Malt is processed grain seeds—any grain seeds—that have been modified from their natural state by a multistep procedure called malting. Along with water, hops, and yeast, malt is one of the four key ingredients in virtually all beers. The basic malting steps are steeping, germinating, and kilning. For many specialty malts, two other steps—stewing and/or roasting, preferably both in a roasting drum—are used as well. The duration and temperature of these malting steps affect the technical characteristics of the different malts, as well as their flavor and color. Malt varieties range from very pale and sweet to amber and biscuit-like to almost black and coffee-like. The brewer usually selects a combination of malts to formulate a particular beer. Like wine grapes, malting barley is varietal and tastes different depending upon its strain and where it is grown.

Most brewing malt is made from barley and wheat. Taxonomically, these grains belong to the Gramineae family, a group of cereal grasses that also includes bamboo, bluegrass, corn, kamut, millet, oats, rice, rye, spelt (also called dinkel), switch grass, and Timothy hay. One of the few nongrains that can be malted is buckwheat, which, despite its name, is actually the edible fruit of a family of herbs called Fagopyrum. See buckwheat. Grain seeds contain all of the nutrients—mostly carbohydrates, proteins, and fats, as well as many trace elements—that the grasses need for their reproduction. Malting makes these nutrients usable for the brew house.

Seed kernels are hardy, compact enclosures. In barley, they have a shell of two overlapping husk leaves. Unless harvested by humans or eaten by wild life, seeds drop to the ground in the fall and lie dormant during the winter. For compactness, the nutrients inside the kernels have complex molecular structures; and the husks contain tannins— astringent polyphenols that act as preservatives to protect the nutrients against such adversities as mold, rot, and pests. As the rays of the returning sun in springtime melt the snow and warm the soil, the kernels come to life by rapidly absorbing ambient moisture. During this hydration, biochemical changes take place inside the kernels, which initiate the development of roots and shoots, the beginning of new plants. The malting process attempts to harness exactly the same processes that occur in the field—in this case not for growing new plants, but for making beer. Of course there have been many technological changes over the millennia that humans have been malting grains, but the basics of the process remain the same.

After the harvest, the maltster stores the grains in silos and carefully regulates the latter’s internal temperature, moisture, and aeration to prevent spoilage. In the case of barley, the kernel’s moisture content at harvest time is between 12% and 17% by weight. In the silo, it must be kept to below 14%. Although the kernels are dormant in storage, as they would be in the field, they are alive and will need to absorb small amounts of oxygen, while releasing small of amounts of CO₂; this is the reason for the aeration. Naturally, rodents and other pests must be kept out of the silos as well.

There are strict selection criteria that separate brewing grains from feed grains. In the case of barley, the kernels should be at least 90% homogeneous per sample, preferably with none of the kernels below 2.2 mm in diameter. A batch in which 95% or more of kernels have a diameter of at least 2.5 mm is considered excellent. The average protein content of these kernels should be higher than 10% and lower than 12%. In the case of brewing wheat, the protein content should not exceed 13%.

When the grain is ready to be malted, it is cleaned of any foreign matter such as dust and debris. Next, it is hydrated. This occurs in giant vats that are alternately filled with water and drained. The “wet” phase may last 8 h and the “dry” phase 12 h. Different maltsters use slightly different wet/dry cycles. The steeping temperature is normally between 12°C and 15°C (54°F and 59°F). During the grain’s wet immersion phases, air is blown into the vat from the bottom to supply the kernels with plenty of oxygen for respiration and to agitate the content; this action also cleans the grain. The vat is allowed to overflow to float off any dislodged dirt and particulate matter. During the grain’s dry phase, air is sucked out of the vat at the bottom to carry away CO₂ exhaled by the respiring kernels and to replace it with air from above. After 20 to 48 h in the steeping vat, the grain has a moisture content of 38% to 46% and tiny rootlets have appeared at each kernel’s base. The grain is now considered fully hydrated and is moved into a moisture- and temperature-controlled germination chamber.

