Unitanks (Universal Tanks)
In the late 1800s, developments continued with upright cylindrical vessels, some having sloping or conical bottoms. In the 1960s, a breakthrough in technology led to a new principle: primary fermentation and maturation could be carried out in the same vessel. This breakthrough gave birth to the modern “cylindroconical” tank, which is an upright cylindrical tank with a slanted, cone-shaped bottom. These tanks were called “universal tanks,” or unitanks for short. Unitanks became widely used in the 1970s, and over the past 40 years, the cylindroconical tank has replaced all other types of fermentation vessels, except for a few exceptions in Belgium and the British Isles where traditional beer styles are still brewed. In the brewing industry, it is widely accepted that cylindroconical tanks offer several advantages over older tank designs:
- They eliminate the need to move beer during the process, which greatly benefits beer quality, process times, space utilization, and production efficiency.
- Cleaning, sanitization, and microbiological control are far superior in cylindroconical tanks compared to older tank designs.
- They can be individually and affordably insulated.
- The degree of automation required in larger, modern breweries is much easier and less expensive compared to older tank designs.
- Collecting CO2 during primary fermentation for regeneration (and reuse/sale) is easy in cylindroconical tanks, unlike the old open fermenters.
- Yeast cropping is efficient, more selective, and more sanitary in cylindroconical tanks compared to older tank types.
Interestingly, the roots of the cylindroconical tank can be traced back to patents dating from 1908 and 1927. However, the revolution in stainless steel fabrication was necessary to make the manufacturing and use of such tanks practical. The proliferation of cylindroconical tanks began in Ireland in the early 1960s, where vertical stainless steel tanks (standing upright) with a capacity of up to 11,500 hectoliters (9,798 US barrels) were already in use. In 1965, the Japanese brewery Asahi patented a large vertical tank with a sloped bottom, allowing the yeast to be harvested from the low side. This was the first step towards what we now know as the cylindroconical tank. The Rainier brewery in Seattle, Washington, was the first large-scale brewery to install a large number of cylindroconical tanks around 1970. These tanks, known as Rainier tanks, had a conical bottom with a very shallow cone angle of only 25°. It was soon discovered that a steeper cone angle (70° is the modern standard) allowed for easier yeast cropping from the settled yeast in the cone bottom after fermentation. Further advancements focused on the number and placement of cooling jackets. Cooling jackets are hollow belts welded onto the outer surface (but inside the layer of insulation and outer cladding) of the tank, through which coolant (either subfreezing glycol or expanding ammonia) can circulate, cooling the contents of the tank. The placement and size of cooling jackets are critical to ensure optimal movement and mixing of the beer during fermentation, ensuring homogeneity, which is particularly important for large tanks. The evolution of CO2 during fermentation creates powerful currents in the fermenting beer. When jackets toward the top of the tank are cooled, the cooled liquid drops toward the bottom, while beer toward the bottom, warmed by fermentation, rises toward the top, creating circulation and mixing of the liquid, promoting faster, healthier, and more complete fermentations.
The overall geometry of tanks was also explored in the 1970s and 1980s, resulting in taller and slimmer cylindroconical tanks that saved floor space. However, it was found that once these tanks, referred to as “rocket tanks” due to their appearance, exceeded a height-to-diameter ratio of 5:1, good mixing could not be achieved, and excessive hydrostatic pressure affected the yeast. Therefore, modern cylindroconical tanks are generally built with height-to-diameter ratios between 1:1 and 5:1.
Modern cylindroconical tanks typically have multiple cooling jackets. This serves two purposes: optimizing thermal movement during fermentation and allowing the tank to operate at less than full capacity. Cylindroconical tanks also have a permanently installed “cleaning device” in the top, facilitating easy and automated internal washing of the tank. Additionally, there may be more than one outlet from the tank bottom, enabling beer to be drawn off without including the settled yeast in the cone bottom.
Cylindroconical tanks are well-suited for outdoor placement, which saves space and building costs for larger breweries. These tanks are usually located in tank farms, where they are placed closely together. Manual operations and servicing of the tanks do not take place in the tank farm itself. Instead, all piping to and from the tanks comes from the tank farm through an adjacent wall to the beer-processing area inside the brewery. Automatic pumps, valves, or swing arms needed for tank operation are placed here, reducing the need for long hoses.
State of the Art in Fermentation Vessels
Even today, the most modern breweries still utilize cylindroconical tanks. The next step in process development could be the introduction of continuous fermentation with immobilized yeast. The latest advancements in tank technology involve the accessories of the tanks, such as mechanical mixing (recirculation by pumping from bottom to top) of the fermenting beer and advanced control systems that automatically measure and regulate the fermentation process.
Despite the introduction of cylindroconical tanks, not all traditional tank types have disappeared. Many large breweries that brew traditional English ales continue to use square tanks with a penetrable ceiling, through which top-fermenting yeasts accumulate at the end of primary fermentation and can be automatically harvested using valves and jets. These tanks are known as “Yorkshire Square” tanks, named after their shape and origin in Yorkshire, where they became widespread during the 20th century. Modern Yorkshire squares can be as large in volume as cylindroconical tanks and are also closed and equipped with CIP facilities, making them just as easy to automate. However, they are more complex, expensive, take up more space, and are typically not suitable for outdoor placement.
Many British breweries still practice open fermentation, as they believe it has positive effects on yeast health and beer flavor. In Bavaria, open fermentation is occasionally used for the production of Weissbier and lager. Brewers there believe that open fermentation creates a more intense aroma profile in Weissbier compared to closed fermentation.
Marston’s Brewery in Burton-on-Trent, England, is known for championing the once-common Burton Union system, where fermentation takes place in oak vats and expelled yeast travels between the vats in open troughs. This system is still in use today, and Marston’s brewers claim that the flavor of their flagship beer would not be the same without it.