Guide to Coffee


Barista making coffee

Acidity

Coffee Acidity is the bright and dry taste that adds life to a coffee. Acidity is unmistakable in most Kenyan coffees and should be evident in any wet-processed coffees. Perceived acidity in coffee does not necessarily correlate to the pH of a coffee, but is believed to be the result of the acids present. The acidity of coffee is akin to the dry but bright sensation experienced on the back sides of your tongue while drinking a red wine.

Dry-processed coffees will have a low-toned and subtle acidity. It is important to note that as the roast develops, coffee acidity decreases. Acidity is not a favorable quality in high amounts in espresso. For this reason, most American roasters will roast espresso very darkly. Unfortunately at darker roasts, the aroma and sweetness diminish, which is equally detrimental to the espresso.

Body

Body is the weight of the coffee that can best be sensed by allowing the coffee to rest on the tongue and by rubbing the tongue against the roof of the mouth. Coffee body ranges from thin, to light, to heavy and is a result of the fat content. The viscosity, however, results from proteins and fibers in the brew. Medium and dark coffee roast styles will have a heavier body than lighter roasted coffees, but conversely will have less acidity.

Coffee flavor is a term that encompasses all of the other coffee cupping parameters. It is an overall evaluation of the coffee taste. The SCAA created a coffee flavor wheel, which is used as a helpful guide during coffee cupping. The full poster is in color and includes another wheel to describe flavor and aroma taints.


Roasting

Coffee roasting is a chemical process by which aromatics, acids, and other flavor components are either created, balanced, or altered in a way that should augment the flavor, acidity, aftertaste and body of the coffee as desired by the roaster.

Roasting Coffee Beans

The first stage is endothermic. The green beans are slowly dried to become a yellow color and the beans begin to smell like toast or popcorn.

The second step, often called the first crack, occurs at approximately 205 degrees celcius (400 degrees farenheit) in which the bean doubles in size, becomes a light brown color, and experiences a weight loss of approximately 5%. The corresponding Agtron number for this color is between 95-90.

In the next step the temperature rises from 205 degrees celcius to approximately 220 degrees celcius, the color changes from light brown to medium brown, and a weight loss of approximately 13% occurs. The resulting chemical process is called pyrolysis and is characterized by a change in the chemical composition of the bean as well as a release of CO2.

The second step is followed by a short endothermic period which is followed by another exothermic step called the second crack. This second pyrolysis occurs between 225-230 degrees celcius, and the roast color is defined as medium-dark brown. The second pop is much quicker sounding and the beans take on an oily sheen.

Espresso potential is maximized in roasting when you maximize the sweetness and aroma of the coffee while minimizing the bitterness and acidity. Most people focus on the latter and therefore roast extremely dark, yet without sweetness and aroma the espresso will never be palatable. This explains the unpopularity of straight espresso and the popularity of espresso based drinks where either milk or other flavors are used to replace the sweetness that was lost by roasting darkly.

From 170-200 degrees celcius the sugars in coffee begin to caramelize. From tasting pure sugar versus its caramelized component it is evident that uncaramelized sugar is much sweeter. The dark color of coffee is directly related to the caramelization of the sucrose in coffee. Therefore, to maximize sweetness you want to minimize the carmelization of sucrose, yet you do not want to roast too lightly or bitter tasting compounds will not thermally degrade. Stop the roast somewhere between the end of the first crack and less than half way through the second crack. Do not roast well into or past the second crack. We recommend a roasting chamber temperature somewhere between 205-215 degrees celcius. Realizing the danger of the following suggestion we might recommend a color similar to the one below. Note: All monitors, computers, and internet browsers will display the color slightly different. This is only a recommendation to point out that this color is preferable to the almost black color you will frequently observe for espresso. To get a better idea of roasting colors order the Agtron roasting classification kit from the Specialty Coffee Association (SCAA).


Chemistry

Cause of Bitter Coffee

Coffee bitterness is sometimes a negative, but omnipresent, aspect of the beverage. At low levels, bitterness helps tame coffee acidity and adds another favorable dimension to the brew. However, at high levels, a bitter coffee compound can overpower the other components present in coffee producing an undesirable effect. Bitter coffee results from the interaction of certain compounds with the circumvallate papillae on the back of the tongue. Astringency, on the other hand is caused by compounds that can precipitate salivary proteins on the tongue. Consumers will often mistakenly attribute astringency and any other potent characteristic of the coffee to the bitterness. Therefore, this article will discuss those compounds that are responsible for contributing to the bitterness of the coffee and those compounds that cause astringency in the coffee.

Why Does Coffee Taste Bitter?

