The best treatise I have read about the development and use of unitank fermenters is in the textbook Brewing Yeast and Fermentation by Boulton and Quain. Although the subject is covered in other brewing texts intended for the commercial brewer, for example Technology Brewing and Malting by Wolfgang Kunze, Boulton and Quain cover many of the practical challenges of unitank fermentation, such as temperature stratification upon cooling, in great detail and discuss solutions to these very real challenges. D. R. Maule wrote an excellent historical account of fermentation design in his “A Century of Fermenter Design” published as a Centenary Review in the Journal of the Institute of Brewing (J. lnst. Brew., March-April, 1986, Vol. 92, pp. 137-145) that covers the evolution of fermenter design from the late 1800s through the publication date. I will offer some thoughts about your question from the perspective of a small, craft brewer.
The development of the modern, single-tank, fermentation and aging system can be traced back to Dr. Leopold Nathan’s cylindroconical tank designs that were patented in 1908 and 1927. Nathan was a Swiss chemist and his designs differed from conventional fermenters at the time because his fermenter design was closed, had a conical bottom, and incorporated a method to strip sulfur volatiles from beer after fermentation with gas. One of his patented designs incorporated an agitator and resembles a modern yeast propagation vessel. The primary design challenge with these vessels was the inability to easily and effectively clean the inside. Clean-in-place (CIP) developments in the early 1960s are credited to broader use of Nathan-type fermenters. Today, the cylindroconical fermenter (CCV) or unitank fermenter is the norm for breweries of all sizes. It is important to note that the term “unitank” was first used by the Rainier Brewery in 1968 to describe a new twist on the Nathan fermenter; the main differences were a 1:1 aspect ratio (shorter and squatter than the Nathan fermenter) and shallower cone. Although small-scale brewers chuckle about the concern of hydrostatic head in fermenters, tanks that have liquid columns taller than about 30 ft. (9.1 m) do present challenges, and this was one of the issues addressed with the Rainier unitank (and Asahi tank design from 1965).
The main features of these designs are that they are closed, permit for fermentation and aging in the same vessel, include sufficient cooling area to quickly chill beer to near-freezing temperatures, and allow yeast to be separated from beer. The following is a description of how many brewers use unitanks.
- Wort is cooled and aerated between the brewhouse and the fermenter. The cooling set point is targeted to about 2 °F (1 °C) below the fermentation temperature set point. This is important because the tank is mixed as fermentation begins. If coolant is allowed to flow into the cooling jackets before fermentation causes liquid movement, temperature stratification will occur where the bottom of the tank becomes very cold. This can cause major delays in fermentation.
- Although it is common to see foam, yeast, and beer flowing from unitank vent lines during fermentation, most commercial brewers try to avoid this from occurring because it represents loss and is just another thing to clean. Most unitanks are designed to be filled to 70–80% of the vessel’s gross volume. In general, skinnier tanks with higher aspect ratios (height:diameter) require more headspace than squattier tanks because the skinnier tanks have more tank wall contact area per volume of beer, and foam stability increases with wall area. Many breweries use anti-foaming agents, like polysiloxane, to keep foaming to a minimum during fermentation, allowing brewers to maximize tank space.
- The progress of fermentation is tracked on a regular schedule by taking a sample, degassing, and measuring specific gravity. Regular sampling provides information about how things are going, allows for sensory evaluation, and is especially important for beers that are spunded, or capped, for natural carbonation. Spunding is a great way to naturally carbonate beer and is very simple given the proper tools. If you want to use this method, it is imperative that the tank’s design pressure is not exceeded and that the tank is equipped with a pressure relief valve.
- Into hoppy IPAs? No problem! Unitanks are easy to dry hop, and the most common method is to simply add hop pellets into the fermentation from the top of the tank before fermentation ends. The main benefit to dry hopping at this stage is oxidation mitigation (OMG = oxygen mitigation is good, and it’s really cool to see this uber beer geek term used so often in social media); since carbon dioxide is still being produced any oxygen entering the tank headspace will be scrubbed out, and oxygen in the hop pellets will be consumed by the yeast cells. Dry hopping after fermentation is complete requires precautions. Hopping during fermentation also allows for biotransformation of hop compounds.
- Once everything to do with fermentation is complete, the unitank is chilled. Although some brewers cool to an intermediate temperature, most brewers crash cool the tank down to about 32 °F (0 °C). This step accomplishes a few important things. The first is that chilling the beer increases yeast flocculation and initiates yeast sedimentation as the floc sizes grow the floc density increases. Yeast can be harvested and re-used, and most breweries prefer cropping the yeast as soon as it settles into the tank cone. If you don’t plan on re-using this yeast, it is still important to remove it from the tank cone because yeast autolysis may lead to off flavors, especially in larger fermenters where the yeast mass in the cone of the fermenter can become very warm from continued metabolic activity.
Deep chilling also causes chill haze to form. Provided sufficient time, these haze particles will settle. The challenge with large, commercial fermenters is that the tall tank height means that gravity clarification can take a very long time to occur. Finings, filtration, and centrifugation are the common methods used to speed beer clarification. At home, very clear beer can be produced with or without finings depending on the grist bill and yeast strain being used.
- Unitanks are so-named because there is no other vessel used for fermentation and aging. This name is a bit of a misnomer in that many brewers use them. The majority of beer produced by craft breweries is not completely carbonated in unitanks. In fact, many brewers do not spund their fermentations and rely on in-line or in-tank carbonation in a separate tank to increase beer carbonation before packaging. As long as your unitank is designed for pressure, in-tank carbonation is very easy; simply increase the headspace pressure in accordance with a carbonation table based on the beer temperature. Given sufficient time, which is not a problem if using gravity clarification, the beer will absorb carbon dioxide from the tank headspace.