Quantitative Studies on the Formation of Key Odorants in Thermally Treated Yeast Extracts Using Stable Isotope Dilution Assays

Eighteen compounds recently identified as key odorants in yeast extracts were quantified by stable isotope dilution assays, and their odor activity values (OAV; ratio of concentration to odor threshold) were calculated. In a thermally treated commercial yeast extract (CYE) as well as in a self-prepared bakers' yeast extract (SPYE), both exhibiting roasty, meat-like odors, 2-methyl-3-furanthiol, 2-furfurylthiol, 3-methylbutanal, and methional showed the highest OAVs. Phenylacetaldehyde was high in the CYE but not in the SPYE, whereas the reverse was true for 2,3-diethyl-5-methylpyrazine. Thermal processing of a yeast extract containing the water-soluble, low molecular weight compounds (LMW-YE) isolated from mechanically disrupted fresh bakers' yeast cells resulted in a bread-crust-like odor. In this extract high OAVs for 3-methylbutanal (malty) and 2-acetyl-1-pyrroline(popcorn-like) were measured, suggesting a significant aroma contribution of both odorants. Both were much increased when the bakers' yeast had been pretreated with sodium chloride and nutrients (12 h at 32 °C). The results were well correlated with a significant increase in the respective precursor amino acids leucine (3-methylbutanal) and ornithine and proline (2-acetyl-1-pyrroline). Results of further model studies suggested Strecker degradation as one of the key reaction pathways in the formation of key odorants during thermal treatment of yeast extracts.