How Is Beer Brewed? Malt, Hops, Yeast and Time

Barley is starch packed into a seed for the use of the seedling, and the seed carries enzymes to break the starch into sugar when it germinates. Malting borrows those enzymes. The barley is steeped in water until it starts to sprout, kept warm and damp for a few days while the enzymes develop and the grain's structure softens, and then dried in a kiln to stop the growth. The kiln temperature sets the colour and flavour: a low, gentle drying gives pale malt with its enzymes intact; a hotter kiln gives amber and brown malts that taste of biscuit and toffee; and a roast gives the black malt of stout, which tastes of coffee and has no enzymes left at all. Most beers use pale malt for the sugar and a little of the darker kinds for colour and taste.

The brewer crushes the malt and mixes it with hot water at around 65 degrees in a vessel called a mash tun, and for an hour the enzymes work, converting the starch into a mix of sugars, mainly maltose. The sweet liquid, the wort, is drained off through the bed of spent grain and boiled for an hour or so with hops. The boil sterilises the wort, drives off unwanted compounds, and extracts from the hops the bitter acids that balance the malt's sweetness and the oils that give aroma, of citrus, pine, flowers or grass depending on the variety. Hops added early give bitterness; hops added late give aroma; and hops added after the boil, dry hopping, give the intense fruitiness of modern pale ales. Hops also preserve, which is why the India Pale Ale of the 1820s was hopped heavily to survive the voyage east. The steps in order:

• •Malting: steeping, germinating and kilning the barley
• •Milling: cracking the malt to expose the starch
• •Mashing: steeping the crushed malt in hot water so the enzymes make sugar
• •Boiling: sterilising the wort and adding hops
• •Fermenting: cooling the wort and adding yeast, which eats the sugar
• •Conditioning: maturing the beer, then filtering, carbonating and packaging

The boiled wort is cooled and yeast is added, and for a week or more the yeast consumes the sugars and produces alcohol, carbon dioxide and hundreds of flavour compounds. Which yeast decides what kind of beer it is. Ale yeast works warm, at 18 to 24 degrees, quickly and at the top of the vessel, and produces fruity esters and spicy compounds; it makes the pale ales, bitters, stouts, porters and wheat beers of Britain, Belgium and the American craft movement. Lager yeast, a hybrid species that appeared in the cold cellars of Bavaria in the fifteenth century and whose other parent was traced in 2011 to Patagonia, works cool, at 8 to 13 degrees, slowly and at the bottom, and produces almost no esters; the beer is then stored cold for weeks, which is what lager means in German, and comes out clean, crisp and neutral. About nine tenths of the world's beer is lager, of the pale, Czech-derived pilsner style, and almost all of the variety is in the other tenth.

Beer is more than ninety percent water, and the water's minerals shaped the classic styles before anyone could adjust them: the hard, sulphate-rich water of Burton-on-Trent made its pale ales sharp and dry, the soft water of Pilsen made the first golden lager delicate, and the carbonate water of Dublin and London suited dark, roasted beers whose acidity balanced it. Modern brewers adjust the water to taste. Other grains enter too, wheat for haze and a light body, rice and maize in the mass-market American lagers, oats for silkiness in stouts, and the Bavarian purity law of 1516, which permitted only barley, hops and water, was as much a grain-price regulation as a matter of quality. Yeast was not on the list because brewers did not yet understand it; scientists identified it as a living organism in the 1830s, and Pasteur's work from 1857 showed that fermentation was a living process, after which pure yeast strains, first isolated at the Carlsberg laboratory in 1883, made beer reproducible.

The earliest chemical evidence of brewing is from Iran around 3500 BC, and a hymn to the Sumerian beer goddess Ninkasi from 1800 BC is a recipe. Egyptian workers on the pyramids were paid partly in beer; medieval monasteries brewed for pilgrims and are still doing so in Belgium; hops replaced the older herb mixtures across Europe between the twelfth and sixteenth centuries; and industrial brewing, with thermometers, hydrometers and steam engines, began in eighteenth-century London with porter, the first beer made in bulk. Refrigeration in the 1870s freed lager from the cave and the winter and made it the world's drink. The craft movement of the last forty years, which began in Britain as a defence of cask ale and in America as a revolt against blandness, has brought back nearly every style the industry had discarded, and a few, such as the heavily dry-hopped hazy pale ale, that nobody had made before.

Brewers work out how strong a beer will be by measuring how dense the liquid is before and after fermentation. Sugar dissolved in water makes it denser, so the wort before the yeast goes in has a density a few percent higher than plain water, called the original gravity, and it is measured with a floating instrument called a hydrometer, which sits higher in denser liquid. As the yeast eats the sugar and turns it into alcohol, which is less dense than water, the reading falls. When fermentation finishes, the brewer takes a second reading, the final gravity.

The drop between the two readings tells you how much sugar was converted, and a standard rule of thumb turns it into strength: multiply the difference by about 131 to get the percentage of alcohol by volume. A wort starting at 1.050 and finishing at 1.010, for example, gives a difference of 0.040 and a beer of roughly 5.2 percent. The final gravity also tells the brewer about taste. A beer that finishes high has more unfermented sugar left in it and will feel fuller and sweeter, while one that finishes close to 1.000 will be thin, dry and crisp.

Hops give beer one of its best known faults. The bitter iso-alpha acids formed during the boil are broken apart by light, especially blue and ultraviolet wavelengths, and one of the fragments combines with sulphur in the beer to form 3-methyl-2-butene-1-thiol, a close chemical cousin of the compounds in a skunk's spray. Your nose can detect it at concentrations measured in parts per trillion, so a few minutes in bright sunlight can be enough to make a beer taste 'light-struck'. Brown glass blocks most of the harmful wavelengths, green glass far less, and clear glass almost none.

Brewers who sell beer in clear or green bottles either accept a little of the effect as part of the product's familiar taste or use chemically modified hop extracts whose bitter acids do not break down in the same way. Cans avoid the problem completely. The other common fault is staling by oxygen. Even small amounts picked up during packaging slowly react with compounds from the malt and hops, and after some months a pale lager can develop a flat, papery taste that tasters describe as wet cardboard. This is why beer keeps best cold and why hoppy pale ales, whose aroma fades first, are best drunk fresh.

It is often said that people in the Middle Ages drank beer because water was unsafe. Historians have found little support for this. Medieval writers discussed drinking water, towns built conduits and wells to supply it, and many poorer people drank water routinely, while beer was valued mainly as food, a source of calories and a pleasure.

Ordinary household ale was often weak, and boiling during brewing does kill many microbes, so beer could be safer than a polluted source, but people did not generally understand that. The idea that everyone avoided water seems to be a modern simplification. It is a good example of how a plausible story about the past can spread widely without anyone checking the sources.

Beer is made by malting barley to wake its starch-splitting enzymes, mashing the crushed malt in hot water to make a sugary wort, boiling the wort with hops for bitterness, aroma and preservation, and fermenting it with yeast, which turns the sugar into alcohol and carbon dioxide. Warm-working ale yeast gives fruity, varied beers and cold-working lager yeast gives the clean, pale style that nine tenths of the world drinks, and the whole sequence has been recognisable for four thousand years.