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 94°C 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
|
70°C
(mg/L)
|
94°C
(mg/L)
|
100°C
(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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