Method for Assaying Volatile Compounds by Headspace Gas Chromatography and Application to Growing Starter Cultures

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Abstract
Automatic headspace gas chromatography was used to assay volatile compounds in fermented milks, including diacetyl, which is of particular interest because of its aromatic properties. Alpha-acetolactic acid, produced by lactic acid bacteria, is an unstable compound that is chemically transformed to acetoin and diacetyl during incubation at 85-degrees-C in the headspace sampler, leading to an over-estimation of diacetyl concentrations in fermented milks. The oxidative decarboxylation of synthetic alpha-acetolactic acid to diacetyl during the assay decreased as the pH increased: at pH 4.0, 40% of the ce-acetolactic acid present was transformed to diacetyl, but this reaction was limited to 6% at pH 7.0. When the assay mixture was degassed and heated to 100-degrees-C before analysis, the reaction was limited to 2%, leading to a more precise assay of diacetyl in the presence of alpha-acetolactic acid. The method for assaying diacetyl was applied to a mixed culture of Leuconostoc mesenteroides ssp. cremoris and Lactococcus lactis ssp. lactis in milk. Acetoin (1.07 mM) and alpha-acetolactic acid (.18 mM) were produced, but not diacetyl. In our culture conditions, the redox potential dropped rapidly at the beginning of fermentation, which prevented diacetyl production by the oxidative decarboxylation of alpha-acetolactic acid. When the same fermentation was carried out with agitation, the redox potential remained high, and diacetyl production was significant, reaching .032 mM (2.8 mg/L)