Many pilot scale fermentation systems have been built specifically to model, on a semi-industrial scale, experimental designs derived from smaller laboratory scale prototypes. Some of these have been described already in previous sections of this chapter. For example, pilot scale fermentation systems have been much employed in the development of continuous and immobilised fermentation systems.
In addition to these experimental systems, more conventional but relatively small-scale fermenters are used as part of pilot scale breweries. These breweries may be used to investigate new processes at a scale intermediate between laboratory and production plant but are probably used most often to develop new beers. In this sense, the fermenters need to be capable of replicating, on a small scale, the behaviour of production scale fermenters. The requirement to produce, on a small scale, beers of the same quality as those made in large-scale breweries is now even more pertinent, further to the current upsurge in the 'brewpub' or 'microbrewery' (Cottone, 1985; Lewis & Lewis, 1996). It is true that many of these enterprises produce top-fermented ales where small volume and shallow vessels do not present any difficulties.
The design of scaled-down versions of cylindroconical vessels, as used for the production of bottom-fermented beers, in particular the optimum aspect ratio, is perhaps more problematic. Since there is a relationship between degree of agitation, vessel height and extent of yeast growth during fermentation it would be predicted that to achieve a match with production scale fermenters, small vessels should have a relatively higher aspect ratio. Thus, Meisel and Huggins (1989) concluded that an appropriate aspect ratio for a 32 litre capacity cylindroconical fermenter was 10 to 6:1. Fermentation performance and beer quality obtained with this design compared very well with a 660 hi production vessel.
Cooling jacket performance should be rated in accordance with the capacity of the vessel. It is not possible to duplicate accurately the effects due to the high hydrostatic pressures associated with tall vessels although elevated pressure may be applied to the contents of a small fermenter by restricting the escape of carbon dioxide. If possible the mains attached to pilot scale vessels should be of a diameter appropriate to the capacity of the vessel. Where pilot scale fermenters are devoted largely to experimental work, it is useful to provide several ports, at various heights, suitable for the installation of various probes. When not in use, the ports may be sealed with hygienic blanking devices. In addition, it is useful to be able to remove samples from several heights within the vessel. However, it should be appreciated that a multiplicity of over-intrusive probes will disrupt the normal patterns of mixing and agitation to a greater extent in a small vessel than would be the case in a production scale fermenter.
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