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.
Fermentation Vessel Construction
Now we have considered what we need from a fermenter, we look at fermenter designs, beginning with the use and choice of different construction materials.
Fermentation vessels have been in use for a long time. Understandably, some of the older designs employ traditional materials in their construction. These include wood, stone, ceramics, concrete, and various metals such as copper, iron, and aluminium. Wooden vessels are often coopered using the same skills needed to build casks.
More recently, some craft brewers, usually working at quite small scale, have chosen plastics. Most modern vessels, however, are made from stainless steel.
Whichever material is chosen, it must have certain properties, partly in order to provide the essential features that we looked at in the previous section. The chosen material must be:
● Capable of being worked to form the required shape.
● Of sufficient mechanical strength to withstand the weight of the contents.
● Inert, with surfaces which are easily cleaned, are not susceptible to build-up of scales, do not impart taints, and are food grade.
● Resistant to corrosion caused either by cleaning agents or the relatively low pH of wort and beer.
● Good insulators, if cooling is via internal attemperation, or efficient heat conductors, if external cooling is used.
Many traditional materials do not fit this list of requirements very well and to overcome some shortcomings a combination might be used, for example an outer carcase of wood or stone providing mechanical strength might be lined with another material such as a metal which is more suitable for contact with the fermenting wort.
Many traditional fermenters have been adjusted, and readjusted, over the years to eliminate some of their weaker properties.
Traditional open squares, used in the UK for ale fermentations, were often made from wood with a liner of copper. This metal has a lot of excellent properties but is hard to work into the right shape and very difficult to keep clean and so when stainless steel became easily available at the right price this was often used to replace the copper liners.
After the change to stainless steel, it was often observed that the vessels were a lot easier to clean but that the beers acquired a strong hint of ‘rotten egg’ because of high levels of hydrogen sulphide (H2S).
The brewers quickly realised that the original copper liner was very efficient at mopping up sulphides and that changing to stainless steel had introduced an unexpected quality problem. This was solved by introducing a sacrificial copper electrode into the brewhouse.
Stainless steel is the material of choice for most brewing vessels. It is easily worked into a variety of shapes, has great mechanical strength, is a good conductor of heat, can be polished to give a good cleanable surface, and (providing it is looked after) is inert.
Austenitic grades (types 316, 304) are used; these contain relatively high levels of nickel and chromium but low levels of carbon. Before use, surfaces are treated in a process called passivation. This process deposits a very thin protective layer of oxide on the surface of the stainless steel. The oxide layer must be entire and so prior to treatment all surfaces must be clean and degreased by a first treatment with caustic soda. The passivation treatment involves washing with an acid such as nitric or citric.
Stainless steel is susceptible to corrosion if not looked after properly. The layer of oxide must be protected. High levels of chloride at temperatures above 40°C must be avoided and welders must take care to avoid setting up areas liable to corrosion.
We now move on to explore specific types of fermenting vessel. For convenience, this lesson divides them into two groups based on the cropping behaviour of brewing yeast. The next section looks at those for top cropping yeasts.