2009_Book_FoodChemistry

21.1 Coffee and Coffee Substitutes 947
developed in Italy, coffee is extracted briefly by
superheated water (100–110 ◦ C), while filtration
is accelerated by steam at a pressure of 4–5 bar.
The exceptionally strong drink is usually turbid
and is made of freshly ground, darkly roasted
coffee. The water temperature should not exceed
85–95 ◦ C in order to obtain an aromatic drink
with most of the volatile substances retained.
Water quality obviously plays a role, especially
water with an unusual composition (some
mineral spring waters, excessively hard water,
and chlorinated water) might reduce the quality
of the coffee brew. Brewed coffee allowed to
stand for a longer time undergoes a change in
flavor.
For regular brewed coffee, 50 g of roasted
coffee/l (7.5g/150 ml cup) is used; for mocca,
100 g/l; and for Italian espresso, 150 g/l. Depending
on the particle size and brewing procedure,
18–35% of the roasted coffee is solubilized. The
dry matter content of coffee beverages is 1–3%.
The composition is presented in Table 21.12.
The taste of coffee depends greatly on the pH of
the brew. The pH using 42.5g/l of mild roasted
coffee should be 4.9–5.2. At pH < 4.9 the coffee
tastessour;atpH> 5.2 it is flat and bitter. Coffees
of different origins provide extracts with different
pH’s. Generally, the pH’s of Robusta var-
Table 21.12. Composition of coffee beverages a
Constituent
Protein b 6
Polysaccharides 24
Saccharose 0.8
Monosaccharides 0.4
Lipids 0.8
Volatile acids 1.4
Nonvolatile acids 1.6
Chlorogenic acids 14.8
Caffeine 4.8
Trigonelline 1.6
Nicotinic acid 0.08
Volatile aroma compounds 0.4
Minerals 14
Unidentified constituents 29.4
(pigments, bitter compounds etc.)
Content (% dry
weight basis)
a Arabica-coffee, medium roast, 50 g/l.
b Calculated as sum of the amino acids after acid hydrolysis.
Fig. 21.3. The flavor of roasted coffee brew as related
to pH value (according to Vitzthum, 1976)
ieties are higher than those of Arabica varieties.
Figure 21.3 shows the relationship between pH
and extract taste for some coffees of known origin.
The difference between the aroma of the beverage
and that of ground coffee is the more intensive
phenolic, buttery, caramel-like note and a weaker
roasty note. These changes are caused by shifts in
the concentrations of the aroma substances during
brewing (Table 21.8). Compounds like 2,3-
butandione, the furanones 6, 7 and 27, 2-ethyl-
3,5-pyrazine, the thiols 17 and 18 are extracted
with yields of >75%, while only 25% or less
of 2-ethenyl-3-ethyl-5-methylpyrazine, 3-isobutyl-2-methoxypyrazine,
2-furfurylthiol and
β-damascenone pass into the beverage. The low
yield of 2-furfurylthiol is partly due to reactions
which occur during percolation of the coffee
powder.
Caffeine and the quinic acid lactones listed in Table
21.13 are the bitter substances in the coffee
drink. Accordingly, these lactones are almost exclusively
responsible for the bitter note of a decaffeinated
coffee drink (Table 21.13). Although the
concentrations of the lactones III–VII, IX and X
in the drink are lower than their threshold concentrations
(cf. Table 21.13), they still additively
contribute to the bitter taste (cf. 5.1.2: additive effect).