Study of the effects of temperature, pH and yeast extract on growth and exopolysaccharides production by Propionibacterium acidi-propionici on milk microfiltrate using a response surface methodology

Abstract

Aims: To study the effects of temperature, pH and yeast extract (YE) concentration on growth and exopolysaccharide (EPS) production by Propionibacterium acidi-propionici DSM 4900 cultivated on milk microfiltrate. Methods and Results: A multifactorial approach using a Response Surface Methodology (RSM) was followed. The results indicated that both growth, and EPS and organic acids production, were influenced by pH, temperature and YE concentration. Biomass and organic acids production occurred in all the tested domains, whereas EPS production was only possible in a narrow pH range (5·3–6·5). The results clearly showed that the optimal conditions for EPS production were different to those for optimal growth. The effect of YE on EPS production was not only due to an increase in growth but also to a direct effect on the production of EPS. The temperature played a major role. A decrease of temperature induced a slowing down of both growth and organic acids production, making the essential factors of the medium and the precursors of EPS biosynthesis more available and hence, leading to an increase in EPS production. Conclusions: The effects of pH, temperature and YE were determined, allowing the definition of favourable, though non-optimal, conditions for EPS production: 23°C, pH 6 and 3 g l^–1 YE concentration. Significance and Impact of the Study: The use of a multifactorial approach for investigating the effect of fermentation conditions on EPS production has been demonstrated.