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Coffee Chemistry: 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.

The acids found in roasted coffee may be classified into three groups: aliphatic, chlorogenic, and alicyclic carboxylic and phenolic acids (Clarke, 25). Table 1 shows the main acids that have been reported in coffee.

Table 1.  Coffee Acidity Chart: Acids Present in Coffee.  Click on compound names to see structural and physical data.  
Acids Present in Coffee 3
Notes 3 Comments
Formic a pKa = 3.75, 130-159 µmole/100 mL.1 0.05-0.1% dry matter at med roast. Max at light roast.2

Acetic

a pKa = 4.75, 74-226 µmole/100 mL.1 0.12-0.4% dry matter, max concentration at light roast.2 Derived from carbohydrate degredation.2
Propanoic a  
Butanoic a  
Methylpropanoic a  
Pentanoic a  

2-Methylbutanoic

a  
3-Methylbutanoic a  
Hexanoic a  
Heptanoic a  
Octanoic a  
Nonanoic a  
Decanoic a  
Lactic

b

pKa = 3.08, 22 µmole/100 mL.1 0.11% dry matter. Concentration independent of roast.2
Pyruvic b 0.06% dry matter. Concentration independent of roast.2
cis- and trans-but-e-enoic b  
cis- and trans-2-methylbut- 2-enoic b  
3-methylbut-2-enoic b  
methylpropenoic b  
oxalic b  
malonic b  
succinic b  
3-methylene butanedioic b  
glutaric b  
Malic b pKa = 3.40 / 5.11, 58-76 µmole/100 mL.1 0.17-0.5% dry matter at med roast. Max at light roast.2
tartaric b  
cis- and trans-butenedioic b  
cis- and trans-methylbutenedioic b  
methylenebutanedioic b  
citric b pKa = 3.14 / 4.77 / 6.39, 75-189 µmole/100 mL.1 0.37-0.5% dry matter, max at light roast.2
propene- 1,2,3-tricarboxylic b  
2-furoic c  
3-monocaffeoylquinic acid d pKa = 3.40, 96-291 µmole/100 mL.1
4-monocaffeoylquinic acid d The chlorogenic acids have an astringent taste due to its ability to precipitate salivary proteins onto the mucous membranes. Therefore it may also be responsible for heightened body.2
5-monocaffeoylquinic acid d At dark roasts, 80% of the CGA's may be lost resulting in a residual CGA content of 2.2-2.4%.2
3,4-dicaffeoylquinic acid d  
3,5-dicaffeoylquinic acid d  
4,5-dicaffeoylquinic acid d  
3-feruloylquinic acid d  
4-feruloylquinic acid d  
5-feruloylquinic acid d  
3,4-p-coumaroylquinic acid d  
3,5-p-coumaroylquinic acid d  
4,5-p-coumaroylquinic acid d  
Quinic e pKa = 3.40, 123-242 µmole/100 mL.1 0.6-0.8% dry matter at med. roast. Concentration increases inversely with chlorogenic acid.2
Ferulic e  
Caffeic e  
Phosphoric f

pKa = 2.12 / 7.21 / 12.67, 65-108 µmole/100 mL.1 0.54% of dry matter.2

Notes

a. Volatile Aliphatic Carboxylic

b. Non-Volatile Aliphatic Carboxylic

c. Heterocyclic furanoid carboxylic

d. Chlorogenic

e. Alicyclic/phenolic

e. 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.

 

In regards to the concentration of citric, malic, lactic, pyruvic and acetic acid, Blank found that a typical medium roast coffee consisted of 0.30%, 0.22%, 0.13%, 0.07%, and 0.27% of each acid, respectively (Clarke, 25).  At very light roasts, Blank found that the total concentration of these acids was around 1.58%, while at dark roasts these acids could drop down to 0.71%.

Chlorogenic acids have been found to make up around 7% of the dry basis weight of Arabica coffee.  The 3-CQA isomer-the largest isomer present-is found at 4-5%.  Clifford and Jarvis found over 17 chlorogenic acid-like substances in 42 robusta samples.  The chlorogenic acids are largely degraded during the roasting process mainly into quinic acid.  Excessive quinic acid has been associated with unfavorable sourness when coffees are roasted too dark or brewed coffee is left on a heater plate.  This sourness, however, is contradictory to the rise in pH and reduced perceived acidity at darker roasts and is likely operating under a different mechanism.

