Exopolysaccharide production by lactobacillus acidophilus for potential applications in fresh cheese

Many strains belonging to the group of lactic acid bacteria (LAB) produce exopolysaccharides (EPS). The ability of EPS to act as viscosifying, stabilizing, and/or water-binding agents in various foods makes it an effective natural alternative to commercial synthetic stabilizers, so it may play an important role in the dairy industry. In fact, EPS-producing LAB are not only important in manufacture of yoghurt and fermented milks, but also in cheese production. Recent research efforts have shown that EPS cultures are useful to increase moisture retention and improve functional properties of low-fat Mozzarella cheese and whey cheese. In this work, the ability of Lb. acidophilus to produce EPS, and the performance of two different methods for EPS isolation was investigated. Fermentations were carried out in a Braun Biostat B 2-L fermentor, filled with 1.5 L of MRS broth containing 20 g/L lactose. The yield of the EPS isolation method was tested by adding xanthan gum to the medium. The experiments were carried out at 37 °C, 150 rpm, pH 5.5 under an N2 atmosphere. A 50 mL/L standard inoculum was prepared from a subculture of Lb. acidophilus, previously grown in the corresponding medium for 20 h at 37 °C, and was used to start-up every fermentation batch. Fermentation batches took 48 h, and samples were taken periodically. Growth was monitored spectrophotometrically and by plate enumeration. To optimise EPS isolation, cells and residual polypeptides were removed by centrifugation (4000 rpm for 20 min) upon precipitation via two different processes: addition of pronase E solution and one volume of 20 % trichloroacetic acid, or precipitation with 20 g/kg 5-sulfosalysilic acid. The EPS was precipitated by three volumes of cold ethanol and was collected by centrifugation. The weight of isolated and dried polymers was measured, and the total amount of carbohydrates was determined by the phenol-sulphuric method. Exponential growth took place for ca. 12 h, and EPS was produced mainly during the stationary phase.