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Fermentation Temperatures

TroubleShooting

Thomas Crawford • Tallahassee, Florida asks,
Q

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.

A

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.

Response by Ashton Lewis.