. Just remember that the temperature of the beer in the carboy will never be the same as the air temperature as long as the yeast is producing heat. This means that temperature of the wort will increase as yeast begin to ferment. When activity peaks and the rate of fermentation wanes, the temperature will begin to drop and will eventually equilibrate with the ambient temperature of your cooler when fermentation ceases. If you measure the temperature of the fermentation, you can get a good feel for where your thermostat should be adjusted.