Quantitative instrumental and sensory studies on aroma compounds contributing to a metallic flavour defect in buttermilk

(E,Z)-2,6-Nonadienol (metallic), 4,5-epoxy-(E)-2-decenal (metallic), 3-methylindole (mothball-like), acetaldehyde (pungent) and 2,3-butanedione (buttery) were quantified by stable-isotope dilution assays in samples of fresh fermented sweet-cream buttermilk (FB) and stored (4 d at 8°C) sour-cream buttermilk (SCB), the latter exhibiting an intense metallic off-flavour. With the exception of acetaldehyde, the concentrations of the four other odorants were higher in the SCB. A calculation of odour activity values (OAV: ratio of concentration to odour threshold) on the basis of odour thresholds determined in tap water indicated that 2,3-butanedione had a greater aroma impact in SCB than in FB, whereas the OAVs of acetaldehyde were nearly identical in both samples. A calculation of OAV on the basis of odour thresholds in buttermilk suggested that (E,Z)-2,6-nonadienol was the key contributor to the metallic off-odour in the SCB sample. Sensory experiments, in which this odorant had been added to fresh FB, showed that additions above 1.3 μg L^-1 completely mimicked the metallic off-odour of the stored SCB.