Fermentation is the process whereby “sugars” are converted by yeast to alcohol, carbon dioxide, and heat. In the brewing of most traditional beer, the sugars are derived mainly from malted barley, although other cereal sources and other plant sugars can also be used. These materials also contribute proteinaceous substances, which in concert with the sugars and added flavoring agents, notably hops, generate the alcohol, flavors, and aromas that we know and love as beer. The fermentation process has been practiced over many thousands of years, with the predilection for consuming alcohol as a common feature of practically all civilizations throughout history. In ancient societies, drinking beer had obvious physiological and psychological benefits (at least with moderate consumption) and also public health advantages; it was safe to drink, unlike many sources of water. Aside from this, the apparently mysterious nature of fermentation lent itself to exaltation in various religious, cult, and ritual ceremonies. Beverages that we can broadly classify as beers have been produced throughout the world for thousands of years. Despite the important place of beer in so many cultures for thousands of years, the nature of the fermentation process remained a mystery until the second half of the 19th century. The role of yeast in the biological transformation of sugars to beer was not fully recognized until microscopists were able to associate their observations with both the production of alcohol by yeast and the spoilage of beer by other microorganisms. However, for centuries before, it had been recognized that what was called “Godisgood” in early English—effectively yeast—was important in the brewing of beer. Nevertheless, despite these empirical observations, the prevailing scientific view was that fermentation was an inanimate, strictly chemical process. Indeed, the word “fermentation” is derived from the Latin fevere, meaning to boil, with the implication that the vigorous carbonation arising from fermentation, visually akin to boiling, caused the production of the intoxicating nature and flavors of beer. Louis Pasteur was decisive in persuading the sceptical scientific community of the mid to late1800s that fermentation was the result of the action of yeast on sugars, although by that time many practicing brewers and scientists involved in brewing were already well aware of the role of yeasts, even if they did not entirely understand the process. Furthermore, Pasteur was able to show that other microorganisms were the cause of “diseased” fermentations and that hygienic conditions were essential in the production of beer (and wine) of sound quality. Pasteur’s work stimulated a surge in the scientific investigation of fermentation in Europe. At the Carlsberg Laboratories in Copenhagen, the pioneering studies of Emil Hansen on pure culture brewing techniques were quickly adopted by brewers throughout the world. In England, and most notably in Burton-on-Trent, leading scientists of the time including Cornelius O’Sullivan, Johann Peter Griess, and the half-brothers Adrian John and Horace Tabberer Brown were developing the understanding of the scientific basis of brewing and fermentation, much of which underpinned the new science of biochemistry. Throughout the 20th century, research on yeasts at a biochemical and genetic level continued at pace, with the characterization of the type species, Saccharomyces cerevisiae, being of economic importance in brewing, baking, and winemaking. Although it is a single-celled organism, S. cerevisiae is a simple form of a eukaryotic cell in that it has a nucleus enclosing genetic material as chromosomes, which is defined by a membrane. Animal cells, including our own, are also eukaryotic, albeit of greater sophistication. However, the eukaryotic relationship was exploited in that the S. cerevisiae genome was the first eukaryote to be gene sequenced in 1996, paving the way for the sequencing of the human genome 10 years later. It is remarkable that from observations relating to the fermentation of beer, our understanding of our own genetic makeup and biochemical functioning at cellular and molecular level has developed to its current levels, directly impacting our medical well-being. In that context, we should also note that the developments of industrial scale fermentation used in brewing beer were utilized and adapted to the production of antibiotics from the 1940s onward. It is fair to argue that fermentation, and that of beer in particular, has profoundly influenced our physical well-being more than any other development in our social history, perhaps with the exception of the provision of safe water supplies and public health sanitation.

