Organic Acids in Coffee in Relation to the Degree of Roast

Abstract

The chemistry of flavor development during the roasting of coffee is not well understood. Acidity is often considered an important factor in the flavor of brewed coffee. Consequently, a study of the acidic compounds of coffee might contribute to a better understanding of the taste and flavor of coffee beverages. Kaufman (8) made water extracts of both green coffee and successive samples withdrawn during roasting, and observed that the p H fell about one pH unit, and then began to rise. These changes were confirmed here. The optimum roast is often considered to terminate shortly after the p H of the water extract begins to increase. The titratable acidity is nearly doubled at this point (12,16). Kaufman (8) suggested that these changes were due to the formation and volatilization of acetic acid from decomposition of sugars, decarboxylation of acids formed by rearrangement of sugars and decarboxylation of chlorogenic acid. In a heavy roast approximately 50% of the original chlorogenic acid was reported to be destroyed (14). Since partition chromatography on silica gel was particularly adaptable for a separation of organic acids in coffee (9,11,19) a quantitative study of several organic acids in coffee during roasting has been carried out. EXPERIMENTALCoffee samples were obtained from the Coffee Brewing Institute, New York. The samples were vacuum packed in one-pound cans as whole beans and stored at -20" C. Table 1 describes the samples and gives the code.Methods. The weight of 100 beans was obtained as a mean value, weighing 4 groups of 200 beans each. One hundred grams of coffee beans were ground coarsely with a laboratory mill. Moisture was determined by drying 2 g. for 5 hours at 105'-110" C. Thirty-gram samples were shaken 5 minutes with 400 ml. distilled water of 95" C. in a 1000-ml. Erlenmeyer flask. Next, 100 nil. of distilled water was added by rinsing the walls of the flask; the flask was then cooled under running tap water. The extract was centrifuged for 20 minutes and the solution transferred to a 500-ml. volumetric flask and filled to the mark. Using 25 ml. of this extract the titratable acidity was detrmined by titrating with 0.1 N sodium hydroxide to pH 8.2 as indicated by a Beckman pH meter model G. The remaining extract was brought to pH 7.9 with 0.1 sodium hydroxide and after filling again to the mark 20 ml. aliquots were freeze-dried. The weight of solids was recorded and then the solid extract after acidifying to pH 1 was transferred to the top of a column of silica gel. The organic acids were then separated into 6 bands by a method similar to that reported from this laboratoe (11). The purity of these bands was checked by paper chromatographic methods using the Ri values of Table 2 in the case of phenolic acids. Some of the compounds were identified by physico-chemical methods.The individual fractions from the column were evaporated to dryness. If the presence of volatile acids was suspected, the fractions were neutralized with ammonia before drying. The residues were dissolved in ethanol or ...

Roasting Kinetics and Chemical Composition Changes of Robusta Coffee Beans During Hot Air and Superheated Steam Roasting

“…This is probably because roasting induced disruption of plant tissues, resulting in the release of these acids (Yodkaew et al., ). However, significant decrease or even disappearance of malic acid was observed upon prolonged roasting due most probably to its heat sensitive nature (Lentner & Deatherage, ; Wei et al., ).…”

The effect of bean origin and temperature on grinding roasted coffee

“…Three illustrative decomposition reactions are shown that are representative processes throughout the heating process. At lower temperature a chlorogenic acid (left) may decompose through either hydrolysis or pyrolysis into quinic acid, acetic acid and the phenolic compound 3,4-dihydroxybenzyl alcohol 40 , or quinic acid, carbon dioxide and 3,4-dihydroxystyrene 41 42 . Oxalic acid (centre) may decarboxylate to either CO 2 or in the case of incomplete combustion CO 2 and formic acid 19 .…”

Importance of nonvolatile compounds to the flavor of coffee

“…CHANGES IN ACIDS DURING ROASTING Lentner and Deatherage (1959) stated that during roasting of coffee beans, the amounts of formic and acetic acids increased. Between 32 and 52% of the chlorogenic acid was destroyed during roasting, whereas the loss in citric acid was 33 to 56% and for malic acid the loss was 16 to 40%, depending on the particular coffee used and the final roast temperature.…”