Effect of exopolysaccharide-producing Lactococcus lactis strains and temperature on the permeability of skim milk gels.

Abstract

Abstract In fermented milks such as yoghurt, lactic acid bacteria (LAB) produce lactic acid, which induces aggregation of the milk proteins, leading to gel formation. Exopolysaccharide (EPS)-producing LAB may improve the texture of fermented milks, depending on the strain. In this study the effect of EPS-producing strains on the permeability of fermented milk gels was investigated. Permeability is related to the roughness and serum separation in fermented milk products, which are important quality characteristics. Milk was fermented at 20, 25 and 30 °C with one non-EPS-producing Lactococcus lactis strain, coded SK110, and with four EPS-producing L. lactis strains, coded B35, B39, B40 and B891. Permeability of the gels was measured by permeation of serum through the gels and for the calculation of the permeability coefficient differences in kinematic viscosity of the various sera were taken into account. The permeability of the gels made with the EPS-producing strains was higher than the permeability of the gels made with SK110, although no clear differences could be observed in the protein network structure as visualised with confocal scanning laser microscopy (CSLM). Therefore the flow properties of the sera were studied in more detail. A new method was used: the flow of serum through a porous glass medium (glass filter). It appeared that the relative differences in flow time through the glass filter of the various sera were much larger than the relative differences in kinematic viscosity. This implies that the effect of EPS on the permeability coefficient originated mainly from the flow behaviour of the sera and not from an effect on the structure of the protein network. At higher fermentation temperature, the permeability coefficient of the gels was higher due to larger pores in the protein network, especially at 30 °C. The only exception was strain B891. The permeability coefficients of the gels made with this strain were almost the same at 20, 25 and 30 °C. The low permeability coefficient at 30 °C is probably due to the high resistance against flow of the serum and the formation of EPS relatively late during the fermentation (high concentration in the pores). At 20 °C clusters of bacteria were seen on CSLM pictures, which might have caused a relatively high permeability. This effect will counteract the temperature effect on the protein network structure (smaller pores at lower temperature).