Tuesday, June 5, 2012

Reasons Behind the Bitterness of Coffee

Source: www.coffeeresearch.org

Coffee 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).
Making Coffee Less Bitter
Based upon the previous analyses, a number of steps could be introduced for making coffee less bitter:
  1. Medium roasted coffee has less soluble solids, a higher acid content, and a potent aroma when compared to darkly roasted coffee.  All of these factors are known to reduce perceived bitterness. 
  2. Decaffeination slightly reduces the perceived coffee bitterness.
  3. Allowing the coffee to soak in fresh water for approximately twenty-four hours after the fermentation process--as is done in Kenya--is said to reduce coffee bitterness.
  4. Brewing via a drip system reduces coffee bitterness relative to French press or other soaking methods, but this is likely due to the decrease in soluble solids, which is positively correlated with bitterness.
  5. A coarser grind reduces coffee bitterness. However, the proper grind size should always be used to ensure proper extraction.
Table 1. Compounds contributing to biterness found in coffee. Table adapted from Table10.1 in McCamey, 173. Click on compound name for more details.
Concentration in Roasted Coffee (mg/L)
Taste Threshold (mg/mL)
2-Methyl Furan0.05
Furfuryl Alcohol30019, 24, 40
Chlorogenic Acid20-10020,26,27
Caffeic Acid10-90
Citric Acid1,800-8,70096-590
Malic Acid1,900-3,900107-350
Lactic Acid0-3,200144-400
Pyruvic Acid400-1,700
Acetic Acid900-4,00022-70
Phenyl pyridine
Alicyclic Ketones
Aromatic Ketones

Hardwick, W. A. Interactive Flavor Influence of Some Materials in Different Food and Beverages. In "Flavor: Its Chemical, Behavioral, and Commercial Aspects." Westview Press. Boulder: 1977.
McCamey, D. A.; Thorpe, T. M.; and McCarthy, J. P. Coffee Bitterness. In "Developments in Food Science." Vol 25. 169-182. 1990.
Ohiokpehai, O.; Brumen, G.; and Clifford, M. N. 10th ASIC Colloq. Salvador, 1982.
Ordynsky, G. Z. fur Ernahrungswissenscaft, 5: 3-4, 1965.
Pangborn, R. M. Lebensm.-Wiss. U. Techno. 15, 1982.
Shibamoto, T.; Harada, K.; Mihara, S.; Mishimura, O.; Yamaguchi, K.; Aitoku, A.; and Fukada, T. Application of HPLC for Evaluation of Coffee Flavor Quality. In "The Quality of Foods and Beverages." Vol 2. Academic P. New York: 1981.
Voilley, A.; Sauvageot, F.; and Durand, D. 8th ASIC Colloq. Abidjan, 1979.
Voilley, A.; Sauvageot, F.; and Pierret, P. Eval. 9th ASIC Colloq. London, 1980.

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