Fermentation Temperature Control
Fermentation temperature is a critical brewing variable. Temperature directly influences the metabolic rate of the yeast and the rate of the biochemical reactions associated with fermentation. Fermentation temperature has a significant impact on the quality of the finished beer, so control of this variable is very important for the brewer.
There are numerous ways for a homebrewer to control fermentation temperature. One very common method used by homebrewers is to place the fermentation vessel inside a temperature-controlled chamber (e.g. a converted refrigerator or chest freezer). The temperature control system that is most commonly used is an external controller with a temperature sensor probe that is installed in the system in such a way as to override the built-in thermostat on the refrigerator or freezer. The temperature sensor that is used for this type of controller is commonly a thermocouple, thermistor or sealed-bulb-type probe. The temperature sensor is designed to detect the temperature of whatever it is in contact with, and transmit this information to the controller. The controller receives the temperature information from the sensor and takes action depending upon the difference between the temperature setpoint on the controller and the temperature data that is being received from the temperature sensor.
In a system such as this, the placement of the temperature sensor is an important consideration. The temperature sensor detects the temperature of its immediate surroundings. If the sensor is mounted to the wall of the chamber, it is detecting (mostly) the temperature of the chamber wall. If the sensor is dangling from the wire and hanging in the air within the chamber, it is detecting the temperature of the air. If the sensor is immersed in the wort, it is detecting the temperature of the wort in contact with the sensor. The temperature controller responds to the signal that is being sent by the sensor. The controller doesn’t know or care where the sensor probe is located. It only sees the temperature output signal from the sensor and takes action based on deviation from the temperature setpoint in a way that is governed by the logic programmed into the controller.
Comparison of temperature responses
For the purposes of our discussion, assume that the temperature controlled chamber is a cooling chamber (i.e. a refrigerator or chest freezer). Two common locations for a temperature sensor in a homebrewer’s temperature-controlled chamber are in the air of the chamber and submerged in the wort being fermented. When a signal is sent by the sensor indicating that the temperature is higher than the controller set-point, the temperature controller sends a signal to the system to activate the cooling unit. The cooling unit then begins to lower the temperature within the chamber until such time as the temperature is reduced below the controller set-point. The controller then tells the cooling unit to stop cooling. Depending upon the location of the temperature sensor, the on/off cycle duration of the cooling unit will be different because the controller will be responding to a temperature sensor signal that will be different in the two different system configurations.
The air temperature within the chamber will quickly become cooler when the cooling unit is operating. If the sensor is reading the temperature of the air, the controller will respond accordingly and will stop the cooling cycle fairly quickly when the air temperature falls below the set point.
For a fixed temperature differential, wort in the fermenter will take a much longer time to become cooler as compared to the temperature of the air. This is because the cooling unit cools the air, which then cools the fermenter, which then cools the fermenting wort. All of these cooling steps take time. Additionally, it generally takes much more time to reduce the temperature of solids (the fermenter) and liquids (the wort) due to the relatively higher heat capacity of solids and liquids vs. gasses. Heat capacity is defined as the amount of energy required to change the temperature of a unit-amount of a substance. The heat capacity for air is approximately 0.24 BTU/lb. The heat capacity for water is approximately 1.0 BTU/lb.
Additionally, there is not a large mass of air in a sealed refrigerator or freezer. There is a much greater mass of water (wort) inside our example sealed refrigerator or freezer (even though the volume of air is greater). Cooling the smaller mass of air happens much more quickly than cooling the larger mass of water (wort).
The cooling unit cools the chamber until such a time as the temperature of the sensor is equal to the setpoint temperature on the controller. If the sensor is immersed in the wort, it will tell the cooling unit to continue lowering the temperature of the chamber more and more, even to a point such that the temperature of the air in the chamber is much lower than the temperature of the sensor immersed in the wort.
In the system configuration where the sensor measures wort temperature, the chamber air temperature is allowed to become much cooler than that of the controller setpoint.
Important considerations for the brewer
Yeast generates heat as a by-product during fermentation. Because the wort within the fermenter is cooled from the outside by the cold air in the chamber, temperature gradients will always be present to some extent within the wort (even though the wort is slightly mixed by the action of evolution of CO2 during fermentation). The wort nearest to the fermenter wall will have the lowest temperature and the wort in the center of the fermenter will have the highest temperature. The graphic on page 61 illustrates this temperature gradient concept.
This means that if the temperature sensor is placed in the wort in the center of the fermenter, all of the other wort within the fermenter will be at a lower temperature than the temperature the sensor is reporting. The wort adjacent to the fermenter wall will be the coolest and the wort at the center will be the warmest.
If the temperature sensor is placed in the air within the chamber, the sensor will indicate a temperature that is lower than the temperature of the wort. In this instance, the wort in the center will still be warmer than the wort near the fermenter wall.
Since the biological activity of the yeast is strongly temperature dependent, it is important to understand and manage these thermal details within the fermenter.
Practical considerations
Management of the temperature within the fermenter is a very critical control variable for a brewer. To brew great beer a brewer must keep the yeast happy and try to optimize their environment in a way that is appropriate for the style of beer that is being produced. As homebrewers, we have limited methods available to allow us to control fermentation temperature, and we are forced to brew our beer within the realities of the physical constraints of the system that we have. Since temperature gradients within the fermenter exist, we must manage them as best we can. Here are some recommendations that might be useful.
Sensor in wort configuration
This configuration is generally preferred over the sensor-in-air configuration because the sensor is actually measuring the temperature of the wort, not the temperature of the air. Understand that the wort near the wall of the fermentation vessel will be cooler than indicated by the sensor. Use LCD adhesive-strip-type thermometers on the exterior of the fermentation vessel in order to monitor the temperature of the fermenter wall. Adjust the setpoint on the temperature controller as needed to ensure that the wort near the fermenter vessel wall does not become too cold.
Sensor in air configuration
Understand that the temperature the sensor is reporting to the controller will always be cooler than the actual temperature of the wort. Use LCD adhesive-strip-type thermometers on the exterior of the fermentation vessel in order to monitor the temperature of the fermenter wall. Take note of the differential between the sensor temperature and the temperature shown by the thermostrip. Adjust the setpoint on the controller so that the temperature shown by the adhesive-strip is approximately 1–2 °F (0.5–1 °C) lower than your desired fermentation temperature. An even better approach is to immerse a separate thermometer into the wort and adjust the setpoint on the controller to sustain the desired temperature on the other thermometer.
