metals from the steel which would interfere with the fermentation. AISI Grade 304, which contains 18.5% chromium and 10% nickel, is now used extensively for brewing equipment.
The thickness of the construction material will increase with scale. At 300,000 to 400,000 dm3 capacity, 7-mm plate may be used for the side of a vessel and 10 mm plate for the top and bottom, which should be hemispherical to withstand pressures.
At this stage it is important to consider the ways in which a reliable aseptic seal is made between glass and glass, glass and metal or metal and metal joints such as between a fermenter vessel and a detachable top or base plate. With glass and metal, a seal can be made with a compressible gasket, a lip seal or an 'O' ring (Fig. 7.8). With metal to metal joints only an 'O' ring is suitable. This is placed in a groove, machined in either the end plate, the fermenter body or both. This seal ensures that a good liquid- and/or gas-tight joint is maintained in spite of the glass or metal expanding or contracting at different rates with changes in temperature during a sterilization cycle or an incubation cycle. Nitryl or butyl rubbers are normally used for these seals as they will withstand fermentation process conditions. The properties of different rubbers for seals are discussed by Buchter (1979) and Martini (1984). These rubber seals have a finite life and should be checked regularly for damage or perishing. Before purchasing a fermenter it is important to check that standard sized 'O' rings can be purchased cheaply from local suppliers.
A single 'O' ring seal is adequate for GILSP and levels 1 and B2, a double 'O' ring seal is required for levels 2 and B3 and a double 'O' ring seal with steam between the seals (steam tracing) is necessary for levels 3 and B4 (Chapman, 1989; Hambleton et al., 1991). In the U.S.A., however, simple seals are used at contain-
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