Characterization of Key Odorants in Dry-Heated Cysteine-Carbohydrate Mixtures: Comparison with Aqueous Reaction Systems

The most odor-active compounds in dry-heated mixtures of cysteine/ribose (I) and cysteine/rhamnose (II) were identified by application of aroma extract dilution analyses and the odor activity value concept. A comparison with the key odorants generated in I with those present in the same mixture thermally treated under aqueous conditions revealed that dry-heating yielded higher amounts of the key odorants 2-furfurylthiol (roasty, coffee-like), 2-acetyl-2-thiazoline (roasty, popcorn-like), 2-ethyl- and 2-ethenyl-3,5-dimethylpyrazine (roasty, potato-like), whereas the yields of 5-acetyl-2,3-dihydro-1,4-thiazine (roasty, popcorn-like) and 3-mercapto-2-pentanone (sulfury) were significantly decreased. In II, the amount of 5-methyl-2-furfurylthiol (roasty, coffee-like) was significantly increased, whereas 4-hydroxy-2,5-dimethyl-3(2H)-furanone (caramel-like) and 3-hydroxy-6-methyl-2(2H)-pyranone (seasoning-like) were drastically decreased under dry-heating conditions. The odorants 2-ethenyl- and (Z)-2-propenyl-3,5-dimethylpyrazine were shown to be formed exclusively by dry-heating in I or II, respectively. A formation mechanism indicating the respective 1-desoxyosones as important intermediates in pyrazine formation is presented.