GAS-CHROMATOGRAPHY MASS-SPECTROMETRY INVESTIGATION OF AROMA COMPOUNDS FROM THE REACTION OF CYSTEIN AND RHAMNOSE UNDER ROASTING CONDITIONS

The model system cystein/rhamnose produced a complex mixture of compounds under roasting conditions (200 to 220-degrees-C). This mixture was separated by adsorption chromatography on silica gel into six fractions according to the increasing polarity of the components. The fractions were investigated by capillary gas chromatography with and without mass spectrometry. Of the approx 180 compounds detected, 125 were identified and quantified: 10 alkyl- and alkenylfurans, 7 condensed furan systems, 6 other furans, 8 sulphur-substituted furans. 5 furanones and lactones, 7 alkyl- and 2 acylpyrroles, 5 N-furfurylpyrroles, 11 alkyl- and 4 cyclopentapyrazines. 6 pyridines, 10 alkyl- and 8 acylthiophenes, 4 cyclic methylenepolysulphides, thialdine, 1,2-dimercaptoethane and 30 thiazoles, thiazolines and thiazolidines. 5-Methyl furfural and furaneol were detected in a ratio of 1.5:1 (approx 33% of the total reaction). However, a considerable amount of the 5-methyl furfural reacted with H2S which was released from cysteine to 5-methyl furfuryl mercaptan. Approximately 50% of the furaneol was reduced to 2,5-dimethyl-3(2H)-furanon. The mercapto compounds derived from furaneol and 2,5-dimethyl-3(2H)-furanon were formed in only trace amounts. The formation of thiophenes proves that under roasting conditions the furanoxygen is substituted by sulphur from H2S.