There are fewer compounds affecting gustatory perception than there are aromatic compounds. These sensations are perceived by our taste buds in very different ways. Further, these compounds also interact with the brain at different rates and in different patterns. Early in their training, professional cuppers gain knowledge that some parts of the mouth, palate and tongue are more sensitive to certain stimuli, and that some sensations occur faster than others. How these individual sensations combine to cancel each other out or amplify their mutual effect must also be taken into consideration.
When it comes to perceiving aroma, the possibility exists that aromatic molecules will not make contact with the olfactory lobe, or the contact will be too brief to be perceived by the olfactory sensors de to dissipation or breakdown of the aroma. Gustatory sensations are more regular because contact between the liquid and receptor occurs for a longer period (at least a couple of seconds) of time. However, what is perceived depends to a great extent to what has previously been tasted. if a sweet solution has just been imbibed, what is tasted next will not have as great an effect in terms of sweetness. There is also the possibility of over-saturation, especially with the bitterness receptors, because if a molecule is already attached to a receptor, it will not be able to pick up further stimuli.
Gustatory sensors are constantly bathed in saliva, itself a solution that contains water, salts, acids, proteins, sugars and more. This saliva is fed and maintained by the blood, and even more complex solution that is constantly in a state of flux. Further, the chemical state of the saliva will also affect perception.
In discussing gustatory responses, some experts refer to as many as 12 different possible perceptions. However, the most familiar and well-studied gustatory sensations are salt, sour, sweet and bitter. The sense of taste is referred to as a chemosensory perception because each perceived taste has a particular chemical compound associated with it. For example, the H+ ion is perceived (in the form of the hydronium ion, H3O+) as sour, while the sucrose atom, a totally different covalently bonded compound, is perceived as sweet. In terms of size, salty tasting ion are smallest, and are therefore more difficult to remove through filtration, followed by sour-tasting substances, then sweet and finally bitter, which are the largest.
Still larger molecules are tasteless since there are no taste bud receptors capable of sensing them, but they can be perceived as aromatic by the olfactory lobe, viscosity, or as body by the tactile senses.
Acidity, Acids and Salts
A major focus of cuppers when evaluating coffee is acidity. In solution an acid is defined as an ionically bonded compound capable of donating a proton (designated H+, a hydrogen atom without its electron) to a water molecule, which forms the hydronium atom H3O+. Upon contact with the appropriate taste bud, the hydronium molecule is perceived as sour.
Liquids are often discussed in terms of being acid (having a higher concentration of H+ ions) or alkaline (referred to as having a higher concentration of OH- ions). The concentrations of these ions in solutions are measured as pH. If the pH is above 7, the solution has an excess of H+ ions and is referred to as acidic.
Some acids, while capable of dissociating in solution, are more tightly bound together and tend not to dissociate. These are referred to as weak acids. When pH is measured, only the concentration of free H+ ions in solution (i.e., those having dissociated and given up a proton) are measured. When tasting, you can sense not only the dissociated atoms, but also other acids that are non-dissociated in the originally ingested solution. Although these are not immediately measured in terms of pH, they dissociate in the mouth upon contact with saliva, or on the tongue itself, and are perceived as acidic. This phenomenon gives certain coffees completely different acid perceptions, and thus, a different taste than other coffees.
Weak acids that do not dissociate in solution can contribute to perceived acidity by dissociating upon ingesting due to mouth pH, which is typically a pH of 6. As H+ forms hydronium and binds to a receptor (i.e., taste bud) in the mouth, the balance of the solution is disturbed and more anions (including H+) are released. Acids will either continue to bind to receptors in this way, or be neutralized by salivary bicarbonate, a naturally occurring buffer solution.
As an example of how this works, consider that citric acid is a reasonably strong acid present in all coffees, though it degrades as the coffee roast is darker. And malic acid, as in the acid found in apples, is often present, particularly in higher grown coffees. But malic acid is a weaker acid than citric. When you taste a coffee containing both acids, you first perceive the citric acid before it is neutralized by your saliva. Then, the malic acid can be perceived. (A study from Englehardt and Maier (2001) found 22 different acids in coffee, and other recent studies have found many more.)
Brewed coffee also contains salts, the ionically bound substances that consist of a positively charged metal and an associated negatively charged anion. These are found in the brewing water as potassium and other salts in coffee. These salts contribute to how acidity and other tastes are perceived. In solution, interaction effects occur when strong acids, weak acids and salts are all present. Stronger acids ionize readily and delay the ionization of the weaker acids, while the salts present in the waters can inhibit ionization of either strong or weak acids. The greater the difference in strengths of their ionic bonding, the more this effect comes into play and will affect the flavor of the brewed coffee. This is one reason why the amount of salts present in the water, whether originating from the water supply or from softening, will have a major effect on coffee flavor.
