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THE CHEMISTRY OF COFFEE FLAVOR
THE ISSUE OF
ACID
Paul Songer was eager to dig into the crucial aspects of acid. “Acidity is a complex issue,” he says. “What cuppers identify as ‘acidity’ is really only part of the big picture.” And these parts of the picture are indeed acids, containing an H+ ion in combination with a balancing “-” ionic chemical or group of chemicals. These join with water to form hydronium, H3O+, which is perceived by certain taste sensors as acid or sour. The issue is complex, but here are the highlights according to Songer: As their name implies, these can be smelled as well as sensed as acid. They include lactic acid and acetic acid, which is the main one and is the chemical name for vinegar. Acetic acid is a product of production: some is developed when the fruit is separated from the seed, and some during roasting as the result of sugar browning. The interesting thing about acetic is that how it is perceived has to do with its concentration. A little bit adds to the pleasurable nature of acidity. A little higher concentration and the coffee tastes like over-ripe fruit or a bit grape-y/wine-y. A little higher yet and it’s like onions. Still higher and it is disagreeably sour, until finally it is a main contributor (with other aromatics) to the defect known as ferment. Some is always present, however. “AromAtIC” ACIDs:
These are the ones that mainly contribute to “fine acidity” and are the subject of most discussions at the cupping table. Predominant is citric acid, a fairly strong tasting acid, but fruit acids are also modified by others such as malic (also found in apples), tartaric (also found in grapes) and others. An acid profile is usually regarded more highly if there is a more complex matrix of these acids present in a beverage. This sort is usually more developed at high altitudes, which is why high-altitude coffees tend to have the best acidity. the fruIt ACIDs:
ChlorogenIC AnD quInIC group: Coffee has a high concentration of chlorogenic acids of a number of types. Upon roasting, these degenerate into cafeic and quinic acids and phenols. One type, di-chlorogenic, is mainly found in less ripe coffee, black beans and immature beans, and it is perceived as astringent and greenish. Chlorogenic itself is perceived as more bitter; in the form of quinic, it is sour/bitter (think of the quinine in tonic water). A little adds to the quality and interest of the acids; too much is unpleasant.
This includes phosphoric acid, which definitely adds to the positive perception of acidity. (Kenyas with high concentrations of phosphoric acid are usually said to have “fine acidity.”) It is a strong acid that acts in complex with the other acids and salts to give a particularly pleasant and lingering note. InorgAnIC ACIDs:
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But what are the basics of how these hundreds of chemical compounds produce the wonderful experience of drinking fine coffee? I asked Paul Songer, technical director for the Association of Coffee Excellence and a consultant specializing in the sensory analysis of coffee. “The first thing is the old saw, ‘The whole is more than the sum of its parts.’ Many coffee chemicals are undesirable on their own or in too great a concentration.” Songer also points out that chemistry is devoted to looking at individual chemicals and their interactions, and he cautions that this alone cannot fully explain coffee flavor. “Even one class of chemicals, such as acids, can’t fully explain coffee flavor,” he says. “Having understood that, coffee is a complex combination of acids, volatiles, oils and other lipids, salts, nitrogenous substances (including caffeine), and carbohydrates.” By way of contrast, I wanted to know why truck-stop coffee, made from low-quality robusta, old and mishandled, is still recognizable as coffee. And what else does specialty coffee bring to this? According to Songer, “Though a lot of volatiles evaporate as coffee sits, most of brewed coffee’s volatiles are contained in the oils. These are released in the mouth and perceived ‘retronasally’”— that is, through the nose after the coffee is swallowed. “Remaining browned carbohydrates, acids, salts and especially chlorogenic acid give us the impression that the vapid sour brew might have some resemblance sensorially to coffee.” However, Songer says that as humans, we don’t mind high concentrations of flavors in our foods and beverages if the flavors are positively perceived. “Making a robusta ‘truck-stop’ coffee at high concentrations would take the top of your tongue off and leave unpleasant astringency and bitterness that might last for days, while making a Cup of Excellence selection at double the strength would give the impression of a damn strong cup, but not necessarily unpleasant.” It is the balance of the chemicals present that are different, as well as the concentration in solution.
