Fermented cocoa beans of various countries of origin (Ivory Coast, Ghana, Sulawesi), cocoa beans roasted under defined conditions
(100–150 °C; 30–120 min), low and high fat cocoa powder, various brands of chocolate, and cocoa shells were analyzed for their
contents of free L-and D-amino acids.
Amino acids were isolated from defatted products using a cation exchanger and converted into volatile N(O)-pentafluoropropionyl amino acid 2-propyl esters which were analyzed by enantioselective gas chromatography mass spectrometry
on a Chirasil®-L-Val capillary column. Besides common protein L-amino acids low amounts of D-amino acids were detected in fermented cocoa
beans. Quantities of D-amino acids increased on heating. On roasting cocoa beans of the Forastero type from the Ivory Coast
at 150 °C for 2 h, relative quantities of D-amino acids approached 17.0% D-Ala, 11.7% D-Ile, 11.1% D-Asx (Asp + Asn), 7.9%
D-Tyr, 5.8% D-Ser, 4.8% D-Leu, 4.3% D-Phe, 37.0% D-Pro, and 1.2% D-Val. In cocoa powder and chocolate relative quantities
amounted to 14.5% D-Ala, 10.6% D-Tyr, 9.8% D-Phe, 8.1% L-Asx, and 7.2% D-Ile. Lower quantities of other D-amino acids were
also detected. In order to corroborate our hypothesis that D-amino acids are generated from Amadori compounds (fructose amino
acids) formed in the course of the Maillard reaction, fructose-L-phenylalanine and fructose-D-phenylalanine were synthesized
and heated at 200 °C for 5–60 min. Already after 5 min release of 11.7% D-Phe and 11.8% L-Phe in the free form could be analyzed.
Based on the data a racemization mechanism is presented founded on the intermediate and reversible formation of an amino acid
carbanion in the Amadori compounds.
Keywords: Amino acid enantiomers – Racemization – Gas chromatography mass-spectrometry – Chirasil®-Val – Theobroma cacao L. – Maillard reaction – Amadori rearrangement products