Fermentation is the second of the three principal stages in the brewing of beer and as such cannot be considered in isolation. The first stage involves the preparation of wort, an aqueous medium comprising mainly fermentable sugars derived usually from starch-rich cereals but also assimilable nitrogen, oxygen, sources of sulfur and phosphates, the vitamin biotin, calcium, and magnesium ions, together with trace elements such as copper and zinc. The exact quantities of these substances will vary depending on the source and proportions of the raw materials used. For example, worts derived from grists with a high proportion of nonmalted starch may need supplementing with sources of nitrogen, biotin, and some trace elements to compensate for the dilution of the malt material, which is usually rich in these components. These supplements are called “yeast food” or “yeast nutrients,” a reflection of the need to sustain growth of the yeast, at least during the early stages of fermentation. Most worts would be expected to contain about 70%–75% fermentable sugars, glucose, sucrose, and fructose, but mainly maltose and maltotriose. The remaining carbohydrate comprises nonfermentable material, mainly longer chain and branched glucose polymers. Nitrogen requirements for wort are usually measured in terms of free amino nitrogen (FAN); for a wort of specific gravity 1.040 (10° Plato) a typical FAN level would be about 150 mg/l. In addition to variations in raw material sources, the relative level of sugars and nitrogenous materials is profoundly influenced by the conditions of mashing and, to a lesser extent, wort boiling. Low temperatures during mashing (45°C–50°C [113°F–122°F]) favor protein breakdown (proteolysis) and therefore increases in FAN levels. On the other hand, higher mash temperatures (60°C–65°C [140°F–149°F]) reduce proteolysis but increase the activity of amylase enzymes, leading to an increase in fermentable sugars. Thus, by manipulating raw material content and processing conditions, the brewer can adjust wort composition to produce a consistent material ready for the addition of yeast and subsequent fermentation. However, the dissolved oxygen content of the wort is a critical parameter in sustaining yeast growth in the early stages of fermentation. To some extent different yeast strains have differing requirements for oxygen, and worts of varying strengths will also demand different levels. Too much oxygen results in particularly vigorous fermentations, which not only affects beer flavor but also causes excessive yeast growth at the expense of alcohol production. Too little oxygen can cause limited yeast growth, which will result in not only incomplete fermentation but also poor yeast vitality and viability, to the detriment of subsequent repitching of that batch of yeast. Traditionally worts of 1.040 specific gravity would be aerated prior to yeast pitching, giving dissolved oxygen levels of about 6 parts per million (ppm) at 20°C. More modern fermentation systems use oxygen levels at 8–12 ppm, generated by direct injection of oxygen rather than air into the wort stream. The wort prior to pitching should ideally be bright and clear, although it is argued that small amounts of precipitated protein and polyphenol material, called trub or break, can be beneficial in supplying lipids for yeast growth. See trub. Brewers vary in their attitudes toward bright worts, with some preferring extremely clear worts, whereas others are content with a slight haze. At the end of the day, it is the success of the fermentation and the stability of the resulting beer that will determine the wort requirements.

Management of the fermentation process is dependent on a number of factors, including the composition and oxygen content of the wort, the quality and quantity of the yeast used for pitching, temperature control in the fermenter, time, and the fermenter design.

Healthy yeast is at the heart of sound fermentation. Unlike traditional winemaking, with the exception of relatively rare “spontaneously fermented” beer styles, brewing depends upon the yeast added by the brewer. The specific strain of yeast is critical to the outcome of the fermentation, not only in the ability of the yeast to metabolize the wort contents to produce alcohol and distinctive flavor characteristics but also in the capacity of the yeast to tolerate its own products of metabolism, most notably alcohol, and the particular attribute of aggregation (flocculation) or otherwise that the yeast strain may normally exhibit. See flocculation. Brewers jealously guard their yeast strains although catalogs of so-called brewing strains are held in various collections throughout the world and can be obtained commercially. Some brewers, particularly traditional ale brewers in the UK, have used the same brewing strain (or strains) unchanged for decades, relying on serial repitching of the collected yeast at the end of fermentation. However, with repeated repitching some yeast strains exhibit subtle changes in character, particularly in flocculation characteristics, and a decline in viability and vitality. Most commercial brewers will repitch for up to 10 cycles or generations before replacing the yeast with a freshly propagated culture of the yeast grown from a starter culture. Brewing strains can utilize a wide variety of carbohydrate sources, although individual strains will vary in their particular appetites. Ale strains of S. cerevisiae are able to ferment glucose, sucrose, fructose, galactose, raffinose, maltotriose, and occasionally trehalose. Lager strains of S. cerevisiae (sometimes also called Saccharomyces carlsbergensis) are also able to ferment the disaccharide melibiose, whereas S. cerevisiae var diastaticus is also able to utilize some of the higher glucose polymers, called limit dextrins, which are out of reach of the other strains. Ale strains are generically described as “top fermenters” on account of their tendency to form a head or crust on the top of traditional open fermenters at the end of fermentation. Lager strains on the other hand, tend to separate out at the bottom of the fermentation vessel and hence enjoy the description of “bottom fermenters.” With modern fermentation systems employing cylindroconical fermenters for both ales and lagers, this traditional differentiation is these days less clear-cut. See ale yeast and lager yeast.

