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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:
- 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.
- Decaffeination slightly reduces the perceived coffee bitterness.
- 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.
- 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.
- 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. |
Compound |
Concentration
in Roasted Coffee (mg/L) |
Taste Threshold
(mg/mL) |
Quinic |
3200-8700 |
10 |
5-hydroxymethylfurfural |
10-35 |
200 |
2-Methyl
Furan |
0.05 |
|
Furfuryl
Alcohol |
300 |
19, 24, 40 |
Trigonelline |
3,000-10,000 |
|
Chlorogenic
Acid |
20-100 |
20,26,27 |
Caffeic
Acid |
|
10-90 |
Citric
Acid |
1,800-8,700 |
96-590 |
Malic
Acid |
1,900-3,900 |
107-350 |
Lactic
Acid |
0-3,200 |
144-400 |
Pyruvic
Acid |
400-1,700 |
|
Acetic
Acid |
900-4,000 |
22-70 |
Pyrazine |
17-40 |
1 |
Thiazole |
|
|
Quinoline |
|
|
Phenyl pyridine |
|
|
Caffeine |
10,000-20,000 |
78-155 |
Peptides |
|
|
Proteins |
|
|
Alicyclic Ketones |
|
|
Aromatic Ketones |
|
|
References
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.
Related
Resources
Coffee
Chemistry- Aroma
Coffee
Chemistry- Acidity
Coffee
Cupping
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