Now we contemplate one of the most powerful ingredients available to brewers—powerful in its ability to deliver dramatic flavors as well as striking appearance. Now we contemplate the dark arts.
The last time I wrote here about dark beers, I focused on everything except the darkest malts (“Beyond ‘Roasty’: The Surprising Psychology of Stout,” beerandbrewing.com). Here, the focus is on the malts that deliver much of that striking color and those roasted flavors for which stouts and porters are famous.
Almost all malt flavor derives from a couple of chemical reaction systems: Maillard browning (aka non-enzymatic browning) and a similar but distinct process known simply as “caramelization.” Maillard involves many forms of carbohydrates plus nitrogen-containing amino acids found in malt. Caramelization mainly involves sugar—typically in a dense and liquid state, like when you put sugar in a pan and heat it until it browns and changes flavor. At the highest temperatures, however, a process called “pyrolysis” takes over, which is just a fancy technical way of saying “burning”— of sugar molecules, specifically.
Maillard reactions are central to malt flavor, and they are … complicated. For us non-chemists, looking at even a simplified diagram of these reactions (Figure 1) is paralyzing in its complexity.
Not to worry: There are some basics that are pretty easy to grasp and inform our understanding of how malt flavors are created and to explain the differences between malts of similar color as well as those in different parts of the color range.
Maillard processes are highly sensitive to:
- starting ingredients—specific carbs and amino acids
- temperature and time
- moisture content
- almost every other imaginable parameter—concentration, pH, pressure, and more
The output is a huge range of cooked aromatic compounds to which humans are uniquely sensitive. This means there are almost unlimited opportunities to manipulate the process, so that even malts of identical color can have dramatically different flavors. Thus, any assumption that the color number is all you need to know is wrong. You’ll have to get some hands-on experience.
Flavor Development During Kilning and Roasting
Let’s follow the process, starting at low temperatures all the way up to its near-fiery conclusion. Maillard browning can take place even at room temperatures, but in malt kilning, it doesn’t get started until about 122°F (50°C). At low temperatures, nitrogen-containing fragments of amino acids function as catalysts, reducing the energy necessary for chemical reactions to proceed. Caramelization reactions take place only at higher temperatures, starting at 248°F (120°C). At the highest temperatures, pyrolysis takes over at 392°F (200°C) and above. This is essentially carbonization, or flameless burning.
As flavor is formed, it’s also simultaneously lost. The bright, grassy aldehyde notes in barley are lost early in kilning, but they may survive in extra-pale pilsner malt. As the temperature increases, flavors may be volatilized, lost to further reactions, or—at the very end—simply burned up.
Low temperatures and high moisture levels favor the formation of compounds with sweet caramel flavors. Chemicals called furans and furanones add spun-sugar or kettle corn–type aromas with sweet personalities.
At higher temperatures (248–356°F/120–180°C), in both Maillard and caramelization reactions, Strecker aldehydes form. These yield potent caramel flavors as well as nutty ones—they include maltol (sweet caramel, cotton candy, jam, fruit, baked bread) and isomaltol (burnt caramel, fruit), as well as furfural (sweet, woody, almond, baked bread). These are characteristic in crystal malts; some remain in dark malts but may be volatilized as roasting temperatures continue to rise. Furfural, in particular, remains in chocolate malt, but it fades at the higher temperatures used for roasting black malts.
Then we come to the largest and most characteristic group of malt flavor chemicals: heterocyclic compounds. The name simply means ring-shaped molecules with some additional atom(s)—sulfur, nitrogen, or oxygen—glommed onto one or more points in the ring. Typical descriptions include nuts and bread crust, plus burnt and roasted notes.
Production of heterocyclic compounds ramps up around 356°F (180°C), and lack of moisture accelerates their formation. Tellingly, many of these have names with the Greek root word for fire, “pyr,” in the name: pyridine, pyrone, pyrrole. However, pyrazines are especially ubiquitous, and “the most important and widespread group of flavor substances in food chemistry,” as one researcher puts it. There may be 10 times more of these in black malt than in crystal malt.
The ratio of maltol to pyrazines has been proposed as a rough measure of the balance between the sweet caramel flavors produced by lower-temperature kilning and the higher ones in roasting. Crystal malt may contain 36 times more maltol than black malt and 125 times more than roasted barley. I think it’s also useful to think of a beer’s balance in that way.
Recall how heterocyclic compounds can have different atoms attached? Oxygen-containing heterocyclic compounds are more prominent in light caramel and colored base malts, and they are absent in in darker ones. Nitrogen-containing heterocyclic compounds are also common in crystal and roasted malts, with aromas reminiscent of Cracker Jack: popcorn, peanuts, and caramel, with accents of bread, tobacco, and aromatic rice. Heterocyclic compounds with sulfur are among the most potent odor molecules known for humans—even if other mammals can barely smell them.