Interactions between acids and bases can affect the perception of acidity in another way. Undissociated weak acids in the presence of a base can form salts, with the remaining OH- and H+ ions combining to produce water. Likewise, if a strong acid ionizes in the presence of a strong base, the dissociated H+ ions combine with dissociated OH- ions to form water. As a result, a hydronium atom is not formed, no change in pH occurs and the acidity is not perceived. The capacity of a solution, such as coffee, to neutralize acidity is referred to as the buffering capacity. This is determined by measuring the total alkalinity (i.e., the concentration of negative ions, or anions), which in turn help determine the capacity of a water supply to brew flavorful coffee. Total alkalinity also has a physical effect on coffee bed expansion due to the particle expansion previously noted.
A solution that can remain at a fairly constant pH, despite changes in relative concentrations of specific ions, is known as a buffer system. A buffer system either protects against strong acids, because salt is present in a base, or protects against strong bases, because of the reserve acidity in an undissociated weak acid. The balance of all these ions, including those measured by pH and total alkalinity, will affect the perceived acidity in the cup.
As with aromas, the variety and concentration of acids and salts differs according to origin and roast parameters of the particular coffee. For instance, the interaction of phosphoric acid (a particularly strong acid) and potassium (a particularly strong base) appears to be important to perceived acidity. The amount of phosphoric acid appears not only to make a direct contribution to beverage acidity, but also binds to the potassium, giving a less salty or rough taste.
Bitterness and Sweetness
The perception of sweetness in foods in mainly due to low molecular weight carbohydrates such as sucrose (i.e., table sugar). These not only contribute to the gustatory sensation of sweetness, but also can add to the perception of body.
Most of sucrose and other simple sugars originating in green beans are sublimated during roasting (97% in light roasts and 99% in dark roasts), emerging mainly as aromatics, caramelized sugars and organic acids. This is enjoyed as a subtly sweet gustatory note that is often present, especially in the best coffees.
The caramelization of sucrose that occurs during roasting forms other compounds that will be perceived as a combination of sweetness and sugar-browning aromatics such as caramel. Maillard reactions are more complex sugar-browning reactions in which amino acids and carbohydrates combine to form new aromatic compounds. More complex carbohydrates found in the green bean can degrade into simple sugars upon roasting. Some combination of these effects most likely accounts for coffee’s sweet taste sensation.
The small amount of sucrose that remains in brewed coffee (between 0.11% and 0.08% dry weight) is below the taste threshold, the level at which most people can perceive. These sub-threshold concentrations of sucrose can improve flavor without directly causing a sweet sensation, but at the same time, suppressing to some extent the bitter, acid and salty tastes. From these observations, it is reasonable to infer that a key function of the sweet compounds in coffee is not to impart a sweet taste directly, but to subtly balance the other gustatory sensations. The maximum extraction of sucrose and other simple carbohydrates, despite their low levels, may be important to brewed coffee flavor and can be affected by water quality.
The most common complaint heard about the coffee beverage relates to bitterness. As with many foods, some bitterness in coffee is desirable, but it should not be overwhelming. Caffeine, bitter tasting in its purified form, was initially thought to be responsible for coffee’s bitter taste, but decaffeinated coffees can also taste bitter. Recent sensory studies have shown that caffeine accounts for about 10% of the bitterness perceived in coffee. As in many foods, bitterness affects other gustatory perceptions, accentuating sweet and sour gustatory sensations. Bitterness is also sometimes confused with astringency, which is a mouth-drying sensation.
Current studies on taste buds hypothesize that a close relationship exists between sweet and bitter taste receptors. These studies suggest that certain compounds have a sweet side to their molecules as well as a bitter side, processing in both the sweet and bitter molecular taste buds. A familiar example is that some sugar substitutes have a definite bitter aftertaste.
Bitter compounds found in coffee include chlorogenic acids, diterpenes, trigonelline and caffeine. All are water soluble, but most require a longer contact time, greater agitation during brewing or higher temperatures to fully dissolve than acids, salts and simple sugars. For this reason, the rate of extraction of flavors, which is directly affected by water quality and coffee particle porosity, will determine the balance of tastes in a brewed coffee.
