Comparison of Key Odorants Generated by Thermal Treatment of Commercial and Self-Prepared Yeast Extracts: Influence of the Amino Acid Composition on Odorant Formation

Application of an aroma extract dilution analysis (AEDA) on a flavor concentrate isolated from a heat-processed (145°C, 20 min) aqueous solution of a commercial yeast extract (CYE) revealed 2-furanmethanethiol, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, 2-methoxyphenol, and 3- and 2-methylbutanoic acid as the key odorants among the 16 odor-active compounds of the intensely roasty, sweet smelling solution. Compared with CYE, in a thermally treated autolysate prepared under laboratory conditions from baker's yeast (SPYA) several odorants, e.g., methional, 2-acetyl-2-thiazoline, 3-hydroxy-4,5-dimethyl-2(5H)-furanone (Sotolon), phenylacetic acid, and 2,3-diethyl-5-methylpyrazine, showed higher flavor dilution (FD) factors, whereas the reverse was found for 2-furanmethanethiol (FMT). The amounts of its precursor amino acid cysteine in the CYE and the SPYA were well correlated with the different odor activities of the FMT in both solutions. Detailed model studies on the formation pathways of FMT indicated the binary mixtures 2-furaldehyde/ cysteine as well as mercapto-2-propanone/hydroxyacetaldehyde as important intermediates in FMT formation. Heat treatment of a water-soluble, low molecular weight fraction isolated from baker's yeast cells predominantly generated the roast odorant 2-acetyl-1-pyrroline (ACPY). Under certain fermentation conditions, the amounts of its precursor ornithine in the yeast were increased, leading to higher odor activities of ACPY after thermal treatment of the extract.