Yeast and the Biology of Fermentation

A small, single-celled fungus, given a warm bath of sugar water, will quietly convert that sugar into ethanol and carbon dioxide and, almost as an afterthought, produce most of what people taste in beer. The fungus has been doing this for considerably longer than anyone has been watching, and the watching itself only really got going in the nineteenth century. Everything about modern brewing — the temperature controls, the stainless steel, the laboratory plating — exists in service of giving this organism a predictable place to do what it was already going to do.

The organism, formally

The yeast used in nearly all commercial beer belongs to the genus Saccharomyces, which translates, with some accuracy, as "sugar fungus." Two species do almost all of the work. Saccharomyces cerevisiae ferments warm, near the top of the vessel, and produces the family of beers commonly called ales. Saccharomyces pastorianus, a hybrid that includes S. cerevisiae and the cold-tolerant S. eubayanus, ferments cool, settles to the bottom, and produces the family of beers commonly called lagers. According to a peer-reviewed review hosted by NCBI PubMed Central on Saccharomyces cerevisiae and beer flavor, the flavor compounds produced by these organisms — esters, higher alcohols, vicinal diketones, sulfur compounds — are the dominant non-malt, non-hop contributors to a finished beer's character.

There are other fermenting microbes that matter to brewers, and they matter a great deal in specific traditions. Brettanomyces yeasts, sometimes called wild yeasts though they are perfectly domesticated when a brewer wants them to be, produce the barnyard and tropical-fruit notes associated with certain Belgian and American sour and farmhouse styles. Lactobacillus and Pediococcus are bacteria, not yeasts, but they share fermentation vessels in mixed cultures and produce lactic acid, which is what makes a Berliner Weisse or a gueuze taste sour. HORAL, the Belgian council that oversees traditional lambic production, treats the spontaneous mixed fermentation of these organisms as a defining feature of the style.

What fermentation actually does

In aerobic conditions — that is, with oxygen available — yeast prefers to respire, breaking sugar all the way down to carbon dioxide and water and getting a great deal of energy out of the transaction. In anaerobic conditions, with oxygen scarce, the same yeast switches to fermentation, breaking sugar only partway down and stopping at ethanol. The yeast gets less energy out of this, but it gets enough, and the brewer gets ethanol, which is the entire point.

The simplified equation, the one written on chalkboards, is that one molecule of glucose yields two molecules of ethanol and two molecules of carbon dioxide. The actual biochemistry is the Embden-Meyerhof-Parnas pathway followed by the conversion of pyruvate to acetaldehyde and then to ethanol, but the chalkboard version is honest about the inputs and outputs. Wort, the sugary liquid produced by mashing malted barley, contains a mixture of fermentable sugars — chiefly maltose, with smaller amounts of glucose, fructose, sucrose, and maltotriose — and yeast consumes them in a fairly predictable order, glucose first, then maltose and the rest.

What yeast cannot consume is the fraction of carbohydrates called dextrins, which are larger sugar molecules left over from the mash. Dextrins contribute body and a faint sweetness to the finished beer. A brewer who wants a fuller-bodied beer mashes at a higher temperature, which leaves more dextrins behind. A brewer who wants a drier beer mashes lower and gets more fermentable sugars and a thinner finish. The yeast does not particularly care either way.

Flavor as a byproduct

If yeast produced only ethanol and carbon dioxide, beer would be a much duller drink and brewing would be a much simpler craft. It produces a great many other things, in small quantities, and those things are what distinguish a Bavarian hefeweizen from a Czech pilsner from a London bitter, even when the malt and hops are not wildly different.

Esters are produced when yeast combines an organic acid with an alcohol. They smell like fruit. Isoamyl acetate smells like banana and is the signature of a Bavarian weissbier. Ethyl acetate, in moderation, smells like pear; in excess, like nail polish remover. Ester production rises with fermentation temperature, which is why ales, fermented warm, smell more fruity than lagers, fermented cold.

Phenols are a separate family of aromatic compounds. The clove note in a hefeweizen is 4-vinyl guaiacol, produced by a specific phenolic-off-flavor-positive yeast strain that brewers of other styles spend considerable effort excluding. In a hefeweizen it is desirable. In a pale ale it is a flaw.

Higher alcohols, sometimes called fusel alcohols, are heavier than ethanol and contribute solvent or warming notes. Small amounts add complexity. Larger amounts produce headaches and unpleasant flavors and indicate that the fermentation ran too warm or that the yeast was stressed.

Vicinal diketones, primarily diacetyl, smell and taste like buttered popcorn. Yeast produces diacetyl during active fermentation and then, given time and warmth, reabsorbs and reduces it. A lager held at fermentation temperature for a few days at the end of primary fermentation — what brewers call a diacetyl rest — comes out clean. A lager rushed off the yeast comes out tasting like a movie theater.

Sulfur compounds, including hydrogen sulfide and sulfur dioxide, are normal intermediates of lager fermentation. They mostly blow off with the carbon dioxide during conditioning. A faint sulfur note in a young lager is not a defect; a strong one in a finished beer is.

A brewer choosing a yeast strain is, in effect, choosing a flavor budget. The Brewers Association's Best Practices Library and the technical resources published by the Master Brewers Association of the Americas (MBAA) treat yeast selection and management as one of the small number of decisions with disproportionate impact on the finished product.

