How Beer Is Made: The Brewing Process, from Barley to Glass

Everything starts with a grain of barley, but the grain itself never ferments — malt does. During malting, the barley is steeped in water for two or three days, according to the Beer Judge Certification Program’s (BJCP) judge training course, then left to germinate for six to ten days. This is when the grain produces the enzymes, alpha-amylase among them, that will later break starch down into simple sugars. Right after, germination is stopped by raising the temperature: first to around 32°C for a day, then up to 49°C for another twelve hours. That’s kilning, and how hot it gets decides the malt’s color: Vienna malt stops around 63°C, Munich malt climbs to 99-118°C, and the roasted malts used for stouts and porters reach 138-149°C. It’s the malt’s roast level, not an added coloring, that darkens the beer.

Practical example: a stout and a pale lager can start from the very same barley; what changes is only how long, and at what temperature, the malt gets dried.

The dried malt is then milled and mixed with hot water: that’s mashing. Here, the enzymes developed during malting work on the malt’s starch, turning it into sugars the yeast will later ferment — the same biochemical principle behind bread leavening, even though there the main actor is a dough rather than a filtered liquid. The resulting sugary liquid, the wort, is then separated from the spent grain, the solid malt remains, by filtration.

At this point the wort gets boiled, and that’s when the hops come in. The alpha acids held in the hops’ resin, lupulin, are what give beer its characteristic bitterness, but according to the Oxford Companion to Beer they stay practically insoluble in water until the wort reaches a boil: only heat isomerizes them, making them soluble and bitter. The most aromatic hop varieties contain 2.5-6% alpha acids, dual-purpose ones 6-10%, bittering varieties 10-15%, and super-bittering varieties up to 18%; the share of these acids that actually ends up in the glass still stays low, rarely above 30%, and drops further the later the hops go into the boil.

Once the wort has cooled, the yeast takes over, and this is where the type of beer gets decided. Ales ferment with Saccharomyces cerevisiae, a yeast that, according to the American Homebrewers Association, works best between 16 and 26°C and is called “top-fermenting” because its activity shows on the surface; lagers ferment with Saccharomyces pastorianus, slower, active between 9 and 14°C and barely visible while it works, followed by weeks of maturation at near-freezing temperatures, known as lagering. The warmer temperatures ales ferment at produce more esters and phenols, meaning fruity or spicy notes; lagers, fermenting cold, come out cleaner on the palate.

The line between the two families has blurred somewhat in recent decades. Hybrid strains have emerged that combine traits of both, while still keeping distinct fermentation times and flavor profiles.

There’s also a third path, one that skips a selected, deliberately added yeast altogether: spontaneous fermentation. In Belgian lambic, as documented in a 2014 study published in the journal PLoS ONE, the boiled wort is left to cool overnight in shallow, open vessels, the coolship, where it gets colonized by microorganisms arriving from the air and from the wooden barrels the beer will mature in. The sequence follows precise stages: first, bacteria from the Enterobacteriaceae family, for the first 3-40 days; then, from the third or fourth week until the third or fourth month, yeasts of the genus Saccharomyces; next, for three to four months, lactic bacteria such as Pediococcus, which sour the wort; and finally, starting around four to eight months in, the genus Brettanomyces. The whole process, from coolship to finished beer, takes one to three years. Anyone familiar with fermentation in other foods, or with winemaking, where grape must meets yeast inside closed tanks, will recognize the same logic here, applied instead to a wide-open, uncontrolled environment.

The split between top and bottom fermentation isn’t as old as the usual telling suggests. According to the Oxford Companion to Beer, the term “bottom fermentation” appeared in Bavaria in 1420, once brewers discovered that cold Alpine caves allowed a slower, cleaner fermentation, one in which spoilage bacteria struggled to take hold. The yeast that made that fermentation possible wasn’t isolated in a lab until 1883, nearly five centuries later, by Danish chemist Emil Christian Hansen, who named it Saccharomyces carlsbergensis — later reclassified as Saccharomyces pastorianus.

Beer regulation has just as layered a history. On 23 April 1516, Bavarian duke William IV issued, during an assembly of the Estates in Ingolstadt, a decree limiting beer’s ingredients to barley, hops and water. Yeast went unmentioned, simply because it hadn’t yet been discovered scientifically, even though brewers were already relying on it without knowing it. The name given to that law today, “Reinheitsgebot”, didn’t exist in 1516: it was coined only in 1918, by Hans Rauch. In the meantime, the rule itself had already changed more than once: in 1873 northern Germany legally allowed malt substitutes, in 1906 the German Empire extended the rule to the whole country, and in 1987 the European Court of Justice ruled it an unlawful restriction on the free movement of goods within the common market. Two versions remain in force, a stricter Bavarian one and a more permissive federal German one, both folded into German tax law.