There, just as in the field, root development accelerates, and plant shoots appear as well. Both start at the kernel’s base. The shoot, called the acrospire, grows upward between the husks and the kernel’s body, which is called the endosperm. Germination also involves the activation of enzymes under the kernel’s husks. Enzymes reside on a thin skin, called the aleurone layer, which covers the endosperm. They are specialized organic catalysts that initiate—but are not part of—particular biochemical reactions that occur only under narrowly prescribed temperature, moisture, and pH conditions. Enzymes break down the endosperm’s complex nutrients into simpler ones so that these can easily be absorbed as building blocks and energy sources by the embryonic new plant. In the brew house, of course, we don’t seek to grow new plants—instead we seek to extract these nutrients for making beer. This enzymatic degradation is called “conversion” or “modification.” It softens the hard and starchy endosperm, which makes for easier milling in the brew house. See friability and milling.

There are three major categories of grain enzymes—cytolytic (cellulose-converting), proteolytic (protein-converting), and diastatic (starch-converting). Cytolytic enzymes such as beta- glucanase break down the kernel’s internal gum-like cellulose walls, which hide and protect the kernel’s starchy and proteinous nutrients. Once these walls are degraded, proteolytic enzymes such as endopeptidase and carboxypeptidase modify large molecular proteins into smaller ones. The larger the protein molecules, the more likely they are to precipitate out into the trub during wort boiling; the smaller the protein molecules, the more likely they are to make it into the finished beer, where they can contribute to its body, mouthfeel, and foam. After cytolysis, starches become accessible as well. These starches are complex carbohydrate molecules that occur in long chains or as branched structures. They are converted by diastatic enzymes, such as alpha- and beta-amylase, into smaller carbohydrate molecules, that is, into starch fractions called sugars. Alpha-amylase breaks down starches into nonfermentable sugars such as dextrins, whereas beta-amylase breaks down starches as well as dextrins into fermentable sugars such as maltose. This starch-to-sugar conversion is also called “saccharification.”

Other significant but minor enzymes in the grain include phytase and lipidase. Phytase breaks down phytic acid and releases phosphorus, whereas lipidase breaks down lipids (fats, oils, and waxes). All grains have germ sacks that contain oil—some more, some less—and barley, fortunately, has unusually low lipid levels. These oils are eliminated in the malting process and do not survive into the brewhouse, where they would cause considerable trouble for brewers.

Mechanically, there are several types of germination chambers. Until roughly the beginning of the 20th century, most germination chambers would have been large, tiled floors with adjustable louvers on all sides for aeration. The moist grain would be spread out on the floor in a layer of perhaps 15 cm (7 inches). There, it would typically stay for 4 to 6 days and begin to sprout. Because germination is an exothermic biological process, the grain needs to be turned constantly—now mechanically, but by malt shovel before the machine age—to release heat given off by the grain. Turning also dissipates CO₂, keeps the grain properly oxygenated, and prevents the rootlets from matting together. For malting to proceed properly on a floor, this grain bed had to be kept at or below an outside temperature of no higher than 13°C to 17°C (55°F to 63°F). In most climates, therefore, malting was possible only in the winter. This labor-intensive malting method is still used by a few boutique malting companies today, and malt produced this way is still referred to as “floor-malted.” Many brewers feel that this old method produces malt with superior brewing and flavor characteristics.

In modern malting plants, the germination floor has been replaced by sophisticated, mechanized, and usually automated installations of various designs that allow for malting to happen year round. Modern germinators are equipped with intake and exit grain transfer systems, environmental controls, mechanical malt turners (usually augers and/or rakes), perforated false bottoms for air intake, and fans for air and moisture evacuation. While germinating, the grain is kept in stable condition by humidified cool air blown through the chamber’s false bottom. In rectangular germination chambers—such as the Saladin Box, a 19th-century French invention—the malting grain bed may have a depth of roughly 1.20 m (4 ft), and the malt turners move slowly back and forth through the grain bed. See saladin box. In more recent, circular designs, the malt turner pivots slowly around a vertical axis in the center of the chamber or the malt turner is stationary, and the entire floor that carries the grain revolves instead. Outside Germany, maltsters may spray enzymes grown on biological media onto the germinating grain. This accelerates the process by 2 to 3 days, which reduces production costs. It also allows for the malting of enzyme-poor, lower-quality grains that otherwise may have gone to the feed lot. In Germany, however, such malt treatments are forbidden by the country’s Beer Purity Law. See reinheitsgebot.

