Determination of α-Acetolactic Acid and Volatile Compounds by Headspace Gas Chromatography

Abstract

Methods based solely on automatic headspace gas chromatography are described for the determination of α-acetolactic acid, acetoin, and the important flavor compound, diacetyl, that is produced by lactic acid bacteria. α-Acetolactic acid is an unstable compound that chemically decarboxylates to acetoin and diacetyl during sampling and headspace gas chromatography, leading to an overestimation of both compounds. However, the decarboxylation to diacetyl was minimized to 1% of the α-acetolactic acid concentration when concentrated phosphate buffer at pH 7.0 was added to the sample before headspace gas chromatography; this procedure resulted in a more precise determination of diacetyl. It is not possible to quantify α-acetolactic acid directly by headspace gas chromatography because of its instability and low volatility. However, 90 to 100% of the α-acetolactic acid was oxidatively decarboxylated to measurable amounts of diacetyl when Fe3+ was used as a catalytic activator and the sample pH was adjusted to <3.1 prior to the headspace gas chromatography. The detection limit for α-acetolactic acid using this method of sample preparation was 0.015 mM (2 mg/L). The method also determined acetoin more accurately because the interference from decarboxylation of α-acetolactic acid was strongly reduced. The headspace gas chromatography methods were also useful for determination of other volatile compounds that were produced by lactic acid bacteria.