There are many different fermentation systems that are used worldwide that have evolved based on available technology, brewing materials, and perceived product quality. The following is a brief description of some of the more common fermentation systems in use today.
Cylindroconical fermentation systems are the most commonly used today for producing both lagers and ales. As the name implies, the enclosed vessels are vertical cylinders with a conical base and, normally, a dished top. This design allows for easy yeast collection and clean-in-place (CIP) cleaning. Vessel aspect ratios are typically between 1:3 and 1:5 (diameter to height). Very tall vessels are not generally favored, since the combination of high pressure due to the hydrostatic head and gas stripping may cause adverse flavor effects such as reduced ester formation at the expense of elevated higher alcohols.
The majority of cylindroconical fermenters rely on natural mixing by carbon dioxide evolution and thermal currents. Mechanical agitators are not usually fitted unless vessels are used in a unitanking process or where the yeast is exceptionally flocculent. The assumption is therefore that mixing is sufficiently good throughout primary fermentation to ensure homogeneity. Yeast sedimentation at the end of the process is a result of the disappearance of fermentable sugars and this is aided by crash cooling.
Foam production during fermentation tends to be excessive and is one of the reasons why some large breweries use antifoaming agents. Excess foaming during production has a negative impact on the final product by physically removing the foaming components from the product and denaturing or destabilizing beer-foaming components.
Many important fermenter technologies are engineered into the best craft brewing cylindroconical fermenters. Fermenters are usually engineered with one or many manway doors, which allow for cleaning the tank interior and inspection. Manways vary in size and cost, which is worked into the overall cost of the vessel. Some are equipped with a dry hop port, which is the opening in the tank, usually a Tri Clover (or TC) ferrule, that is used to add dry hops to the fermenters.
When it comes to sanitation, CIP fluids are often introduced through the top plate or perhaps at the bottom if ease of access is better below.
Cylindroconical fermenters lend themselves readily to temperature programming. The fermentation temperature can be automatically controlled on a preset program by means of a temperature recorder-controller operating the vessel jackets on the walls and the cone. Temperature probes are located at different depths to detect variations in temperature due to circulatory currents. A temperature probe should be located low on the tank so that it will be immersed when the tank is only partly full. If the vessel is used for conditioning in which the beer is cooled to near the freezing point, an additional probe higher up on the tank is advisable.
Traditional Ale Top-Skimming System
This is the traditional ale skimming system used in the United Kingdom (Figure 12.7). It utilizes top-fermenting yeast cropped for re-pitching by skimming from the surface. The vessels are normally shallow and flat-bottomed and may be round, square, or rectangular. Traditionally, these vessels are open-topped to facilitate skimming and are located in a well-ventilated room to disperse the carbon dioxide evolved during fermentation.
Open Square Fermenters
Traditional lager systems in Continental Europe and still common in Eastern Europe utilize open square fermenters similar to those used in the traditional ale top system. Enclosed fermenters are more common.
Yorkshire Square System
This system, as shown in Figure 12.8, is used in the north of England to produce ales with a clean, round palate. It is a variation of the ale fermentation system adapted to enable the use of a flocculent yeast strain or a two-strain system with one of the yeasts being flocculent. The vessels are normally square or rectangular in shape, and constructed of stainless steel. The Yorkshire square system consists of a lower compartment with a gently sloping upper deck located just above the fill level of the vessel.
Burton Union System
This system was commonly used in Burton-upon-Trent and produces pale ales with a fruity character by using “powdery” non-flocculent yeast (Figure 12.9). The wort is collected and pitched in a separate vessel and then transferred to the union set after 24 to 48 hours. The union set consists typically of 2 rows of wooden casks, 24 to 50 in number and each of 7 hL capacity, positioned under a cooled trough.
The vessel characteristics that make cylindroconicals so suitable for fermenting vessels also make them ideal for conditioning tanks. This has led to the installation of dual-purpose vessels, i.e., unitanks (short for “universal tanks”) where primary fermentation and conditioning are carried out in the same vessel. Unitanks have cooling jackets located high in the vessel for fermentation and low in the vessel for conditioning. Unlike cylindroconical fermenters, the cooling capacity of unitanks must be sufficient to achieve and hold at sub-zero conditioning temperatures.
Uninoculated wort is liable to contamination by many types of beer spoilage organisms—bacteria such as Pediococcus, spp. and Lactobacillus spp.; and wild yeasts such as Hansenula, Dekkera, Brettanomyces, Candida, and Pichia. Other Saccharomyces species may be present.
Click on the following topics for more information on beer fermentation.