Various coffee scientists have made the following observations concerning bitter coffee, which were presented in a review article by McCamey et al.:

  • The perceived bitter taste in the mouth from coffee is correlated to the extent of extraction. The extent of extraction is dependent upon the roast, the mineral content of the water, water temperature, time, grind size, and brewing procedure.
  • Bitterness is reduced in coffee brewed with either soft or hard water relative to distilled water (Voilley et al., 251).
  • Bitterness is correlated with the total dissolved solids of a coffee.
  • Perceived coffee bitterness is lower when coffee is brewed hot than when cooler water is used. This is hypothesized to be due to the heightened aromatics released in hot coffee, which counteract the bitterness (Voilley et al., Eval., 287).
  • Coffee bitterness is decreased by the addition of sucrose, sodium chloride, or citric acid. Hydrocolloids, in general, were found to decrease the perception of coffee bitterness (Pangborn, 161).
  • Robusta coffee contains higher levels of both caffeine and chlorogenic acids, which are partly responsible for bitterness and astringency in coffee.
  • Several investigators have found that the processing of coffee (wet or dry processing) does not affect the perceived bitterness of coffee even though the overall flavor profile is significantly different (Clarke and Macrae; and Clifford and Wilson).
  • Caffeine has a distinct bitter taste and has a test threshold of only 75-155 mg/L (60-200 mg/L found by Clarke). However, Voilley considers caffeine to only account for around 10% of the perceived bitterness in coffee.
  • Hardwick found that the bitterness of caffeine is weakened when polyphenols are introduced.
  • Maier reported that the sourness of coffee was diminished by increased bitterness.
  • Astringent and metallic tastes in coffee have been attributed to dicaffeoylquinic acids, but not the monocaffeoylquinic acids (Ohiokpehai et al., 177).
  • Trigonelline is perceived as bitter at concentrations of 0.25%, whereas chlorogenic acids necessitate a concentration of 0.4% at pH of 5 to be perceived as bitter (Ordynsky, 206).
  • Trigonelline degradation is proportional to roast degree. Its byproducts include pyridines, which are said to contribute a roasty aroma to the coffee.
  • Quinic acid -- a degradation product of chlorogenic acids -- is present at twenty times its threshold value and is partly responsible for the perceived bitterness in coffee (McCamey, 176).
  • Furfuryl alcohol is thought to contribute a burnt and bitter taste to coffee (Shibamoto et al., 311).

Coffee Aroma

Coffee aroma is responsible for all coffee flavor attributes other than the mouthfeel and sweet, salt, bitter, and sour taste attributes that are perceived by the tongue. Therefore, it might be said that coffee aroma is the most important attribute to specialty coffee. Even instant coffee has the components responsible for stimulation of our taste buds. The difference, however, is that instant coffee lacks most of the aromatic volatile compounds causing a dramatic decrease in the overall coffee flavor.

Perception of Coffee Aroma

Coffee aroma is perceived by two different mechanisms. It can either be sensed nasally via smelling the coffee through the nose or retronasally. Retronasal perception occurs when the coffee is either present in the mouth or has been swallowed and aromatic volatile compounds drift upward into the nasal passage.

The number of aromatic compounds found in coffee increases every year. Today the number is well over 800, and as our analytical methods become more precise, more will be uncovered. Yet, the perception of coffee aroma is dependent upon both the concentration of the compound and its odor threshold. With that said, understanding coffee aroma is not as difficult as understanding how over 800 coffee elements interact with the olfactory epithelium. It is probable that a relatively small group of compounds that share both a high concentration and a low odor threshold make up the fragrance we know as coffee aroma. This article will discuss the recent research that has narrowed in on these aroma impact compounds.

Illy listed the following chemical processses that affect the development of volatile compounds in coffee (112):

  1. Maillard or non-enzymatic browning reaction between nitrogen containing substances, amino acids, proteins, as well as trigonelline, serotonine, and carbohydrates, hydroxy-acids and phenols on the other.
  2. Strecker degradation.
  3. Degradation of individual amino acids, particularly, sulfur amino acids, hydroxy amino acids, and proline.
  4. Degradation of trigonelline.
  5. Degradation of sugar.
  6. Degradation of phenolic acids, particularly the quinic acid moiety.
  7. Minor lipid degradation.
  8. Interaction between intermediate decomposition products.

In a review article published by Clarke, he asserts that various research groups have identified 150 aliphatic compounds including 56 carbonyl compounds and 9 sulfur containing compounds; 20 alicyclic compounds, including 10 ketones; 60 aromatic benzenoid compounds, including 16 phenols; 300 heterocyclic compounds, including 74 furans, 10 hydrofurans, 37 pyrroles, 9 pyridines, 2 quinolines, 70 pyrazines, 10 quinoxalines, 3 indoles, 23 thiophens, 3 thiophenones, 28 thiazoles, and 28 oxazoles (34).


