77%. LAGER YEASTS CAN FERMENT RAFFINOSE IN ADDITION TO THE SUGARS T...

67-77%. Lager yeasts can ferment raffinose in addition to the sugars that are fermentable by ale yeasts. These yeasts have traditionally been called bottom fermenters, since they do not cling together to form colonies on the surface, but instead fall to the bottom of the fermenter. Lager yeasts can be further subdivided into the Frohberg type (also called dusty or “powdery”) which ferment quickly, and do not flocculate as well. Due to the longer time it remains suspended in the wort, this subtype will have a greater attenuation. The other subtype of lager yeast is the Saaz type (also called the S.U. or “break”). These strains tend to flocculate more readily, and hence tend to have a lower attenuation (6). Lager yeasts, in comparison to ale yeasts, produce beers that lack the esters and fusel alcohols, since they are active at cooler temperatures. Lager beer styles should have a cleaner aroma to them, reflecting only the malt and /or hop aromas used to make the wort. Bacteria, specifically Lactobacillus delbrückii, are used in the production of the Berliner Weiss style of wheat beer with an intense lactic sourness. Other microorganisms are also used in the production of some Belgian ales, specifically lambics. Lambics have varying degrees of sourness which is appropriate for their style. Yeasts of the Brettanomyces genus and various bacteria generate these flavors. Bacteria are commonly divided into two broad classes based on a laboratory Gram stain. The Gram-negative bacteria involved in lambic production are Escherichia coli and also various species of Citrobacter and Enterobacter, but fortunately they cannot tolerate even moderate alcohol levels and do not survive in the finished beer. The Gram-positive bacteria involved are from genus Pediococcus and Lactobacillus. These microorganisms use a different pathway than that of Saccharomyces yeast known as a mixed acid fermentation pathway. It involves the esterification of the various alcohols to the corresponding carboxylic acids, thus generating lactic sourness (7). At low contamination levels, these Gram-positive bacteria may also be responsible for the sweet, butterscotch or buttery notes associated with diacetyl and related vicinal diketones. The Yeast Life Cycle When yeast are pitched into fresh wort, the overall process of fermentation can be divided into several stages or phases, all of which are part of the life cycle. While these stages can each be described separately, the transitions between each are continuous and should not be thought of as distinct parts of the life cycle. Also the relative time spent in each phase depends on several factors including the composition of the wort, the environment and the amount of yeast pitched. Most technical brewing references break the yeast life cycle into five phases of growth: lag, accelerating, exponential, decelerating and stationary (8, 9). Readers familiar with earlier versions of the BJCP Study Guide may recall that prior to this revision, the growth phase was referred to as a distinct phase in yeast development. Although that notation is consistent with some homebrewing references, the five phases listed above are more common in microbiology textbooks and technical brewing references. The first phase of the cycle is called the lag phase, which is sometimes referred to as the latent phase. During this time the yeast will adapt to the new environment they are now in and begin to make enzymes they will need to grow and ferment the sugars in the wort. The yeast will be utilizing their internal reserves of energy for this purpose, which is the carbohydrate glycogen. The yeast will acclimatize itself and assess the dissolved oxygen level, the overall and relative amounts of the amino acids and the overall and relative amounts of sugars present. Some of these amino acids, small groups of amino acids called peptides, and sugars will be imported into the cell for cell division. Normally this period is very brief, but if the yeast is not healthy, this period can be very protracted, and ultimately lead to problematic fermentation (1, 10). Based on these factors, the yeast will then move into the next phase of the life cycle, the accelerating phase. This is sometimes referred to as the low kräusen stage. During this time the yeast will start to divide by budding to reach the optimal density necessary for the true fermentation. The rate of cell division continuously increases during this phase. If an adequate amount of healthy yeast has been pitched and the proper nutrients are present, there should only be one to three doublings of the initial innoculum. The oxygen that was used to aerate the wort is absorbed during this time to allow the yeast to generate sterols, which are key components of the cell wall (10). It has also been proposed that cold trub can provide the unsaturated fatty acids needed for sterol synthesis (11, 12). Furthermore, it has been proposed that if an adequate amount of yeast has been pitched, such that cell growth is not necessary, then the oxygenation is not necessary (10, 13). While this theory has not been completely accepted (14, 15), perhaps further research will elucidate other variables which may be involved in this phenomenon. This sterol synthesis is the default pathway used in an all malt wort; however if the wort contains greater than 0.4% glucose then this pathway will not be used and the yeast will instead ferment the glucose, even in the presence of oxygen. This effect is called glucose repression, or the Crabtree effect. During the exponential phase, the growth rate is constant at the maximum rate determined by the yeast strain, temperature and wort composition. This phase is also referred to as the logarithmic (log) or the high kräusen phase. The yeast have now completely adapted to the condition of the wort, and the transport of both amino acids and sugars into the cells for metabolism will be very active. During this period, esters are formed by the esterification of fatty acids by ethanol and also possibly by the esterification of higher alcohols. Fusel alcohols can be produced by the conversion of amino acids to higher alcohols via deamination, decarboxylation and reduction processes. To minimize the formation of esters and fusel alcohols, the brewer should ensure that: (a) a healthy amount of freely available nitrogen (FAN) is available in the wort, (b) the wort is chilled to a maximum of 75 °F (24 °C) for ales and 55 °F (13 °C) for lagers prior to pitching the yeast, (c) the chilled wort is sufficiently but not excessively aerated prior to pitching the yeast, and (d) the fermentation temperature is maintained within the optimum range for the yeast strain. The fourth stage of the yeast life cycle is the deceleration phase or late kräusen phase, during which the growth rate gradually decreases. At this point, ale yeast strains will have metabolized most of the sugars present in the wort. Lager yeast strains, on the other hand, may still be reducing the extract by four gravity points/day, and this is important because it is during this time that the yeast begin to phase. Specifically, a diacetyl rest may be performed to help with the re-absorption and subsequent reduction of the diacetyl and the related diketones during this time. The temperature of the beer may be allowed to rise up to 68 °F (20 °C) during the diacetyl rest. The final stage is the stationary phase, during which the number of yeast cells remains approximately constant. The kräusen begins to fall, and the yeast drop out of suspension, or flocculate. During this deceleration phase, the specific gravity of the beer approaches its terminal point, and the yeast will begin to flocculate. This is the optimum time to rack the beer into a secondary fermenter, which allows for the attenuation of the last remaining extract, usually consisting of the trace sugars. Also removal of the excess yeast and trub will prevent formation of off flavors due to autolysis and/or reactions with trub substrates. For ale styles this period may be very brief, while lager styles may be four to six weeks, or even as long as six months in the case of strong lager styles. When lagering, it is important not to chill the beer too quickly, which might cause premature flocculation before the fermentation has been completed and all the by-products have been reabsorbed. The general rule of thumb is that a temperature drop should be no more than 5 °F (3 °C) per day; otherwise it is possible to cold shock the yeast. It is also important during this time to prevent reintroduction of air, since this can lead to oxidation flavors and may introduce contaminants that can infect the beer. During packaging of the beer, fresh yeast may often be reintroduced, particularly if it has been lagered for an extended period of time and/or the remaining yeast are not that viable. Two common methods are