Yeast and Fermentation

by Chuck Hanning and Scott Bickham

Introduction

Most beer styles are made using one of two unicellular species of microorganisms of the Saccharomyces genus, more commonly called yeast. Generally, either an ale yeast strain (known as S. cerevisiae) or a lager yeast strain (known as S. pastorianus or by older terminology S. carlsbergensis or S. uvarum) is used for the appropriate style. Functionally these yeasts differ in their optimum fermentation temperatures, ability to ferment different sugars, environmental conditions, and ability to settle out upon completion of fermentation, and production and/or metabolism of fermentation by-products. The choice of the strain of ale or lager yeast and how these factors are controlled during the various stages of fermentation will determine how well a beer is made to style. While a list of all the possible strains is beyond the scope of this guide, readers are encouraged to review reference (1) for a more thorough review.  Most yeast suppliers provide detailed descriptions of the various commercial strains they carry (2), as do homebrew shops or other retailers of their products.

One of the common terms used to describe yeast is apparent attenuation. The attenuation of a particular yeast strain describes its ability to decrease the original gravity of wort upon fermentation. It is commonly listed as a percent, in which the numerator is the difference between final and original gravity and denominator is the original gravity. Because the density of ethanol is less than water, when a hydrometer is used to measure this attenuation, it will be measuring the apparent attenuation not the real attenuation (if the alcohol was replaced by water). Another common term used to describe different yeasts is flocculation, which is the ability of the yeast to settle out of the beer upon completion of fermentation; it can vary significantly with strain.

The environmental conditions that differ with each yeast type and strain are alcohol tolerance, oxygen requirements, and sensitivity to wort composition. Alcohol tolerance describes how well a yeast strain will continue to ferment as the alcohol concentration increases during fermentation. Most lager yeast can ferment up to about 8% alcohol by volume, and some ale strains can ferment up to 12% (2, 3). Oxygen requirements can differ with each strain as well; some need much more oxygen to be able to ferment without problems. Lastly, different worts will have different relative amounts of sugars present. The various strains can respond differently to the same wort upon fermentation.

The by-products that are produced (and also metabolized) by the yeast are esters, fusel alcohols, diacetyl, and sulfur compounds. Esters are produced by yeast combining an organic alcohol and an acid. While approximately 90 different esters have been identified in beer, ethyl-acetate, isoamyl-acetate and ethyl-hexanoate are most commonly above their flavor thresholds. These impart a fruity, sweet aroma to the beer. Another by-product of fermentation is fusel alcohols, which contain more carbon atoms than the most common alcohol, ethanol. These are produced by the metabolism of amino acids (4), and tend to add harsher, more solvent-like tones the beer. Yet another by-product is diacetyl, which is generally reduced to more benign compounds during the secondary fermentation, but premature removal of the yeast (among other things) can lead to elevated levels. Its presence imparts a buttery note to the beer. It is produced by an oxidation reaction, which can be repressed by the production of the amino acid valine (5). Lastly, there are several sulfur compounds that can be produced by the yeast. One of these is hydrogen sulfide, which smells like rotten eggs. Other sulfur compounds exist, but their production is not yet completely understood (1).

Ale Yeast, for the purposes of beer fermentation, tend to work best in the 55-75 °F (13-24 °C) temperature range. Apparent attenuation can range from 69 to 80%. These yeasts can fully ferment the common sugars glucose, fructose, maltose, sucrose, maltotriose and the trace sugars xylulose, mannose, and galactose. They can partially ferment raffinose. These yeasts have traditionally been called top fermenting because they form colonies (groups of yeast that cling together) that are supported by the surface tension of the beer. Ale yeasts produce esters since they require higher temperatures to remain active. Styles that use these yeasts have varying degrees of fruity and sweet smelling aromas. It should be noted that the yeast used to produce the German weizen style are special strains that generate high concentrations of the clove-like phenols and “bubblegum” and “banana” esters, which are the signatures of this style.

Lager Yeast generally tend to work best between 46-56 °F (8-13 °C), but California Common Lager yeast is an exception, having a range of 58-68 °F (14-20 °C). Apparent attenuation usually ranges from 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 metabolize some of the fermentation by-products they had previously excreted during the low kräusen 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 1) bottle conditioning, or the addition of a fresh yeast starter and corn sugar (glucose), as is commonly done for Trappist-style Belgian ales, and 2) kräusening, or the addition of freshly fermenting beer as is often practiced with German lagers. For bottle conditioned beers, a 250 ml starter is usually added for a five gallon (20 liter) batch along with the sugar; which provides fresh yeast to metabolize the added sugar. In the case of kräusening, an actively fermenting batch at high kräusen stage is added to the beer being primed. The volume of kräusen added is typically 20% by volume of the beer being primed. Adding this actively-fermenting beer serves two purposes; it carbonates and also helps clean up any off flavors generated from the previous fermentation.