Coffee Acidity

The perceived acidity of coffee results from the proton donation of acids to receptors on the human tongue. Coffee acidity is typically a highly valued quality especially in Central American and some East African coffee. Sourness, however, is an extreme of acidity and can be considered a coffee defect. Acidity has been correlated with coffees grown at very high altitudes and in mineral rich volcanic soils. The perceived acidity of washed coffees is also significantly higher than the acidity found in naturally (dry) processed coffee. This is likely due to an increase in the body of naturally processed coffees relative to wet processed coffees since body masks the acidity in coffee. The coffee acid content in a brew is also greatly dependent upon the coffee roasting degree, type of roaster, and coffee brewing method.

The pH of a coffee has been found to correlate with the perceived acidity in coffee by Pangborn, Sivetz and Desrosier, and Griffin and Blauch; whereas Voilley et al. suggests that titratable acidity produces a better correlation to perceived coffee acidity.

Notes
  1. Volatile Aliphatic Carboxylic
  2. Non-Volatile Aliphatic Carboxylic
  3. Heterocyclic furanoid carboxylic
  4. Chlorogenic
  5. Alicyclic/phenolic
  6. Inorganic
Sources
  1. Clifford, M. Tea and Coffee Trade J. 159: 8. 1987. 35-39.
  2. Illy, A. and Viani, R. Espresso Coffee: The Chemistry of Quality. 107-110.
  3. Clarke, R.J. The Flavour of Coffee. In Dev. Food Science. 3B: 1-47. 1986. 1-47.

Coffee Cupping

Cupping is one of the coffee tasting techniques used by cuppers to evaluate coffee aroma and the flavor profile of a coffee. To understand the minor differences between coffee growing regions, it is important to taste coffee from around the world side-by-side. Cupping is also used to evaluate a defective coffee or to create coffee blends.

Tasting Coffee

In a coffee cupping session, the table is usually set up with 6 to 10 cups per coffee. These are fashioned in a triangular manner. At the top of this triangle you should place a sample of the roasted coffee and a sample of the green coffee. In the center of the table place a cup of room temperature water and an empty cup containing the cupping spoons. Cover both the green sample and roasted sample until the cupping session is over and the coffee aroma, fragrance, and flavor profile have been documented. After this time, the coffee samples could be uncovered and additional comments can be written based on appearance. This method will help reduce the common "eye cupping" technique.

Coffee Sample Preparation

To prepare the coffee samples, place 2 tablespoons of freshly roasted and freshly ground coffee in a 6 oz cup. Ideally one should use 55g of coffee per liter of water. The grind should be between a French press size and a drip coffee size. The coffee should be roasted light (Agtron 65). In the industry we often stop the roast about 30 seconds into the first crack long before the start of the second crack. This allows us to fully evaluate the coffee for defects and for the sweetness and aroma that are burned off at darker roasts. The roast should be similar for all of the coffees evaluated. During an important coffee cupping session the roast similarity can be verified visually by grinding a portion of each sample and lining the coffee samples up next to each other on a black sheet of paper.


Coffee Fragrance and Aroma Analysis

While the filtered water is boiling, smell the coffee grounds and write down your observations. The smell of the grounds (before water is added) is referred to as the fragrance.

Then add hot water -- just off the boil -- to each cup. At this time you should also add hot water to the cup containing the spoons so that the spoons stay at the same temperature as the cups containing the coffee. Smell each cup without disturbing it and write down your initial observations of the coffee aroma.

After 1-2 minutes, break the crust of the coffee using one of the preheated spoons. Put your nose directly over the cup and push the coffee down. This is the most potent burst of aroma you will have during cupping and is the best time to evaluate the coffee aroma. As you break the crust stir the cup a little to make sure all of the coffee is covered in water and to help the coffee sink to the bottom of the cup. Add any further description of the aroma to the description you wrote before breaking the crust.

Rinse the spoon in hot water and move to the next sample. After evaluating the aroma of all of the samples, scoop out any grounds that continue to float. Due to the high density of the lightly roasted coffee most of the grounds will sink.


Coffee Cupping Conclusions

The key to cupping coffee is practice and humility. The best cuppers I know are modest and always eager to learn more. I have served on cupping juries with some of the best in the world and we do not always agree. The beauty is that we agree to disagree while respecting and trying to identify the characteristics that other people find.

Do not be intimidated by people that try to impress you with some abstract description of a coffee. This is more of a romantic tribute to a coffee rather than a reality. Cupping coffee should be fun and interesting, but not a contest of who is more articulate. On the other hand, your description should be more substantial than a reiteration of a textbook definition of a coffee.

Despite the strict, scientific-like protocol to coffee cupping, the method followed in the industry is quite varied and almost every good cupper has his or her own permutation. Cup under conditions you like, but try to stay close to the standards in case you need to cup with other people.

The secret to becoming a good coffee cupper is simple: trust yourself by practicing regularly and be humble enough to continue to learn from others.