S.F. Spoelstra

Anaerobic Conversion of Lactic Acid to Acetic Acid and 1,2-Propanediol by Lactobacillus buchneri

ABSTRACT The degradation of lactic acid under anoxic conditions was studied in several strains of Lactobacillus buchneri and in close relatives such as Lactobacillus parabuchneri,Lactobacillus kefir, and Lactobacillus hilgardii. Of these lactobacilli, L. buchneri andL. parabuchneri were able to degrade lactic acid under anoxic conditions, without requiring an external electron acceptor. Each mole of lactic acid was converted into approximately 0.5 mol of acetic acid, 0.5 mol of 1,2-propanediol, and traces of ethanol. Based on stoichiometry studies and the high levels of NAD-linked 1,2-propanediol-dependent oxidoreductase (530 to 790 nmol min−1 mg of protein−1), a novel pathway for anaerobic lactic acid degradation is proposed. The anaerobic degradation of lactic acid by L. buchneri does not support cell growth and is pH dependent. Acidic conditions are needed to induce the lactic-acid-degrading capacity of the cells and to maintain the lactic-acid-degrading activity. At a pH above 5.8 hardly any lactic acid degradation was observed. The exact function of anaerobic lactic acid degradation by L. buchneri is not certain, but some results indicate that it plays a role in maintaining cell viability.

Anaerobic lactic acid degradation during ensilage of whole crop maize inoculated with Lactobacillus buchneri inhibits yeast growth and improves aerobic stability

Aerobic deterioration of silages is initiated by (facultative) aerobic micro‐organisms, usually yeasts, that oxidize the preserving organic acids. In this study, a Lactobacillus buchneri strain isolated from maize silage was evaluated for its potential as a bacterial inoculant that enhances aerobic stability of silages. In four experiments, chopped whole crop maize (30–43% dry matter (DM)) was inoculated with Lact. buchneri and ensiled in laboratory silos. Uninoculated silages served as controls. Analysis of silages treated with Lact. buchneri at levels of 103−106 cfu g−1 after about 3 months of anaerobic storage showedthat acetic acid and 1‐propanol contents increased with inoculum levels above 104 cfu g−1,whereas lactic acid decreased. Propionic acid, silage pH and DM loss increased withinoculum levels above 105 cfu g−1. Time course experiments with maize inoculated with Lact. buchneri at 4 × 104−2 × 105 cfu g−1 showed that up to 7–14 d after ensiling, Lact. buchneri had no effect on silage characteristics. Thereafter, the lactic acid content of the inoculated silages declined and, simultaneously, acetic acid and, to a lesser extent, propionic acid and 1‐propanol, accumulated. Inoculation reduced survival of yeasts during the anaerobic storage phase and inhibited yeast growth when the silage was exposed to O2, resulting in a substantial improvement in aerobic stability. The results indicate that the use of Lact. buchneri as a silage inoculant can enhance aerobic stability by inhibition of yeasts. The ability of the organism to ferment lactic acid to acetic acid appears to be an important underlying principle of this effect.