Acetaldehyde is often present in wine in low levels. It is present as a metabolite of yeasts, particularly if fermentation conditions aren’t ideal, as a by-product of acetobacter and from the oxidation of ethanol. In wine, it is considered a fault with a sensory threshold of 100-125mg/L. It imparts a cut apple or straw flavour to the wine. In some sherries, however, that same flavour is a known organoleptic characteristic of the beverage, and its production is targeted. Samples can be tested to determine if acetaldehyde is present above sensory threshold levels and suitable for release.
An enzyme assay and spectrophotometry can be used to quantify acetaldehyde in a sample. Acetaldehyde will oxidise in the presence of NAD+ and water to produce acetic acid, NADH + H+. This reaction is catalysed by aldehyde dehydrogenase. To measure the acetaldehyde concentration, the sample is mixed with a known quantity of NAD+ and aldehyde dehydrogenase. A spectrophotometer, which measures the absorbance of light at a particular wavelength is then used. The absorbance of the sample is then measured at 340 nm to determine NADH concentration. The acetaldehyde can then be quantified by determining its stoichiometric ratio to NADH. The test only takes a few minutes to complete analysis.