Dark beer on stump

© 2023, Randy Mosher / Craft Beer & Brewing Magazine

The last time I wrote here about dark beers I focused on everything but the darkest malts. Here, the focus is on the malts that deliver much of the color and roasted flavors for which stouts and porters are famous. 

Nearly all malt flavor derives from a few 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 a process called “pyrolysis” takes over, which is just a fancy technical way of saying “burning,” specifically of sugar molecules.

Maillard is central to malt flavor. For us non-chemists, looking at even a simplified diagram of these reactions 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 those of similar color as well as those in different parts of the color range. Shown is a simplified schematic of Maillard reactions.

Maillard processes are highly sensitive to:

1) Starting ingredients: specific carbs and amino acids
2) Temperature and time
3) Moisture content
4) Nearly every other imaginable parameter: concentration, pH, pressure and others

Output is a huge range of cooked aromatic compounds to which humans are uniquely sensitive. This means there are nearly unlimited opportunities to manipulate the process, so that even malts of identical color can have dramatically different flavors. This means you have to get some hands-on experience rather than brewing based on the incorrect assumption that the color number is all you need to know.

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 only  take place at higher temperatures, starting at 248 °F (120 °C). At the highest temperatures, a process called pyrolysis takes over at 392 °F (248 °C) and above. This is essentially carbonization, or flameless burning.

As flavor is formed, it’s also simultaneously lost. The bright grassy aldehydes notes in barley are lost early in kilning, but may be preserved in extra-pale pils 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, caramelly flavors. Chemicals called furans and furanones add spun sugar or kettle corn-type aromas with sweet personalities. 

At higher temperatures (248 °F/120°C to 356 °F/180°C) in both Maillard and caramelization reactions, Strecker aldehydes form, yielding potent caramelly flavors as well as nutty ones. These include maltol (sweet caramel cotton candy, jam, fruity, baked bread) and isomaltol (burnt caramellic fruity), as well as furfural (sweet, woody, almond, bread, baked). 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 fades at the higher temperatures used for roasting black malts. 

Then we come to the largest and most characteristic group of malt flavor chemicals,  heterocyclics: The name simply means ring-shaped molecules with some additional atom(s) glommed onto one or more points in the ring: sulfur, nitrogen or oxygen. Typical descriptions are nuts and bread crust, plus burnt and roasted notes.

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 between 36 and 125 times higher concentrations of maltol than black malt and roasted barley respectively. I think it’s also useful to think of a beer’s balance in that way.

Production of heterocyclics 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. Pyrazines are especially ubiquitous, “the most important and widespread group of flavor substances in food chemistry,” as one researcher put it. There may be ten times more of these in black malts than in crystal malt. 

Oxygen-containing heterocyclics are more prominent in light caramel and colored base malts and are absent in in darker ones. Nitrogen-containing heterocyclics are also common in crystal and roasted malts, with aromas reminiscent of Crackerjacks: popcorn, peanuts and caramel, with accents of bread, tobacco and aromatic rice. Heterocyclics with sulfur are among the most potent odor molecules known for humans, but can barely be smelled by other mammals. 

An interesting side-note 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 an infection like COVID, 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 re-learn the familiar smell patterns: banana, ham sandwich, 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 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, a carbonization process called pyrolysis starts burning up the sugar rings that form the skeletons for the heterocyclics, replacing some of them 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-vinylguiacol, a phenolic compound that gives hefeweizen its characteristic clove aroma. The fruity and coconut notes of β-damascenone and γ-nonalactone formed at lower temperatures simply boil off.

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 can add reddish notes to mid-colored crystal malts.

Roasted barley was invented to save the effort of malting something that was being roasted to within an inch of its life. Compared to black malt, the flavors are different. Without malting, there is far less amino acid content, upon which most of the Maillard reactions are dependent. 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).

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, 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” 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 and not assume any two black malts, for example are truly equivalent.

Very dark malts may have a fair amount of polyphenols that 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 “de-bitterized,” which also helps.

Evaluating 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 I feel 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 an ounce (28 g) to perhaps three or four times that of not-quite-boiling water. Allow to stand for ten 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 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.