In a study by the Technical Unit of the International Coffee Organization, they reported on the acid concentration in coffee at different particle sizes, water temperature, and extraction times.  The results have been detailed in Table 2, 3, and 4, respectively.

Table 2.  Acid concentrations at different grind sizes.  All brewing was carried out at 94C for 5 minutes.  (Source: ICO Sensory).

Acids

Course Grind Size (mg/L)

Fine Grind (mg/L)

Extra Fine Grind (mg/L)

Lactic Acid

109.67

194.50

308.33

Acetic Acid

242.67

225.67

209.00

Citric Acid

325.00

461.00

440.00

Malic Acid

119.33

137.00

163.67

Phosphoric 

68.33

77.33

82.00

Quinic Acid

435.33

495.00

510.00

Chlorogenic 

700.00

1,064.67

1,177.00

Palmitic Acid

5.03

5.90

3.63

Linoleic Acid

6.27

5.97

4.50

 

Table 3. Concentration of extracted acids at different brewing temperatures. All coffees were brewed using a fine grind for 5 minutes.  (Source: ICO Sensory).

Acids

70C (mg/L)

94C (mg/L)

100C (mg/L)

Lactic Acid

121.00

194.50

187.33

Acetic Acid

151.33

225.67

187.00

Citric Acid

388.33

461.00

332.00

Malic Acid

131.00

137.00

122.00

Phosphoric Acid

86.33

77.33

80.00

Quinic Acid

348.33

495.00

383.33

Chlorogenic Acids

872.67

1,064.67

1,067.67

Palmitic Acid

3.26

5.90

6.53

Linoleic Acid

3.83

5.97

8.30

 

Table 4. Concentration of extracted acids at different brewing times. All coffees brewed were of a fine grind and were brewed at 94°C.  (Source: ICO Sensory).

Acids

1 Minute (mg/L)

5 Minutes (mg/L)

14 Minutes (mg/L)

Lactic Acid

56.67

194.50

125.67

Acetic Acid

261.00

225.67

242.00

Citric Acid

343.33

461.00

355.33

Malic Acid

109.33

137.00

100.33

Phosphoric 

75.00

77.33

75.67

Quinic Acid

525.00

495.00

556.67

Chlorogenic 

955.00

1,064.67

988.33

Palmitic Acid

4.97

5.90

5.87

Linoleic Acid

6.70

5.97

6.37

 

Virtually no free amino acids are still present after roasting for 5 min at 220°C (Maier, 568). However, the amino acids are important during the Maillard reaction in the production of aromatics early in the roasting process.

Phosphoric acid has been implicated as a major contributor to perceived acidity by Maier and Rivera, but an alternate conclusion has been reached by Griffin and Blauch who suggest that phosphoric acid might contribute, but is not directly correlated to the perceived acidity. The phosphate concentrations found in the latter study are nearly identical to those found for phosphoric acid in the Technical Unit Quality Series No 9 by the ICO.

It is still unknown which acids are imperative to recreate the acidity experienced in coffee.  It is generally understood that citric acid, malic acid, and acetic acid are the most important because they exist in high proportions and have low pKa's.  However, due to highly complex buffering effects and the wide distributions of salts and acids present in coffee, it is difficult to predict the exact mechanism and agents responsible for the perceived acidity in coffee.

Sources
Pangborn, R. M. Lebens. Wiss. And Technol. 1982. 15: 161-168.
Sivetz, M. and Desrosier, N. W. Coffee Technology. Avi Pub. Westport, Conn. 1979.
Griffin, M. J. and Blauch, D. N. ASIC 18th Colloq. Helsinki. 1999. 118-126.
Voilley, A.; Sauvageot, F.; Simatos, D.; and Wojcik, G. 1981. J. Food Processes. Preservation. 5: 135-143.
Maier, H. G. Proc 12th ASIC Colloq. 1987, 229-237.
Rivera, J. Organic Acid Analysis of Kenya SL 28 and Other Cultivars. SCI Technical Papers, 1997, 5-9.
Sensory Evaluation of Coffee: Technical Unit Quality Series. No 9. International Coffee Organization. 1991. 209-243.

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Coffee Chemistry- Aroma

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