METHOD FOR ASSAYING VOLATILE COMPOUNDS BY HEADSPACE GAS-CHROMATOGRAPHY AND APPLICATION TO GROWING STARTER CULTURES

Automatic headspace gas chromatography was used to assay volatile comp ounds in fermented milks, including diacetyl, which is of particular i nterest because of its aromatic properties. Alpha-acetolactic acid, pr oduced by lactic acid bacteria, is an unstable compound that is chemic ally transformed to acetoin and diacetyl during incubation at 85-degre es-C in the headspace sampler, leading to an over-estimation of diacet yl concentrations in fermented milks. The oxidative decarboxylation of synthetic alpha-acetolactic acid to diacetyl during the assay decreas ed as the pH increased: at pH 4.0, 40% of the ce-acetolactic acid pres ent was transformed to diacetyl, but this reaction was limited to 6% a t pH 7.0. When the assay mixture was degassed and heated to 100-degree s-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 Leu conostoc mesenteroides ssp. cremoris and Lactococcus lactis ssp. lacti s in milk. Acetoin (1.07 mM) and alpha-acetolactic acid (.18 mM) were produced, but not diacetyl. In our culture conditions, the redox poten tial dropped rapidly at the beginning of fermentation, which prevented diacetyl production by the oxidative decarboxylation of alpha-acetola ctic acid. When the same fermentation was carried out with agitation, the redox potential remained high, and diacetyl production was signifi cant, reaching .032 mM (2.8 mg/L).