Chapter 4

Brewers Yeast

Yeast Byproducts

The flavor and aroma of beer is very complex, being derived from a vast array of components that arise from a number of sources. Not only do malt, hops, and water have an impact on flavor, so does the synthesis of yeast, which forms byproducts during fermentation and maturation. The most notable of these byproducts are, of course, ethanol and carbon dioxide; but in addition, a large number of other flavor compounds are produced such as:

Aldehydes

There are many flavor-active aldehydes present in beer. These are formed at various stages in the brewing process and are produced by oxidation of alcohols and various fatty substances. Aldehyde levels reach a maximum during primary fermentation or immediately after kraeusening, then decrease (51). Aldehyde is reduced to ethanol by the end of the primary fermentation. If oxygen is introduced back into the process, the ethanol is oxidized back into acetaldehyde.

Esters

Esters are considered the most important aroma compounds in beer. They make up the largest family of beer aroma compounds and in general impart a "fruity" character to beer. Esters are more desirable in ales than in lagers. Kunze reports that ester production is increased by 1) high fermentation temperatures, 2) restricting wort aeration, 3) increasing the attenuation limit, and 4) increasing the wort concentration to above 13% P (30). In addition, the type of yeast affects ester levels. Most of the esters are formed during primary fermentation, and some ester formation occurs during maturation. However, the level of esters could double with a long secondary fermentation (30).

Diacetyl

Diacetyl and 2,3-pentanedione, which are classified as ketones, are important contributions to beer flavor and aroma. Often these two ketones are grouped and reported as the vicinal diketone (VDK) content of beer, which is the primary flavor in differentiating aged beer from green beer. Of the two, diacetyl is more significant because it is produced in larger amounts and has a higher flavor impact than 2,3-pentanedione. A buttery or butterscotch flavor usually indicates the presence of diacetyl, while 2,3-pentanedione has more of a honey flavor.

Dimethyl Sulfide

Another major compound responsible for sulfury flavors in beer is dimethyl sulfide (DMS), which is a desirable flavor component in lager beer but not in ales. In lagers it will lead to a malty/sulfury note. The taste threshold for DMS is considered to be from 50 to 60 µg/liter (30). DMS also enhances the malt character of beer.

Fatty Acids

Fatty acids are minor constituents of wort and increase in concentration during fermentation and maturation. They give rise to goaty, soapy, or fatty flavors and are recognized as common flavor characteristics in both lagers and ales; but they are more prevalent in lagers because of the tendency of some lager yeast strains to produce greater quantities of fatty acids than do strains of ale yeast (34).

Fusel Alcohols

Fusel alcohols are a group of byproducts that are sometimes called "higher alcohols." They contribute directly to beer flavor but are also important because of their involvement in ester formation. Fusel alcohols have strong flavors, producing an "alcoholic" or "solvent-like" aroma. They are known to have a warming effect on the palate. About 80% of fusel alcohols are formed during primary fermentation (31). The yeast strain is very important, with some being able to produce up to three times as much fusel alcohols as others (21).

Nitrogen Compounds

Yeast also excretes some nitrogen compounds during fermentation and maturation as amino acids and lower peptides, which contribute to the rounding of the taste and an increase in palate fullness. Harvesting of the yeast too soon can therefore produce empty, dry beers even when they are subsequently lagered for a long time (31). The beginning of autolysis can be detected by an excessive increase in the amino acid content.

Organic Acids

Some of these organic acids are derived from malt and are present at low levels in wort, with their concentrations increasing during fermentation. Other acids are produced solely as a result of yeast metabolism. Organic acids can directly effect the flavor of beer by lowering its pH.

Sulfur Compounds

Volatile sulfur compounds such as hydrogen sulfide, dimethyl sulfide, sulfur dioxide, and thiols make significant contributions to beer flavor. When present in small concentrations, sulfur compounds may be acceptable or even desirable, but in excess they give rise to unpleasant off-flavors, e.g. rotten-egg flavors. Three main sources of sulfur compounds in beer are raw materials (malt and hops), yeast metabolism, and spoilage organisms – in particular Zymomonas anaerobia, Enterobacter aerogenes and Hafnia protea.

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