Microbial physiology, fermentation kinetics, and process engineering of heteropolysaccharide production by lactic acid bacteria

Abstract

Many species of lactic acid bacteria (LAB) produce extracellular heterotype polysaccharides, the so-called heteropolysaccharides (HePS). Biosynthesis and secretion of the HePS from the LAB occur during different growth phases, and both the amount and type of the polymer is influenced by growth conditions. The total yield of exopolysaccharides produced by the LAB depends on the composition of the medium (carbon and nitrogen sources, growth factors, etc.) and the conditions in which the strains grow, i.e. temperature, pH, oxygen tension, and incubation time. It is never higher than 1.5 g of polymer dry mass per litre of fermentation medium. Whereas mesophilic strains produce maximal amounts of HePS under conditions not optimal for growth, the HePS production from thermophilic LAB strains is growth-associated, i.e. maximum production during growth and under conditions optimal for growth. The HePS degradation often takes place upon prolonged incubation of the HePS-producing LAB strains due to glycohydrolase activity. Primary, secondary, and tertiary modelling unravel the functionality of the HePS-producing LAB strains in a food environment. Finally, appropriate process engineering can lead to an industrial breakthrough of the HePS production and applications: a high and stable high-molecular-mass HePS production by appropriate feeding strategies through fed-batch cultivation on the one hand, and the application of a two-step fermentation process in yoghurt manufacture on the other hand.