The fermentation of the wort occurs in fer-mentor vessels of varying composition, size and configuration, and in either batch or continuous modes.Although most modern fermen-tors are now constructed of stainless steel, the traditional materials were wood, concrete, or copper. Size and shape depend on a number of considerations, but especially on whether the fermentation is top fermenting, as for ales, or bottom fermenting, as for lagers (see below).
For example, lager fermentation vessels are typically cylindric, with cone-shaped bottoms (cylindroconical), so that when the yeasts flocculate and settle, the cells collect within the conical region. Traditional fermentors, at least for ales, were also open or uncovered, so that the CO2 foam that developed during fermentation provided the only protection against the elements (e.g., oxygen in the air, airborne yeasts and bacteria, and other contaminants). Thus, although the yeast can be skimmed from the top and re-used, the CO2 is lost.
Currently, enclosed, pressurized, cylindro-conical fermentors are used for lagers, as well as ales, with capacities of nearly 106 L. Enclosed fermentors obviously have the advantage of reducing exposure to air and airborne contaminants; however, it is not possible in these fermentors to recover the evolved CO2. Whereas open fermentors, whether round or rectangular, are necessarily more horizontal (i.e., width > height), enclosed fermentors are usually constructed in a vertical orientation, so less floor space is required. Regardless of shape, however, most modern fermentors are jacketed to provide efficient cooling.
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