Pitching, growth, and the shape of a fermentation

A fresh pitch of yeast into oxygenated wort goes through phases that brewers describe with reasonable consistency. There is a lag phase, lasting some hours, during which the yeast takes up oxygen, builds sterol and unsaturated fatty acid into its cell membranes, and prepares to divide. There is an exponential growth phase, during which cell counts roughly double every couple of hours and visible activity — krausen, the foam cap on top of the fermenter, and audible CO2 release through the airlock — becomes obvious. There is a stationary phase, during which growth stops but fermentation continues and the yeast cleans up some of the byproducts it produced earlier. And there is a decline, during which the yeast flocculates, settling out of suspension, and the beer clears.

The pitch rate — how many viable yeast cells per milliliter of wort — matters. Underpitching stresses the yeast, drives up ester and fusel production, and can leave fermentations stuck before they finish. Overpitching produces a thinner, less expressive beer because individual cells divide less and produce fewer growth-phase flavor compounds. Standard guidance, drawn from MBAA technical literature, calls for roughly one million viable cells per milliliter per degree Plato of wort gravity for ales, and roughly double that for lagers. The numbers are guidance, not law, and house practice varies.

Oxygen, counterintuitively, matters at exactly one moment: the moment of pitching. Yeast needs dissolved oxygen to build healthy cell membranes for the generation of cells that will do the fermenting. After that early aerobic interval, oxygen becomes an enemy, oxidizing the finished beer and producing stale, papery flavors. Brewers oxygenate wort vigorously before pitching and then exclude oxygen meticulously for the rest of the beer's life.

Top, bottom, and the lager question

The traditional distinction between ale and lager yeast — top-fermenting versus bottom-fermenting — describes where the yeast collects during fermentation, not what it tastes like, though the two correlate. S. cerevisiae strains tend to rise with the CO2 and form a thick krausen, then flocculate down at the end. S. pastorianus strains tend to stay in suspension lower in the vessel and to ferment slowly at temperatures, around 7-13 degrees Celsius, that would leave most ale yeasts dormant.

The cold-tolerant half of S. pastorianus, S. eubayanus, was identified relatively recently, in Patagonia in 2011, which means that the parentage of the world's most commercially significant yeast was uncertain until the present century. The NCBI PubMed Central review of Saccharomyces and beer flavor treats this as an active area of research; the genetics of brewing yeast are not, as a field, finished work.

For a working brewer, the practical implication is that lager fermentation is slower, colder, requires more yeast, and rewards patience. For a trained drinker, the implication is that a clean lager is a more difficult beer to make than its plain appearance suggests, and that flaws — diacetyl, acetaldehyde (green-apple), sulfur — have nowhere to hide.

Wild, mixed, and spontaneous

A separate category of fermentation dispenses with pure-culture yeast altogether. Traditional lambic, made in a defined region southwest of Brussels, is inoculated by exposing cooled wort to ambient air overnight in a shallow vessel called a coolship. The microbes that arrive — a succession of Enterobacter, then Saccharomyces, then Brettanomyces, then Pediococcus and others — ferment the beer over months and years. HORAL maintains the traditional production protocols for lambic and gueuze.

American mixed-fermentation brewers have adopted the principles, if not always the geography. Coolship beers, oak-aged sours, and Brettanomyces-finished saisons all rely on populations of organisms rather than single strains, and the resulting beers change over time in ways that pure-culture beers do not.

The regulatory framing

Yeast is not regulated by name in the federal definition of beer, but its work is. According to 27 CFR Part 25, beer is fermented from malt and hops or their substitutes, and the fermentation itself is what produces the ethanol on which federal excise tax is assessed under 26 USC § 5051. The TTB does not specify which yeast must be used, only that the resulting product meet the compositional and labeling requirements of 27 CFR Part 7 for malt beverages. Spontaneously fermented beers, sour beers, and Brettanomyces-conditioned beers all sit comfortably inside the regulatory definition; the agency cares about alcohol content and labeling, not microbial census.

In Germany, the Reinheitsgebot tradition, overseen in modernized form by the Federal Ministry of Food and Agriculture (BMEL), permits yeast explicitly — yeast was added to the original 1516 list once its role was understood, which took roughly three centuries — and the Deutscher Brauer-Bund publishes the contemporary interpretation.

What this means at the glass

A trained drinker tasting through a flight can, with practice, separate yeast-derived character from malt and hop character. The banana in a hefeweizen is yeast. The clove in the same beer is yeast. The clean, almost neutral background of a German pilsner is also yeast — specifically, a yeast doing its job so unobtrusively that the malt and hops can speak. The funk in a gueuze is yeast and bacteria together. The buttered-popcorn note in a poorly conditioned lager is yeast that did not get enough time to clean up after itself.

Candidates studying for the Certified Cicerone® exam, BJCP judging exams, or IBD qualifications all spend considerable time learning to identify yeast-derived flavors, because correctly attributing a flavor to yeast — rather than to malt, hops, water, or process — is the first step in understanding why a beer tastes the way it does, and the first step in deciding whether what is in the glass is what the brewer intended.

Further reading

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