The temperature of the germination chamber is kept at roughly 13°C to 18°C (55°F to 64°F). The grain remains in the germination facility until the acrospires have grown to roughly two-thirds of the length of the endosperms, which takes anywhere from 4 days to 1 week. At this point, the grain is well on its way to becoming malt, but it is not malt yet. At this stage it is called “green malt.” This green malt is then transferred into a drying device, which, depending on the desired malt type, is either a kiln or a roasting drum. In the kiln, hot air is applied to the moist load of grain, primarily to dry it. This kills the developing acrospires but does not destroy the enzymes. Acrospire growth, which generally comes to a halt at temperatures above 40°C (104°F), must be interrupted, because the maltster and brewer want to preserve the modified malt nutrients to make beer, not new plants. Green malt still tastes rather rough and raw, and it requires further biochemical changes to give it the pleasant malty– aromatic quality that we expect to taste in beer. These changes happen only under the influence of heat. Also, because the soggy green malt is a nutritious medium for spoilage organisms, it is highly perishable and must be dehydrated quickly. The malt’s water content must be reduced from the 40% to 50% it contains at the end of germination to at least 4% to 6% by weight.

In the kiln, the malt is finished in two phases, drying and curing. The total length of the kilning process varies with the kiln’s construction and may take anywhere from 20 h to 2 days. Both stages involve hot air being blown through the malt and the evacuation of the resulting humidity via big fans and exhaust flues. The malt must be completely dry before curing because only dry enzymes can survive the higher subsequent curing temperatures without damage. The malt enzymes need to remain intact so that the brewer can reactivate them later on in the mash. Once the malt’s moisture content has dropped to between 10% and 20%, the temperature is raised gradually from a low of perhaps 13°C (63°F) to a finishing temperature of below about 85°C (185°F) for pale malt, such as Pilsner malt, to perhaps 120°C (approximately 250°F) for darker malts. During the temperature ramp-up, which may take about 8 h, the malt traverses through the various active temperature zones of the cytolytic, proteolytic, and diastatic enzymes, much as it does again later in the brewer’s mash tun during a step mash. The kilning process, therefore, determines the malt’s degree of enzymatic modification. At the final curing phase, at which the malt dries completely, dimethyl sulfide and its precursors are also driven off. See dimethyl sulfide (dms). A few maltsters now use installations that allow for germination and kilning to take place in the same unit. See germination-kilning vessels (gkv). Old-style floor-malting, incidentally, which tends to be less efficient than modern, mechanized malting, generally results in what are now considered slightly “under-modified” malts. After kilning, the finished malt is usually cooled with unheated air to blow off any residual vapors and to prepare it for bulk storage. On its way to the malt silo, the dried-up dead rootlets, called culms, as well as any loose husks and kernel fragments are removed by gently shaking the malt over screens and then passing it through a malt polishing machine.

Another key process that takes place in the kiln is the Maillard reaction, by which sugars and amino acids—the products of diastatic and proteolytic modification—combine to form melanoidins at high temperatures. See maillard reaction. Melanoidins are brown polymers that give malt its typical malty flavor and aroma. This is why the Maillard reaction is often referred to as nonenzymatic browning. Higher curing temperatures for such darker malts as Vienna and Munich malts promote greater melanoidin formation and are thus responsible for the distinctly toffee-like malty notes in such beer styles as Vienna lager, märzen-oktoberfestbier, and bock.