Although the primary function of fermentation is to convert sugars into alcohol, for the yeast to fulfill this requirement it must be present in sufficient quantity to effect the transformation. The yeast used to ferment is usually one or several generations old and, as a consequence of storage prior to pitching and the physiological condition at the end of the previous fermentation, it is depleted of nutrients for growth and fermentation. It is said to be in a stationary phase of growth and requires the stimulus of fresh wort nutrients, particularly oxygen, to rebuild its nutrient store and recommence growth and multiplication. Pitched brewing yeast will normally take several hours to adapt to its new environment before growth begins. This period is known as the lag phase and precedes a period of very active growth and metabolism, known as the exponential or logarithmic phase. The yeast will multiply four- or fivefold by a process of budding and build up its nutrient store, while at the same time commencing the conversion of sugars in the wort. The oxygen present at the start of pitching is rapidly used up by the yeast and is not involved in the fermentative process. The sugars present in the wort are taken in to the yeast cell and broken down into smaller units, ultimately producing alcohol, carbon dioxide, heat, and a vast range of other compounds, many of which contribute distinctive flavors and aromas to beer. At the same time, the nitrogenous compounds in the wort are also assimilated by the yeast, and as well as being used in the growth of yeast, they are metabolized and contribute to the rich flavor spectrum. When all available sugars have been utilized, the yeast will begin to use its own carbohydrate reserves (glycogen and trehalose) and effectively shut down its metabolism. This is known as the stationary phase of growth.

The fermentation temperature is critical in controlling the outcome of fermentation and has a significant impact on the development of flavor. Ales are generally fermented in the temperature range of 16°C to 22°C (61°F–72°F) using top-fermenting strains, whereas lagers are fermented much cooler, 9°C–14°C (48°F–57°F), using bottom-fermenting strains. Some beers, particularly Belgian styles, may be fermented very warm, with temperatures reaching almost 32°C (90°F) for some farmhouse ales. See saison. The combination of specific yeast strains and temperature generates very distinctive flavor profiles in the beers, with the ales and particularly stouts generally producing fruity/estery characters, whereas the lagers feature much lower ester levels, enabling more of the delicate pale malt characters and hop aromas to manifest on the nose. Fermentation is an exothermic process in that heat is produced and control of the heat generated is essential in fermentation control. Brewing vessels are equipped with cooling equipment of varying levels of sophistication, designed to effect cooling at the appropriate times in fermentation. Cooling is important in moderating yeast flocculation in that it tends to encourage yeast to flocculate. This is necessary at the end of fermentation to facilitate yeast separation, but if applied too early, it can cause incomplete fermentation and leave excessive levels of diacetyl in the finished beer. See diacetyl.

The progress of the fermentation is usually monitored by following the specific gravity drop and/or increase in alcohol content. Yeast growth and alcohol production deplete the sugar concentration and the pH falls as nitrogenous materials are used up and the yeast secretes organic acids. Flavor compounds are generated during yeast growth, although some volatile components are lost with the exhaust carbon dioxide, whereas other compounds (notably diacetyl) are absorbed and metabolized by the yeast. Traditional ale fermentations at between 16°C and 20°C (61°F and 68°F) will normally take about 4 days to complete, whereas lagers at 12°C will be up to 10 days.

There are many different types of fermentation vessels used in the brewing of beer. This reflects the beer being brewed, the volume required, tradition, the relative age of the equipment, and the type of yeast being used, particularly in relation to the use of top- or bottom-fermenting yeasts. The earliest fermenters were small and probably reflected the availability of local materials, be it clay for earthenware vessels, wooden barrels, or slate (stone) vessels used in Britain. As brewing operations increased in size, metal, particularly copper, was used although the vessels were generally shallow. At the end of fermentation, the yeast was collected from the top of the fermenter and used to pitch subsequent brews.

