2009_Book_FoodChemistry

964 21 Coffee, Tea, Cocoa
21.3.2.3.7 Volatile Compounds
and Flavor Substances
Cocoa aroma is crucially dependent on harvesting,
fermentation, drying and roasting. The fresh
beans have the odor and taste of vinegar. The
characteristic bitter and astringent taste and the
residual sweet taste of fermented beans might be
impaired by various faults, such as processing of
unripe or overripe fruit, insufficient aeration, lack
of mixing of the fruit, infection with foreign organisms
and/or smoke damage as a result of improper
drying.
The odorants of cocoa powder are listed in Table
21.24. A model (cf. 5.2.7) made by using
the 24 aroma substances on the basis of deodorized
cocoa powder reproduced the aroma of cocoa
very closely.
The taste of cocoa is described by the attributes
bitter, astringent and sour. It can be reproduced
by mixing 41 constituents dissolved in
water (pH 5.5). The key compounds for the
individual notes are the substances listed in
Table 21.23. Apart from theobromine, a series of
diketopiperazines are involved in the bitter taste
(cf. Table 21.23), which are formed during the
thermal degradation of proteins during roasting
(Formula 21.10):
(21.10)
The intensity of the bitter taste of theobromine
is increased by the interaction with certain
diketopiperazines, a molar ratio of 2:1 giving the
highest intensity. However, only those complexes
are synergistically active in which the hydrogen
bridges shown in Formula 21.11 can be formed.
Thus, synergism does not occur when, e. g., in
caffeine, the N(1)-atom in the purine ring is
methylated.
Apart from the compounds mentioned in Table
21.23, epicatechin contributes to the astringent
taste and also influences the bitter note.
(21.11)
Table 21.24. Concentrations of potent odorants in a cocoa
powder a
Compound
Concentration
(mg/kg)
Acetic acid 332
3-Methylbutanal 25.8
2-Methylbutanal 14.3
Phenylacetaldehyde 6.60
3-Methylbutyric acid 8.55
2-Phenylacetic acid 7.70
Methylpropionic acid 2.80
2-Methylbutyric acid 1.75
4-Hydroxy-2,5-dimethyl-3(2H)-
furanone 0.62
2-Phenylethanol 0.59
Butyric acid 0.32
2-Phenylethylacetate 0.32
2-Methoxyphenol 0.23
4-Methylphenol 0.12
Linalool 0.072
2-Ethyl-3,6-dimethylpyrazine 0.070
3-Methylindole 0.055
2-Ethyl-3,5-dimethylpyrazine 0.031
3-Hydroxy-4,5-dimethyl-2(5H)-
furanone 0.015
2,3-Diethyl-5-methylpyrazine 0.008
Dimethyltrisulfide 0.007
2-Acetyl-1-pyrroline 0.006
2-Methyl-3-(methylthio)-furan 0.0005
2-Isobutyl-3-methoxypyrazine 0.0009
a Partially defatted cocoa powder (fat content: 20%),
treated with alkali.
21.3.2.4 Reactions During Fermentation
and Drying
The reactions occurring within the pulp during
fermentation of whole cacao fruit can be distinguished
from those occurring in the nibs or
cotyledons. The pulp sugar is fermented by yeast
to alcohol and CO 2 on the first day. Lactic acid