Here is an article that you and others might find interesting, I have cut in the abstract text below but it is the later section of the paper on the Sensory Evaluation of Dark Chocolate that is relevant to this discussion and is worth a read see attached file
INFLUENCE OF ROASTING CONDITIONS ON VOLATILE FLAVOR OF ROASTED MALAYSIAN COCOA BEAN Journal of Food Processing and Preservation 30, 2006, p280
Abstract:
In this study, commercial Malaysian cocoa beans (SMC1A) were roasted
in a forced airflow-drying oven for 20, 30, 40 and 50 min at 120, 130, 140,
150, 160 and 170C. The products were evaluated for flavor compounds and
sensory evaluation (as dark chocolate). The volatile fraction was isolated
using the combined steam distillationextraction procedure and was identified
by gas chromatographymass spectrometry. A quantitative descriptive analysis
was used to evaluate the flavor intensity of the chocolates using a 9-point
rating scale for selected flavor attributes, namely astringency, bitter taste,
sour taste, cocoa and burnt. Panelists were asked to smell and taste the sample
against a standard chocolate. It was found that there were significant differences
in flavor compounds between the different conditions of roasting. The
main flavoring compounds identified composed of aliphatic and alicyclic
groups such as alcohol and ester, and heterocyclic groups such as pyrazine
and aldehyde. A total of 19 volatile major components were identified:
nine pyrazines (2,5-dimethyl-, 2,3-dimethyl-, 2-ethyl-6-methyl-, trimethyl-,
3-ethyl-2,5-dimethyl-, tetramethyl-, 2-ethenyl-6-methyl- and 3,5-dimethyl-2-
methylpyrazine); five aldehydes (5-methyl-2-phenyl-2-hexenal, benzaldehyde,
benzalacetaldehyde and a-ethyliden-benzenacetaldehyde); one methyl ketone
(2-nonanone); two alcohols (linalool and 2-heptanol); and two esters
(4-ethylphenyl acetate and 2-phenylethyl acetate). Based on the flavor profile
of the compounds identified, an optimum production of the major flavoring
compounds such as pyrazine, aldehyde, ketone, alcohol and ester occurred at
160C for 30 min of roasting. Trimethylpyrazine and tetramethylpyrazine compounds
together with 5-methyl-2-phenyl-2-hexanal were found to be good
indicators for the evaluation of the roasting process. However, based on
chocolate evaluation, the best roasting temperature was 150C for 30 min,
which gave the lowest astringency and at the same time gave the lowest bitter
taste and low level of sour and burnt tastes. At 150C roasting temperature, the
desirable cocoa flavor was at its optimum. Correlation coefficients among
certain volatile flavor and sensory characteristics of cocoa beans and dark
chocolate were significant (P 0.05).