The introduction of taller, narrower vessels facilitated the selection of bottom-fermenting yeast strains but taller vessels demanded more efficient cooling systems and methods for cleaning. Additionally, tall vessels generate differing hydrostatic pressures, which can impact yeast performance and mitigate against homogeneity throughout the vessel. Nevertheless, this type of vessel has developed into the cylindroconical fermenter that is now used by most of the larger brewers throughout the world and many smaller breweries as well. With facilities for in-place cleaning, carbon dioxide collection, automatic temperature control through cooling jackets, and yeast collection, these vessels can be further adapted to store the beer at lower temperatures after primary fermentation and yeast removal, a process known as conditioning or maturation. These vessels are sometimes known as combined fermentation and conditioning tanks and can have capacities up to several thousand hectoliters.

However, despite the use of these larger fermenting vessels, many brewers still use more traditional methods of fermentation. Rectangular or circular shallow tanks are still used in many small and medium-size breweries, particularly for traditional ales, wheat beers, and classic lagers of central Europe. In Britain, two very distinctive fermentation systems still operate, known as the Burton Union system and Yorkshire squares. In the Burton Union system, now sadly operated in the UK only by Marston’s Brewery in Burton-on-Trent, a series of twelve 7-hl oak casks are connected via a “swan neck” to a central trough. Wort is pitched into an open square vessel on the floor above and run into the casks. As fermentation continues, yeast and carbon dioxide are forced through the swan neck and into the top trough. Most of the yeast separates from the part-fermented wort, which is returned to the cask to complete the fermentation. See burton union system. Yorkshire squares are rectangular fermenters, traditionally made of slate but now mostly of stainless steel construction, incorporate a false ceiling. Fermenting yeast rises though a central hole (0.6-m [24 in] diameter with a 5-cm [2 in] rim) and collects on the top of the ceiling, whereas fermenting wort flows back into the fermenter via a series of narrow holes. At the end of fermentation, the collected yeast is removed by suction. See yorkshire square.

For more than a century, brewers have experimented with the principle of continuous fermentation in which a fermenting vessel is fed continuously with wort, with beer produced in a continuous stream at the same rate as wort addition. Although this type of system has operated in New Zealand for over 40 years, it is not widely used because of difficulties in preventing infections and holding the yeast culture in a steady state. Recently, however, holding yeast on a bed of wood chips has proved to be effective in retaining the yeast. This process is known as an “immobilized yeast” system and is now being used commercially. See continuous fermentation and immobilized yeast reactor.

Beer at the end of the primary fermentation process is sometimes referred to as “green beer.” Before it can be packaged and dispensed, the beer is usually subject to a further processing, which can include secondary fermentation, conditioning, and maturation. In secondary fermentation, the green beer is kept in contact with the yeast after the primary fermentation has ended. This process is most commonly practiced in the brewing of cask ales (also known as “real ale” or “traditional draught”). At the end of the primary fermentation, most of the yeast is removed but a small concentration, usually 0.5 to 2 million cells/ml, is left. A small amount of additional sugar, known as “priming sugar” or “primings,” either in the form of sucrose or glucose, is added, which stimulates a secondary fermentation. Very little alcohol is produced (about 0.1%) but the main reason for priming is the generation of additional carbon dioxide, which gives the beer extra carbonation, or “condition,” as it is known. Traditionally this primings addition would have taken place in the cask itself but it is now usually added just prior to filling the casks, in a vessel that is known as the “racking tank.” See cask conditioning and cellarmanship, art of.

A similar process occurs in the production of “bottle-conditioned” beers, where secondary fermentation takes place in the bottle. In addition to giving the beer condition, the residual yeast also scavenges any small amounts of dissolved oxygen picked up at the time of filling the bottle. This antioxidant effect can help to extend the shelf life of the beer. See bottle conditioning.

One traditional form of secondary fermentation practiced, particularly in Germany, is known as “kräusening.” A proportion of actively fermenting wort is added to beer that is maturing in lager tanks to stimulate secondary fermentation and remove diacetyl and aldehydes, as well as provide additional carbonation. See kräusening.