Lesson 6 - Design of Fermentation Vessels
This unit began with the principles that underpin fermentation. This lesson now explores how these are put into practice. It covers how fermentation vessels must be designed to suit the behaviour of the yeast strain being used, as well as the nature, scale, and sophistication of the processes available.
Combined Fermentation and Cold Conditioning
At the end of fermentation, the beer usually undergoes a conditioning process where it is held at low temperature; this, primarily, improves its colloidal stability by allowing a proportion of beer proteins and polyphenols to coagulate and form cold break. The fermentation and cold conditioning processes usually happen in separate tanks but can be combined into one.
Cylindroconical fermenters are very versatile vessels. They can be used for fermentation and cold conditioning, and as bright beer tanks.
We often refer to combined fermentation and conditioning as uni-tanking, and we call the vessels uni-tanks, FVCTs, or dual-purpose vessels.
The process involves:
● Cooling at the end of fermentation to encourage yeast sedimentation
● Yeast crop removal
● Cooling to conditioning temperature
● Possibly adding beer stabilisation aids
● Holding, as required, then removing tank bottoms (tank bottoms being the solid cold break materials that form when beer is held at a low temperature)
● Transfering beer for filtration
Some vessels were specially designed for this purpose. Ashai vessels, for example, widely used in the USA in the 1960s and 1970s, were very large cylindrical vessels with a sloping floor and a recirculation system that was fitted with a heat exchanger to make cooling more efficient.
More commonly, we use cylindroconicals, though they need some adaptations to make them suitable for both process steps. We must increase the cooling capacity to make sure conditioning temperatures can be reached and in as short a time as possible. Often small, side-mounted mechanical stirrers are fitted, close to the top of the cone, to prevent stratification (as mentioned earlier regarding the cooling requirements of cylindroconical fermenters) and to help disperse process aids.
There are arguments both in favour and against the value of this dual process compared with the use of separate fermentation and conditioning vessels (see table below).
Table: Advantages and disadvantages of combined fermentation and conditioning
|
Advantages of DPVs |
Advantages of separate FVs & CTs |
|
Process can be managed with fewer expensive tanks |
Cooling in tank is inefficient and may take many hours |
|
Fewer fills, emptying, and cleans |
Two-tank system is more flexible allowing choice of use, either fermentation or conditioning |
|
Fewer transfers |
Tank transfer allows more efficient in-line cooling |
|
Reduced wetting losses |
Tank transfer gives opportunity for additions of process aids with efficient mixing |
|
Less risk of oxygen pickup during transfer |
Tank transfers allow solids removal via continuous centrifugation |