Rheological, microscopic and primary chemical characterisation of the exopolysaccharide produced by Lactococcus lactis subsp. cremoris DPC6532

Abstract

The exopolysaccharide (EPS) produced by Lactococcus lactis subsp. cremoris DPC6532 has been shown to improve yield and functionality of reduced fat cheese. The aim of this work was to characterise the EPS produced by this strain to elucidate its role in fermented milk. The EPS was isolated from fermented skim milk, purified, quantified and imaged using atomic force microscopy and scanning electron microscopy. The monosaccharide composition and molecular weight of the EPS was also determined. The direct effect of the EPS on gelation and viscosity was investigated by oscillation rheometry on skim milk fermented with DPC6532 and its non-EPS-producing isogenic variant. The results indicated that this EPS gave a yield of 322 mg.L−1 in skim milk, had a molecular weight of ~2.8 × 105 g.mol−1 and was mainly composed of glucose and galactose (1.29:1). When observed by atomic force microscopy and scanning electron microscopy, a large size distribution was observed, with large aggregates consisting of several EPS molecules. Milk fermented with the EPS-producing culture was significantly more viscous than milk fermented with its non-EPS-producing isogenic variant. Gel strength was also significantly higher after 16 h of fermentation when the EPS-producing strain was used. This effect is thought to be due to depletion interactions caused by the EPS, which would stay dissolved in the serum phase, hence affecting gel viscosity and causing phase separation. The information provided by this study provided some insight to the information available on this specific EPS which can help to better understand its role in dairy matrices.