After the diacetyl rest, the beer is cooled to lagering temperature at 2 to 4 degrees C (36–39°F), which encourages the formation of chill haze particles and to allow time for yeast sedimentation. This happens even though the same top-cropping strains of yeast are used. Any deviation could result in the beer freezing—and in doing so, the beer would decarbonate, expand its volume, and potentially cause damage to the heat exchanger, piping, or other equipment. Crash cooling can be achieved with jacketed tanks or a heat exchanger.
Once the yeast has settled, it will begin to uptake diacetyl and 2,3-pentanedione that are excreted early in the fermentation. It is important in brewing lagers not to lower the fermentation temperature prematurely or rack the beer prematurely until the yeast sediment has reabsorbed the diacetyl, or else the beer will end up with a distracting “buttery” aroma and flavor.
Crash cooling in a tall vessel required the use of two or more side wall jackets, one located near the top of the cylinder and the other abutting the junction with the cone along with a cone-cooling jacket. A cone-cooling jacket is necessary for storage at conditioning temperatures to avoid localized heating of the yeast and beer in the cone. The positioning of thermometers is also important. Thermometer probes must be long enough to project into the main body of liquid, and therefore avoid local cold currents close to the cooling jackets.
Formation of Ice During Chilling
A phenomenon commonly observed in fermenters in small breweries is the formation of ice on the tanks during the chilling of beer after fermentation is complete. This is a function of the location of the cooling jackets, the placement of the temperature probe, control of the flow of coolant, and the required temperature of the cooled beer.
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