Malts that have been kilned very quickly are generally considered of lower quality because, during an excessively rapid loss of moisture during the drying phase, the malt’s pores may shrink and close. This hardens the kernel and makes it less mealy and more difficult to mill in the brewhouse. Maltsters and brewers distinguish between several different categories of malt, each with different brewing and culinary characteristics: base malts, caramel/dextrin malts, crystal malts, chocolate/roasted malts, and roasted raw grains. Base malts are gently kilned, as explained above, and are usually pale and highly enzymatic. They account for at least half the grist in most mashes. All other nonbase malts are considered “specialty malts.” These add varying degrees of color, flavor, aroma, and texture to the finished beer.

Malts that are cured in the kiln, after steeping, germinating, and drying at a high temperature of perhaps 140°C (approximately 185°F), are branded as crystal malts.

Caramel malts require a fourth step, stewing, as part of the malting process. For this, the green malt is sent, after germination, into a rotating, drum-type roaster instead of a kiln. There, it is heated immediately to between roughly 64°C and 72°C (147°F and 162°F), which is the temperature range for strong alpha- and beta-amylase activity. It is kept at that temperature for about an hour. This wet stewing ensures that enzymatic saccharification takes place inside each kernel. The stewed malt is then finished off in the kiln or it stays in the roaster for finishing. If it is finished in the kiln, it is subjected to a drying cycle at about 90°C (roughly 195°F), which causes the sugars to caramelize into hard, glassy, and unfermentable dextrins. If the stewed malt is finished in the roaster, on the other hand, it is subjected to a drying cycle at perhaps 200°C (roughly 390°F), which causes not only the sugars to caramelize into unfermentable dextrins but also the entire kernel to become roasted. The roasting times and temperatures vary with the desired degrees of color and roastiness, but the results are always strongly color- and flavor-intensive malts. Because caramelization completely and irretrievably denatures (destroys) the malt’s enzymes, caramel malts rarely make up more than half of a beer’s grist bill. Historically, the first pale caramel malt was made by Weyermann and patented in Germany under the brand name of Carapils in 1903. See weyermann® malting. Caramel malts impart a sweet maltiness to the brews made with them.

Chocolate malts are slightly stronger in color and aroma than caramel and crystal malts. They are produced by moving regularly steeped, germinated, and kilned—that is, not stewed—malts, after a month-long rest, into a roaster. There, these otherwise finished malts are heated to about 250°C (about 480°F). The resulting finished products are very dark to jet black and when concentrated may taste strongly acrid and bitter. Some maltsters make roast malts directly in the kiln, without moving them to a rotating drum. This method, however, results in less homogeneous products, because of the uneven heat distribution from the bottom to the top of the malt bed.

Certain roasted malts are offered in two forms—regular and dehusked. When malts are dehusked, the barley’s tannin-containing husks are removed before the start of the malting process. Husk-free chocolate malts taste very mild, with greatly reduced bitterness and roastiness, but they have the same coloring effects as regular chocolate malts. Roasted chocolate malts, however, must not be confused with roasted barley, roasted wheat, or roasted rye, which are produced like chocolate malts, in a roaster, from raw or malted grains. These slightly harsh-tasting products add not only burnt aromas but also strongly biscuity notes to the finished beer. Neither chocolate malts nor roasted grains—just like caramel and crystal malts—have any enzymes left, and because of their color and flavor intensity, they rarely make up more than 5% of a mash.

Finally, there are several smoke-flavored malts, which are steeped and germinated and then kilned in smoky, direct-fired kilns. These kilns are constructed to allow smoke to filter through the drying grain. Peat smoke is a favorite for whiskey mashes, whereas smoke from aged beechwood logs gives Bamberg-style smoked malt (also known as Rauchmalz) and the Bamberg-style Rauchbier made from it their characteristic flavor. Before the invention of modern kilning and drying techniques, most malts were dried over fires, and many early beers undoubtedly had strong smoky flavors. To this day, malts are major drivers of beer flavor and differences between different beer profiles. The differences in malting are among the reasons why beer has a far wider range of flavor than wine does. Beer can taste of dark chocolate, espresso coffee, caramel, toffee, biscuits, or bread, and malts are the basis for all of these myriad flavors.