T
he chemistry of coffee flavor is really an interaction of the coffee’s chemical makeup with the physiology of the senses. The sense of taste involves the four basic tastes— salt, sweet, sour and bitter—and new research shows that there are receptors on the human tongue for other things, such as amino acids, fats and glutamates (something flavor professionals call umami after the Japanese word for a round, savory flavor found in soy sauce, MSG, tomatoes and Parmesan cheese). Important tastes in coffee include alkaloids, which provide its characteristic bitter aspect, and tannins, which give an astringent feel and crisp delivery to taste, but cannot be smelled. For a substance to be smelled, it must be volatile (tending to evaporate) and oil-soluble (dissolves in oil but not in water). So what actually dissolves in the water is the tasted compounds, while the oil-soluble aromatics are suspended in the coffee. While humans can taste only a handful of these simple tastes, they can smell more than 10,000 distinct aromatics. The combination of taste and aroma is called flavor. continued on page 100
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Taste, according to Songer, is the most consistent way we have of evaluating our experience. While aromas also are important, recent findings indicate that when we imbibe a food or beverage, it is our experience of taste that our brain uses to set up our sensitivities to expected perception of certain aromas. We are capable of sensing thousands of aromas (and other perceptions, such as the temperature of a room we’re in), but we are not always conscious of all these perceptions (though we can get better at this with practice—that’s what a good cupper does). “The taste balance of fruit acidity, sweetness, saltiness (potassium salts in the case of coffee), bitterness, and their balance with one another, are mainly responsible for our evaluation of coffee,” says Songer. When we perceive from a taste perspective a certain combination of sourness and sweetness with which we are familiar, it may cause us to be more sensitive to lemon or other citrus fruits. This phenomenon is why one cupper will find nuts and chocolate aromas, while another will find fruit and flowers. Sweetness in coffee comes from sugars, most of which caramelize during roasting to produce a warm toastiness. The uncaramelized sugars don’t give a distinct sweetness, so the sweetness of coffee is mainly the darker tone of caramel. This is complemented by many aromatics that also provide caramel-like aromas. Bitterness, though in America almost synonymous with an unpleasant taste, is an important factor in the taste of coffee. Indeed, much of the reason some drinkers add sugar to black coffee or espresso is to balance the bitterness; but without the bitterness, the beverage would be unrecognizable as coffee. In fact, adding any of the other tastes has been found to reduce the sensation of bitterness, whether sugar, salt or sour acid; just don’t try this at home. Bitterness itself helps balance the acidity in coffee. Aromatics can also balance bitterness. This is part of the balance of tastes in coffee: The best taste is found in the complementarity of the different factors. Caffeine contributes a bitter taste to coffee, but it is only responsible for about 10 percent of the overall bitterness, to which more than 20 different compounds contribute. Other compounds related to quinine give coffee an element of bitterness similar to that of tonic water. Another important chemical component of coffee is niacin (in coffee chemistry usually called by its chemical name, trigonelline). Niacin is bitter, but roughly 85 percent of it is converted in roasting to pyridines, which taste warm and roasty. The darker the roast, the more this bitterness is turned to roast aroma. The main cause of excessive bitterness is over-extraction, whether by brewing time, fineness of grind, water quality or brewing method.
