Fermentation vessels enclosed

Enclosed vessels are more easily cleaned by in-place methods and indeed some open vessels are covered temporarily by a light metal or plastic canopy, in order to clean them in-place. Again fermentations in enclosed vessels are less likely to become infected by air-borne or water spray-borne microorganisms. The advantages of open vessels are that they are somewhat less expensive, are easier to clean manually, and permit easier dipping of the vessel to judge the volume of wort present. The latter procedure is required by Excise authorities in Britain unless the volume of wort is measured in a collecting vessel used specially for this purpose. 

The Porton Mobile Enclosed Chemostat (POMEC) described by Evans and Harris-Smith was the first fermentation system designed and built to enable stirred batch (20 1) and continuous (2.5 1 vessel) culture of pathogenic bacteria to be carried out without risk of escape of any aerosols released from the fermenter vessel. A specially designed and constructed culture apparatus was contained within a purpose-built Class III type cabinet constituted of glass reinforced polyester resin. The various controls and measurement indicators were panel-mounted and accessible on the exterior of the cabinet. An inclined airlock with two UV lights and a liquid disinfectant lock (dunk tank) were set into the cabinet wall to allow safe... 

Enclosed vessels have several desirable features. Less cooling is needed because the heating of the beer by air is minimized. It has been calculated that refrigeration of enclosed vessels is about 15 % of that needed for open fermenters [22]. 

Carbon dioxide evolution is not dangerous in enclosed vessels (except for those who enter them) whereas it can be a continual hazard in those fermentation rooms having both open vessels and poor air circulation. About 4 % of carbon dioxide in air by volume can be a danger, even for short periods and legislation may limit levels in some countries to below 0-5 %. 

The carbon dioxide can of course be recovered readily from the enclosed vessels. For one unit weight of sugar, about half this weight is theoretically recoverable as carbon dioxide and, considering the cost of the gas and the great use for it in the post-fermentation period of beer treatment, the escape from open fermenters appears to be an expensive waste. In practice, the percentage of carbon dioxide recovered is rarely above 50% of the theoretical (see Chapter 20). There is, however, considerable debate on the economics... 

The most popular system of top fermentation uses a single vessel, which until recently was usually open, but now enclosed vessels have tended to oust them. In very deep vessels such as cylindroconical fermenters, the yeast forms little top crop and is mainly harvested at the base. However with open vessels and the more shallow enclosed fermenters, the yeast rises to the top of the beer at the end of fermentation and is either removed by suction or by skimming. The method is therefore called the skimm ng system . If during the fermentation, the beer is fed by gravity from one vessel to another, the method is called a dropping system . 

Yeast removed from the fermenter by suction is usually collected in an enclosed cylindrical vessel called a yeast back. The yeast is usually pumped from the back to a sheet filter press where entrained beer (barm ale) is recovered and after pasteurizing it is blended with racked beer (Fig. 19.16). Sometimes the yeast is held in the filter which is cooled by water or brine circulating in a jacket around it. In other cases, the yeast is discharged from the filter into trays or trucks which are placed in a refrigerator until it is required for pitching. 

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