The Chemistry of Coffee

What is the formula for the perfect Cup?

Coffee can be a popular topic of study in the chemistry classroom. The word "coffee" is derived from the Arabic word "quahweh," which may have been a poetic term for wine. Since wine was a forbidden drink to some, the name was transferred to coffee. The drink and variations of its name spread across the continents. They drink kahweh in Turkey, café in France, caffe in Italy, kaffee in Germany, koffie in Holland, and coffee in England. Of more than 5oo compounds in coffee that have been identified, only caffeine, its main physiologically active ingredient, has been extensively studied. Since so much is already known about this aspect of coffee, the focus of this investigation is the acidity of this particular beverage when it is brewed.
The Variables of the Experiment

The activity should demonstrate the effect of some variables on the acidity of coffee. Since variable recognition is important to the structure of an experiment, considerable emphasis should be placed on determining what variables would be included. The following are some suggestions:

* Bean Type: Arabica or robusta

* Flavouring: hazelnut, Irish cream, chocolate almond

*Caffeinated or decaffeinated

* Roast type: dark, medium, light or expresso

* French beans (gourmet) or preground beans stored in a vacuum container (supermarket variety)

* Instant, freeze dried, or regular

* Temperature of water necessary for preparation

* Method of preparation: drip or percolation

* Length of brewing time

Information Found about Coffee Beans

Research has indicated that the yield of acids, as judged by the beverage pH, appears to be a function of the origin of the bean, processing, method of roasting, roast colour, and method of brewing. Typically the acidity of the beverage is greater when the beans used are: roasted Arabians instead of roasted robustas, wet processes instead of dry processed, medium roasts instead of dark roasts, and pressure roasted instead of normal roasted. The caffeine content and the flavouring should not affect the pH. Chlorogenic acids (quinic, caffeic, and pyridine-3-carboxylic acids) can greatly affect the pH of coffee. During the roasting process, changes occur in these acids. The percent of chlorogenic acids in an Arabica bean changes from 3.77 (green bean), to 2.74 (light roast), to 2.16 (medium roast), to 0.93 (dark roast). Phosphoric acids on the other hand, is considered by some researchers to be the major source of acidity in coffee. It is influenced by the method and severity of roasting, the method of extraction or brewing, and the brew water used. A pH of 4.9 to 5.2 is the preferred range for a "good cup of coffee."

The Experiment

With the knowledge above, the students can then describe what variables they would keep constant and what variables they would change. They can either devise their own experiment to test the acidity of the coffee beans or they can follow the method outlined below:

Brew each coffee type using the filter method with 75 mL of water, having a temperature of the of 92oC. Separately place 7.0 g of a coffee brand on each filter paper and brew for 2 minutes. Measure the pH value of the coffee once made. The students should have a chart for their results (see Table 1) to record their observations and examine how each of the following affects the pH of coffee:

  • Light, medium and dark roasts
  • Decaffeinated and caffeinated
  • Arabica and robusta beans
  • Light roast and light roast flavored

Table 1

Name of coffee brand / Roast type / Bean type / Decaf / Flavored / pH

This is an excellent way to get the students to practice their research skills and use deductive reasoning to explain the results that they have obtained.

Reference:

Borer, L., Magnusson, C., Fendall, B., Science Teacher, May 1994, volume, 61, no.5.