Modern Industrial Microbiology and Biotechnology

252 Modern Industrial Microbiology and BiotechnologyThe green beer is stored in closed vats at a low temperature (around O°C), for periodswhich used to be as long as six months in some cases to mature and make it ready fordrinking.During lagering secondary fermentation occurs. Yeasts are sometimes added toinduce this secondary fermentation, utilizing some sugars in the green beer. Thesecondary fermentation saturates the beer with CO 2 , indeed the progress of secondaryfermentation is followed by the rate of CO 2 escape from a safety valve. Sometimes activelyfermenting wort or Kraeusen may be added. At other times CO 2 may be added artificiallyinto the lagering beer. Materials which might undesirably affect flavor and which arepresent in green beer e.g. diacetyl, hydrogen sulfide, mercaptans and acetaldehyde aredecreased by evaporation during secondary fermentation. An increase occurs in thedesirable components of the beer such as esters. Any tannins, proteins, and hop resinsstill left are precipitated during the lagering period.Lagering used to take up to nine months in some cases. The time is now considerablyshorter and in some countries the turnover time from brewing, lagering, andconsumption could be as short as three weeks. This reduction has been achieved byartificial carbonation and by the manipulation of the beer due to greater understanding ofthe lagering processes. Thus, in one method used to reduce lagering time, beer is stored athigh temperature (14°C) to drive off volatile compounds e.g. H 2 S, and acetaldehyde. Thebeer is then chilled at – 2°C to remove chill haze materials, and thereafter it is carbonated.In this way lagering could be reduced from 2 months to 10 days.Lagering gives the beer its final desirable organoleptic qualities, but it is hazy due toprotein-tannin complexes and yeast cells. The beer is filtered through kieselghur orthrough membrane filters to remove these. Some properties of lager beer are given in Table12.5.Table 12.5Some properties of lager beerProperty Pilsener United Danish English English Mounich DortmundStates Pilsener ale stout Lowenbraulager beerOriginal 12.1 11.5-12.0 10.6 15.0 21.1 13.3 13.6extractcontent o pcReal extract 5.3 5.5 3.1 5.0 8.7 6.4 5.5content o pcAlcoholic 3.5 3.4-3.8 3.9 5.2 6.7 3.6 4.2content, wt %Protein 0.28-0.35 0.3 0.6 0.6 0.5 0.8content, wt %CO 2 content % 0.53 0.5 0.4 0.41 0.42Color, EBC 10 2.7 5 40 8Air in 1.5 2 8 10 6bottle, mLpH 4.2-4.50 4Real degree of 60-75 69 66 59 48 60attenuation, %