One of the major byproducts produced during wort fermentation by yeast is ethanol. Alcohol impacts the final beer mainly by intensification of the alcoholic taste and aroma and by imparting a warming character. However, it is the types and concentrations of the other yeast byproducts that primarily determine the flavor of the product. The formation of byproducts is influenced by wort composition, brewing practices, and the choice of yeast strain. For example, non-flocculent yeasts tend to produce more volatiles than do flocculent strains. Lager yeasts produce more fatty acids and sulfur byproducts than do ale yeasts. Mutant strains of yeast have been known to produce markedly different levels of vicinal diketones.
There are many flavor-active acetaldehydes present in beer that are formed at various stages in the brewing process. They are produced by oxidation of alcohols and various fatty substances. Acetaldehyde levels peak during the early to mid-stages of primary fermentation or immediately after kräusening, then decrease in concentration (Russell, 1995). By the end of the primary fermentation, aldehyde is reduced to ethanol. If oxygen is introduced back into the process, the ethanol is oxidized back into acetaldehyde.
Diacetyl and 2,3-Pentanedione
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. Quantitatively, diacetyl is the most important since its flavor threshold is approx. 0.1 mg/L and is ten-fold lower than that of 2,3-pentanedione.
Esters are considered the most important aroma compounds in beer. They make up the largest family of beer aroma compounds and in general impart flowery and fruit-like flavors and aromas to beer. In moderate quantities, they can add a pleasant, full-bodied character to beer aroma. When present in excess, however, they give beer aroma an overly fruity quality, which is considered undesirable by most consumers. Esters are more desirable in most ales, and in some dark or amber lagers, lower levels are preferred in pale lagers.
Fatty acids are minor constituents of wort and increase in concentration during fermentation and conditioning. They give rise to “goaty”, “soapy”, or fatty flavors and can cause a decrease in beer foam stability.
The major higher alcohols (also known as fusel alcohols) found in alcoholic beverages are the aliphatic alcohols. These compounds may have both positive and negative impacts on aroma and flavor. Large quantities of these higher alcohols (>300 mg/L) in beer can lead to a strong “alcoholic” or “solvent-like” aroma and taste, whereas optimal levels impart desirable characteristics (Olaniran et al., 2017).
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.
Organic acids contribute to the decrease in pH observed during fermentation and many are flavor-active. The most important organic acids found in beer are acetic, citric, lactic, malic, pyruvic and succinic.
Sulfur compounds make a significant contribution to the flavor of beer. Although small amounts of sulfur compounds can be acceptable or even desirable in beer (e.g., Burton ales), in excess they give rise to unpleasant off-flavors. Many of the sulfur compounds present in beer are not directly associated with fermentation but are derived from the raw materials employed. However, the concentrations of hydrogen sulfide (rotten egg aroma) and sulfur dioxide (burnt match aroma) are dependent on yeast activity. Failure to manage fermentation properly can result in unacceptably high levels of these compounds occurring in the finished beer.
Of the sulfur compounds, hydrogen sulfide is found in the greatest concentrations. Knudsen has reported its threshold at 10 µg/L in lagers and 30 µg/L in ale (Knudsen, 1985a). Hydrogen sulfide is very volatile; it can be reduced during fermentation either by applying carbon dioxide counter-pressure or by ascending carbon dioxide gas during prolonged maturation times.
Sulfur dioxide is also present in beer, although usually in concentrations well below 10 mg/L, at which level it does not have a flavor impact in most beer (Munroe, 1995a).
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. Dimethyl sulfide in lagers it will lead to a malty/sulfury note. The taste threshold for DMS is considered to be from 50 to 60 µg/L (Kunze, 1996). If the concentrations are too high; it has a relatively objectionable taste and aroma of cooked sweet corn.
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