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here are other gustatory factors beyond the basic tastes, such as mouthfeel, astringency and even spiciness, which are actually picked up by pain-sensation nerves in response to the irritating, oily resin called capsicum. This last is not a factor in coffee, though. (And if you do detect some while tasting a coffee, count it as a fault!) 100
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AromAtICs are he os abundan class of aroaics in coffee, and conribue carael aroas fro he sugars broken down by he hea of roasing. furAns
are he nex os abundan, wih roas-y, walnu-y or oasy aroas. they can be selled even in sall aouns, and so conribue uch o he aroa. pyrAzInes
give swee odors of carael or even ushroos. pyrroles
have a eay aroa fro maillard reacions beween sulfurous aino acids and sugars. thIophens
Songer points out, “Recent research points to the importance of body and other issues of mouthfeel in the overall experience of coffee drinking and enjoyment, especially in the case of espresso. They are not purely physical phenomenon as once thought, but involve a complex set of responses that include gustation (taste) responses in order to form a general impression.” The feeling of astringency tells you that a compound is chemically reacting with your saliva, thinning it and making reacted proteins fall onto the tongue. A little of this gives the coffee a more fluid feel and helps the taste reach the tongue; too much of it gives a rough, dried-out feeling to the tongue. As in wine and tea, tannins play a role in coffee’s flavor. According to Joseph Rivera, director of science and technology for the SCAA, “Tannins affect coffee in two ways: body and astringency. What has more body, a sauvignon blanc or a cabernet? The cabernet, due to a greater tannin content from the grape skins and seeds, which also provides the red color. The ‘body’ of coffee depends greatly on the total tannin content and concentration of dissolved short-chain carbohydrates. Perhaps one of the most important tannins, or tannin-like compound, is chlorogenic acid.” This raises the related and often misunderstood question of acid. Altogether, about 50 important acids are found in coffee, including the more familiar lactic (milk) acid, malic (apple) acid and citric acid. How does this quality work? Is a chemical acid tasted as sour? And is the desirable quality of acidity in coffee, in fact, an acid? Rivera teaches a workshop focusing on organic acids. The big picture, according to him, is that, “Although all acids are sour in 102
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nature, some taste better than others. Of course, the key here is moderation, as too much acidity will imbalance the coffee. From a chemical point of view, it makes sense that coffees of higher acidity taste better. A coffee with a higher acidity (i.e., lower pH) will always have more aroma, which in turn means more flavor.”
T
hough the four basic tastes seem simpler than the complex aromatics, these tastes are generated by a wide array of chemical compounds, and it is their full and complementary balance that gives the foundation to a rich flavor. “What really brings life to coffee as we know it,” says Rivera, “is not so much the taste component, but rather what we pick up from our olfactory senses.” “Perhaps one of the most important aspects of coffee flavor originates from the soil,” Rivera adds. “Why? Of the hundreds of aromatic compounds found in coffee, they all contain sulfur as a chief aromatic constituent,” such as in the key coffee aromatic compounds called mercaptans. “Because volcanic soils have a high concentration of sulfurous compounds, the coffees cultivated there tend to have a greater aroma content.”
T
he above aromatics give you the basic coffee aroma profile, combining caramel, toastiness and the roasted flavor similar to other Maillard-reaction effects, like bread crust, seared steak and “tobacco onions.” In culinary science, Maillard reactions are key to many cooking processes, and they were even the source of some controversy when a recent book on the science of cooking pointed out that “caramelized onions” are not caramelized at all, but Maillard-ized. “Maillards,” Songer explains, “are the result of sugar joining up with an amino acid and can smell like anything from a skunk to fruits and flowers. In research, several studies point to mercaptans and other sulfur-based aromatics (mainly Maillard reactions) as being most responsible for what we recognize as coffee.” One pyrazine, which gives an earthy aroma, is also a flavor constituent in bell peppers. Phenols found in coffee aroma are also an important element in the aroma of single-malt Scotch whiskies, especially malts from Islay, in which “phenolic” is a typical characteristic associated with their salty, spicy, seaweedy character. Diacetyl, which is produced by fermentation bacteria during coffee processing, gives a rich, buttery aroma. Coffee also contains the familiar compound vanillin, which brings its delightful scent of vanilla to the brew. Still others carry aroma elements described as sweet and potato-like, or honey-like and fruity. It is really the complexity of these factors that makes good coffee so entrancing and gives such wonderful variety to different beans. Smelling hundreds of pleasant aromas, some overlapping and others complementing each other, is a feast for the senses and deeply engaging for the brain.
