Search

International audience; In order to investigate the pressure-dependent effects of carbon chain growth from methyl radical, pyrolysis experiments of butane-2,3‑dione (also known as diacetyl) were conducted both in a flow reactor over 780–1520 K at low and atmospheric pressures and in a jet-stirred reactor (JSR) over 650–1130 K at 10 bar. Identification and quantification of the intermediates were achieved by synchrotron vacuum ultraviolet photoionization mass spectrometry in a flow reactor and Fourier transform infrared spectrometry and gas chromatography in a JSR. A pyrolysis model of butane-2,3‑dione, incorporating the sub-mechanisms of butane-2,3‑dione, ketene, C1–C4 hydrocarbons and the formation of benzene, was developed from our recently reported C1 model in this work. It was validated against the present pyrolysis data of butane-2,3‑dione and the pyrolysis data of C2–C4 hydrocarbon fuels and ketene in the literature. Generally, the present model could reasonably predict all the experimental targets. Based on the rate of production analysis, the unimolecular decomposition reaction (CH3COCOCH3 (+ M) = 2CH3CO (+ M)) is the dominant pathway to consume the fuel at low pressure. However, at atmospheric and high pressures, both the unimolecular decomposition reaction and the H abstraction reaction of butane-2,3‑dione by methyl radical are important. Ketene, acetone and acetaldehyde were measured in this work and their formation is also pressure dependent. For the methyl-related growth chemistry, addition-elimination reactions play an important role at low pressure. Their contribution becomes less at atmospheric pressure and can be neglected at high pressure. In contrast, the combination reactions are responsible for methyl-related growth at high pressure. For the formation of benzene, the recombination of propargyl radical is the most important pathway at low pressure, while the H-assisted isomerization reaction of fulvene becomes dominant at atmospheric and high pressures.

Haemin, Fe3+ and Cu2+ caused significant decarboxylation of 2-acetolactate (ALA) to diacetyl during steam distillation of milk at pH 6.5. In the absence of these compounds, maximum conversion (28%) of ALA to diacetyl occurred at pH 3.5. Addition of 1.5 m m CuSO4 at this pH increased the conversion to 100%. Based on this result, a new method for the determination of ALA was developed, in which interference from pre-formed acetoin was minimised. Comparison of this method with the previous method (Jordan and Cogan, Irish Journal of Agricultural and Food Research 34, 39–47, 1995), with which high concentrations of acetoin interfere, was excellent (R2=0.96). Breakdown of ALA to diacetyl in the absence of Cu at pH 0.5 and 6.5 were 0.8 and 2.5%, respectively, implying that samples for diacetyl analysis should be adjusted to pH 0.5 before distillation if ALA is also suspected to be present. In quarg (pH 4.5) held at 4°C, ALA was degraded at a rate of 6.9–16.9 μmol L-1 day-1 and from 32 to 50% of the degraded ALA was converted to diacetyl during extended storage.

A method was developed to screen and isolate mutagenized Lactococcus lactis subsp. lactis biovar diacetylactis strains accumulating (alpha)-acetolactate. This compound is accumulated by (alpha)-acetolactate decarboxylase-deficient strains and undergoes spontaneous degradation into diacetyl on agar plates. The diacetyl produced is detected by a colorimetric reaction yielding a red halo around the colonies.

Following treatment with the mutagen N-methyl-N'-nitro-N-nitrosoguanidine, three mutants of Lactococcus lactis subsp. lactis biovar diacetylactis CNRZ 483 that produced diacetyl and acetoin from glucose were isolated. The lactate dehydrogenase activity of these mutants was strongly attenuated, and the mutants produced less lactate than the parental strain. The kinetic properties of lactate dehydrogenase of strain CNRZ 483 and the mutants revealed differences in the affinity of the enzyme for pyruvate, NADH, and fructose-1,6-diphosphate. When cultured aerobically, strain CNRZ 483 transformed 2.3% of glucose to acetoin and produced no diacetyl or 2,3-butanediol. Under the same conditions, mutants 483L1, 483L2, and 483L3 transformed 42.0, 78.9, and 75.8%, respectively, of glucose to C4 compounds (diacetyl, acetoin, and 2,3-butanediol). Anaerobically, strain CNRZ 483 produced no C4 compounds, while mutants 483L1, 483L2, and 483L3 transformed 2.0, 37.0, and 25.8% of glucose to acetoin and 2,3-butanediol. In contrast to the parental strain, the NADH balance showed that the mutants regenerated most of the NAD via NADH oxidase under aerobic conditions and by ethanol production under anaerobic conditions.