Conditioning can occur by one of several methods. In traditional ale manufacture, the beer is pumped from the fermentor into wooden casks ranging in size from 20 L to more than 200 L. Stainless steel casks have replaced many of the wood casks, although the latter are still available. Sugar is added to induce the secondary fermentation, and additional hops and fining agents (see below) may also be added. The casks are held at 12°C to 18°C for up to seven days. Following the maturation period, this cask-conditioned or drought beer is ready for consumption, without any additional clarification steps (thus, the yeast sediment is still present).
For lager beers, conditioning or lagering occurs in tanks held at lower temperatures for a longer time. Typically, temperatures as low as 0°C,for as long as three months, can be used.A special variation of the lager method involves adding a portion of wort obtained at high krausen (i.e., cells in their most active growth phase) to the green beer. Since the krausen also contains wort sugars, this step, called krausening, essentially serves as a source of both fast-growing cells, as well as fermentable
Box 9—7. Flocculation—A Case of Beer Yeasts Sticking Together (Continued)
for the lectin-like protein, FLOlp), was used as the host organism.The expression of FLO1 was made inducible by replacing the FLO1 promoter with an inducible promoter derived from the HSP30 gene.
The latter is a heat shock gene that responds not only to heat, but also to nutrient depletion and high ethanol concentrations. Importantly,the HSP30 promoter (HSP30p) is also induced by stationary phase (i.e., a late-fermentation).Thus, expression of FLO1 behind HSP30p would be expected to occur in the transformant only at the end of the fermentation. Indeed, this strategy was successful, as flocculation was evident in the transformants during stationary phase (at about 95% of the wild-type level).As expected, heat shock or ethanol additional also induced flocculation. The phenotype was stable for 250 generations, and when inoculated into wort, about 90% of the transformed cells sedimented at the end of the fermentation, indicating that flocculation had occurred.
These results indicate that it is now possible to improve the flocculation properties of yeast strains. It is noteworthy that the actual strategy used to modify gene expression in this organism involved a simple self-cloning or benign approach.Thus,the transformants contained no foreign DNA that might warrant legal or public scrutiny.
Halme,A., S. Bumgarner, C. Styles, and G.R. Fink. Genetic and epigenetic regulation of the FLO genefamily generates cell-surface variation in yeasts. Cell 116:405-415. Javadekar,VS., H. Sivaraman, S.R. Sainkar, and M.I. Khan. 2000.A mannose-binding protein from the cell surface of flocculent Saccharomyces cerevisiae (NCIM 3528): its role in flocculation.Yeast. 16:99-110. Kobayashi, O., N. Hayashi, R. Kuroki, and H. Sone. 1998. Region of FlO1 proteins responsible for sugar recognition. J. Bacteriol. 180:6503-6510. Sampermans, S., J. Mortier, and E.V. Soares. 2005. Flocculation onset in Saccharomyces cerevisiae: the role of nutrients.J.Appl. Microbiol. 98:525-531. Verstrepen, K.J., G. Derdelinckx, F.R. Delvaux, J. Winderickx, J.M. Thevelein, F.F. Bauer, and I.S. Pretorius. 2001. Late fermentation expression of FLO1 in Saccharomyces cerevisiae. J. Am. Soc. Brew. Chem. 59:69-76.
Verstrepen, K.J., G. Derdelinckx, H. Verachtert, and F.R. Delvaux. 2003. Yeast flocculation: what brewers should know.App. Microbiol. Biotechnol. 61:197-205.
substrates. Since the vats are closed to the atmosphere, the CO2 that evolves is trapped, and the beer becomes naturally carbonated.
Traditional conditioning of lagers and ales is time-consuming and expensive, and therefore, used mainly for premium beers that can bear higher production costs. Most modern breweries have adopted faster methods, referred to as accelerated lagering or brewery-conditioning. In these systems, the fully attenuated beer is pumped into storage tanks at 0°C to 2°C and held for one week or less. Small wood chips (less than 3 cm X 30 cm) may be layered on the bottom of the tank to promote maturation (e.g., "beechwood aging"), as well as serve a clarification function (see below). During the fermentation and later during conditioning, many volatile flavor compounds are formed, which have a major impact on the overall quality of the beer (Table 9-5).
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