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

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. a- Acetolactic acid, produced by lactic acid bacteria, is an unstable compound that is chemically transformed to acetoin and diacetyl during incubation at 85'C in the headspace sampler, leading to an overestimation of diacetyl concentrations in fermented milks. The oxidative decarboxylation of synthetic a-acetolactic acid to diacetyl during the assay decreased as the pH increased: at pH 4.0, 4096 of the a-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 1WC before analysis, the reaction was limited to 2%, leading to a more precise assay of diacetyl in the presence of a- acetolactic acid. The method for assaying diacetyl was applied to a mixed culture of Leuconostoc mesenteroides ssp. cremoris and Luctococcus Iactis ssp. lactis in milk. Acetoin (1.07 mM) and aacetolactic 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 a-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 m a ) .