Identification of key aroma compounds generated from cysteine and carbohydrates under roasting conditions

Application of the aroma extract dilution analysis to volatile fractions obtained from dry-heated (180°C for 6 min on silica) mixtures of cysteine and ribose (I), glucose (II) or rhamnose (III) revealed the following compounds with the highest flavour dilution factors: 2-furfurylthiol (FFT) and 2-acetyl-2-thiazoline in I; 2-acetyl-2-thiazoline, 2-propionyl-2-thiazoline and FFT in II; 5-methyl-2-furfurylthiol, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, 2-acetyl-2-thiazoline, 2-propionyl-2-thiazoline and (Z)-2-propenyl-3,5-dimethylpyrazine in III. A comparison of the data with results of previous studies, in which the same mixtures had been reacted under aqueous conditions (145°C, 20 min) revealed that the dry-heating process favoured the formation of the roasty, popcorn-like smelling 2-acetyl- and 2-propionyl-2-thiazoline as well as the potato-chip-like 2-ethyl-3,5-dimethylpyrazine and 2,3-diethyl-5-methylpyrazine in all three reaction systems. 2-Ethenyl- and (Z)-2-propenyl-3,5-dimethylpyrazine, both having an intense roast potato smell were, however, selectively formed in the presence of ribose or rhamnose, respectively. Based on quantitative data, pathways generating the latter two pyrazines via their 1-desoxyosones as key intermediates are discussed.