Control of Fermentation By-Products

Esters may be controlled by the choice of yeast strain, wort gravity, wort aeration, and fermentation temperature. In general, ale yeast strains produce higher ester levels, although there are variances among different ale strains. Lager yeast strains can, if fermented too warm, also produce esters as is practiced in the making of French Bière de Garde styles. Wort gravity also is a factor; the hallmark esters of Belgian Trappist styles are not only due to the yeast strains used but also a result of their high gravity wort. Wort aeration also plays an important role, as the ester production pathway directly competes with the absorption of oxygen and metabolism into sterols (16). Lastly, fermentation temperature also plays an important role. A four-fold increase in ester production may be observed as a result of increasing the fermentation temperature from 60 to 68 °F, 16-20 °C (1).

Phenols can be produced by certain wild yeast strains. Hence, their control in styles in which they are not desired is a matter of proper sanitation. One exception to this is German wheat beers, which contain the phenol 4-vinylguaiacol, which give a clove-like flavor.  This phenol is produced by a special strain of S. cerevisiae when its precursor ferulic acid is available in the wort.  The level of this amino acid may be increased by including a ferulic acid rest at 111 °F, 44 °C in the mash (17).  Please refer to the Beer Characteristics section below for more information. 

Fusel alcohols are metabolized from amino acids. As mentioned previously, their production is increased as the fermentation temperature is increased. Also, like esters, fusel alcohols increase with wort gravity. Lastly, various wild yeasts tend to produce excessive amounts of fusel alcohols; hence, proper sanitation is important for their reduction (1).

Diacetyl precursors are produced during yeast metabolism, but they are not converted into diacetyl until the active stages of fermentation.  Factors that increase the probability of this occurring include higher fermentation temperatures and the introduction of oxygen, but the yeast strain is also an important consideration.  Diacetyl and related compounds can only be removed by the yeast, and only through the conversion to the nearly flavorless compound, butanediol.  Several factors can help with this process, including maintaining a sufficiently high fermentation temperature during the decelerating and stationary phases.  As noted above, higher fermentation temperatures also promote the formation of diacetyl, but the effect on the reduction is even greater, so the net result is a lower level of diacetyl in the finished beer.  This is often a confusing dichotomy for beer judges, but it is the key motivation for performing a diacetyl rest for a lager beer style.  Diacetyl reduction is also highly dependent on ensuring that the yeast remains in contact with the beer, so premature removal of the beer from the yeast can also lead to an elevated diacetyl level.  Finally, as noted above, diacetyl can be produced by Gram-positive bacteria, hence proper sanitation and control during yeast propagation will help minimize its presence (1).

References

1. J. Fix & L. A. Fix, An Analysis of Brewing Techniques (Brewers Publications, 1997).
2. Information about specific strains is available from most yeast suppliers, including Wyeast and White Labs.
3. Busch, “A Matter of Immense Gravity”, Brewing Techniques 4(2), 20 (1996).
4. J. Fix, Principles of Brewing Science (Brewers’ Publications, 1989).
5. J. Fix, “Diacetyl: Formation, Reduction and Control” Brewing Techniques 1(2), 20 (1993).
6. J. Noonan, New Brewing Lager Beer, Brewers’ Publications, pp. 89-99 (1996)
7. Liddil, “Practical Strategies for Brewing Lambic at Home”, Brewing Techniques 5(4),38 (1997).
8. Malting and Brewing Science, Vol. II, J.S. Hough, D.E. Briggs, R. Stevens and T.W. Young, p. 617 (Chapman and Hall, London, 1982).
9. Kunze, Technology Malting and Brewing, p. 80 (VLB, Berlin, 1996).
10. -X. Guinard, M. Miranda, & M. J. Lewis, “Yeast Biology and Beer Fermentation”, Zymurgy 12(4), 14 (1989).
11. Aquila, “The Biochemistry of Yeast” Brewing Techniques 5(2) pp. 50-57 (1997)
12. W. Knull, “Readers’ Technical Notes: The Trouble with Trubless Fermentations”, Brewing Techniques 4(5), 14 (1996).
13. Daughty, J. Adkins, and S. Bickham respond to reference 13 in Readers’ Technical Notes, Brewing Techniques 5(1), 16 (1997).
14. J. Noonan, Scotch Ale (Brewers Publications, 1993).
15. Miller, “Readers Technical Notes: Putting in a Good Word for Wort Aeration”, Brewing Techniques 5(3), 10 (1997).
16. Rajotte, Belgian Ale (Brewers’ Publications, 1992).
17. Warner, German Wheat Beer (Brewers Publications,1992