An interesting sidenote is that many of these toasty-roasty aroma compounds—especially pyrazines and sulfur-containing thiols—are central to an olfactory problem called “parosmia.” This can occur after a trauma to the olfactory system, such as a COVID infection, or a head injury in which receptor nerve fibers are damaged. This system has an amazing ability to repair itself, but the nerves may grow back to different targets in the olfactory bulb than before. The new patterns of receptor responses are scrambled, making some things smell weird, even disgusting. Over time, they relearn the familiar smell patterns: banana, ham sandwich, or whatever. For people afflicted with it, the flavors of coffee and chocolate—and, presumably, stouts—are particularly distorted. It’s not clear why these roasty chemicals are the villains here, but it’s likely due to some yet-to-be-identified olfactory receptors.
As the roasting malt passes through different temperature stages, these flavor molecules pile on top of each other—up to a point. To my taste, this seems to be in the range of light-to-medium chocolate malts. At some point, all these volatile chemicals start, well, volatilizing, so that you can look at malt flavor as a combination of flavor development and subsequent losses as the heat increases. In addition, pyrolysis starts burning up the sugar rings that form the skeletons for the heterocyclic compounds, replacing some of them with sharp, not-quite-burnt flavors. High roasting temperatures can create other aroma-active chemicals. Ferulic acid in malt can be thermally degraded into 4-vinylguaiacol, the phenolic compound that gives hefeweizen its characteristic clove aroma. The fruity and coconut-like notes of beta-damascenone and gamma-nonalactone, which form at lower temperatures, simply boil away.
The endpoint of the Maillard reaction is a tarry goo called melanoidins, composed of polymerized sugar and amino-acid residues. It is relatively flavorless, but we value it for coloring in beer. These compounds have an impact on texture and are in part responsible for the creamy mouthfeel and lingering head found in many stouts. There are also some chromophore compounds produced at lower temperatures. These have different yellow or reddish hues, with the redder ones being more sensitive to destruction as the temperatures rise in roasting, but they can add reddish notes to mid-colored crystal malts.
The reason for inventing roasted barley in the first place was to avoid having to malt something that was going to be roasted to within an inch of its life anyway. Compared to black malt, however, the flavors are different. Without malting, there is far less of the amino-acid content upon which most Maillard reactions depend. This ends up creating a large proportion of pyrazines, and their sharp roasty characters are highly characteristic of roast barley and the Irish-style stouts brewed from them. The resulting chemicals include: hydroxyacetone (burnt-like aroma), furfural derivatives (caramel aroma), pyrazines (roasted aroma), and pyrroles (nutty aroma).
A Framework for Malt Flavor
With any kind of ingredient, it’s important to have a mental framework for classification. With the darkest malts, I use a scale with the smooth, soft, roastiness of chocolate on one end, and the sharp twang of espresso on the other, with other forms of coffee close to that end and malts forming a range in the middle.
Because the specific end chemicals of the Maillard process are highly dependent on starting ingredients, the roasted flavors of malt will never exactly match those of coffee or chocolate. Nevertheless, there are enough similarities to make this schema work. Names are only useful for broad classification. Personally, I think “chocolate malt” is a misnomer—I always find those malts more coffee-like. At any rate, each maltster’s interpretation is different to some degree, so you need to get some actual experience. For example, don’t assume any two black malts are truly equivalent.
Very dark malts may have a fair number of polyphenols, which can sometimes create an astringent character in the finished beer. Huskless roasted grains such as wheat and rye often seem smoother. Some malts are processed to be “debitterized,” which also helps.
A Simple Way to Evaluate Dark Malts
Bottom line: It’s impossible to rely solely on manufacturers’ names or their descriptions. Actually sampling malts from a variety of manufacturers is something that I believe brewers should be doing to internalize knowledge of specific ingredients’ characteristics, regardless of their commercial descriptors.
While these very dark malts are pretty unpleasant to put in your mouth, they respond nicely to being made into concentrated “teas” and added to a neutral base beer for evaluation. I generally use one ounce (28 g) of malt to perhaps three or four times that of not-quite-boiling water. Allow the “tea” to stand for 10 minutes, then decant and run that liquid through a coffee filter. (Although I haven’t tried it, an espresso machine should work nicely here.) If you’re looking into cold-steeped grains, somewhere around 18 hours for steeping seems to be optimum—at least that’s what the coffee folks think. These tinctures can be blended to create quick prototypes that inform actual recipes.
If we can look beyond their inky appearance, roasted malts and other grains offer a rich sensory experience, one that is equally useful whether you’re brewing a beer—or just enjoying one.