No abstract

The sensomics approach was used to clarify the formation of the fusty/musty off-flavor of native cold-pressed rapeseed oil. A "positive control" (PC) showing the desired sensory attributes and an oil eliciting a fusty/musty off-flavor (OF) were analyzed. Comparative aroma extract dilution analysis (cAEDA), identification experiments, quantitation by stable isotope dilution assays (SIDAs), calculation of odor activity values (OAVs), and aroma recombination resulted in 11 odorants with an OAV ≥ 1 in PC. Main differences between both oils were obtained for compounds caused by microbial influence revealing significantly higher concentrations in OF, e.g., for ethyl 2-methylbutanoate, 2-methoxyphenol, 3-hydroxy-4,5-dimethylfuran-2(5H)-one (sotolon), 2- and 3-methylbutanoic acid, and 4-methylphenol. Comparison of the key odorants in OF with those of the rapeseeds (OFS), from which it was pressed, showed the same 18 compounds proving that the grade of the seeds and their storage conditions are important criteria for the quality of the final oil. Finally, a further 7 native cold-pressed rapeseed oils, eliciting the same sensory defect, were analyzed to confirm aroma-active marker compounds responsible for the fusty/musty off-flavor.

The conversion of parent free amino acids into alcohols by an enzymatic deamination, decarboxylation, and reduction caused by microbial enzymes was first reported more than 100 years ago and is today known as the Ehrlich pathway. Because the chiral center at the carbon bearing the methyl group in l-isoleucine should not be prone to racemization during the reaction steps, the analysis of the enantiomeric distribution in 2-methylbutanal, 2-methylbutanol, and 2-methylbutanoic acid as well as in the compounds formed by secondary reactions, such as ethyl 2-methylbutanoate and 2-methylbutyl acetate, are an appropriate measure to follow the proposed degradation mechanism in the Ehrlich reaction. On the basis of a newly developed method for quantitation and chiral analysis, the enantiomers of the five metabolites were determined in a great number of fermented foods. Whereas 2-methylbutanol occurred as pure (S)-enantiomer in nearly all samples, a ratio of almost 1:1 of (S)- and (R)-2-methylbutanal was found. These data are not in agreement with the literature suggesting the formation of 2-methylbutanol by an enzymatic reduction of 2-methylbutanal. Also, the enantiomeric distribution in 2-methylbutanoic acid was closer to that in 2-methylbutanol than to that found in 2-methylbutanal, suggesting that also the acid is probably not formed by oxidation of the aldehyde as previously proposed. Additional model studies with (S)-2-methylbutanal did not show a racemization under the conditions of food production or during workup of the sample for volatile analysis. Therefore, the results establish that different mechanisms might be responsible for the formation of aldehydes and acids from the parent amino acids in the Ehrlich pathway.