Metabolism of lactate and sugars by dairy propionibacteria: A review

Dairy propionibacteria are important organisms for the manufacture of Swiss -type cheese, for the biological production of propionate and vitamin B-12 and have probiotic properties. In all these applications, their metabolic a ctivities play a critical role. A complete understanding of propionate ferm entation and of the metabolic routes used is therefore necessary. Dairy pro pionibacteria have a complex metabolism and involves several cycles. Lactat e or sugars utilisation yields pyruvate which can be reduced to produce pro pionate via the transcarboxylase cycles, or oxidised to yield acetate and C O2. During the coupled oxidation-reduction, ATP is produced by an electron transport system and fumarate acts as the final acceptor. Although propioni bacteria are mainly anaerobes, the electron transport system can be used in the presence of oxygen and they possess the citrate cycle, but can not gro w under normal atmospheric oxygen pressure. The proportions of propionate a cetate and CO2 produced vary depending on the strain used and this can be e xplained, to some extent, by their relative ability to utilise pyruvate via reactions of the citrate cycle. The physicochemical environment during gro wth affects propionic acid fermentation; it is impaired by the presence of oxygen and nitrate, but fermentation at acidic pHs enhances propionate prod uction. Fermentation in presence of more than one substrate is complex and still poorly understood. When both L- and D-lactate isomers are available, L-lactate is used preferentially. Although sugar fermentation is more effic ient, in the presence of lactate and sugars, there is evidence that lactate is used preferentially. Propionic acid fermentation is affected by the uti lisation of amino acids, especially aspartate; co-metabolism of lactate and aspartate results in a lower propionate production and a decrease of the r atio propionate:acetate. There is evidence that the utilisation of the prod ucts of proteolysis is an important event in the ripening of Swiss-type che ese and could account for the low propionate:acetate ratios observed in Swi ss-type cheese. (C) Inra/Elsevier, Paris.