Original article Productions and monomer compositions of exopolysaccharides by Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus strains isolated from traditional home-made yoghurts and raw milk Belma Aslim,* Yavuz Beyatli & Zehra Nur Yuksekdag Department of Biology, Faculty of Science and Arts, Gazi University, Teknikokullar, 06500 Ankara, Turkey (Received 16 March 2005; Accepted in revised form 11 October 2005) Summary In this study, a total of forty-five strains of lactobacilli and streptococci were determined exopolysaccharide (EPS) production in skim milk and Man Rogosa and Sharpe (MRS)/M17 medium, viscosity and proteolytic activity. The exopolysaccharide production by lactobacilli strains during growth in MRS medium was twenty-one to 211 mg L )1 , while in skim milk was to thirty-six to 315 mg L )1 . The EPS production by streptococci strains during growth in M17 medium was sixteen to 114 mg L )1 , while in skim milk was to twenty-four to 140 mg L )1 . The EPS production of strains was lower in MRS/M17 medium than skim milk. Results showed that it was not clear correlation between the viscosity and EPS production of some strains. All strains were shown proteolytic activity. Positive correlations between exopolysaccharide production and proteolytic activity in skim milk were found some strains of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. These results indicated that the high exocellular protease-producing strains can produce high EPS in skim milk. The monomer compositions of the EPSs formed by selected five strains were analysed. Mannose dominated (99–100%) on the EPS produced by L. delbrueckii subsp. bulgaricus and S. thermophilusstrains (except L. delbrueckii subsp. bulgaricus 22) in skim milk and MRS/M17 medium. Besides, the EPSs of strains in skim milk contained small amount of lactose. Keywords Exopolysaccharide, lactobacilli, proteolytic activity, streptococci, viscosity. Introduction Lactic acid bacteria (LAB) have been the focus of substantial research because of their economic import- ance in food fermentation. LAB comprise lactococci, lactic streptococci and lactobacilli. Amongst them, a combination of Streptococcus thermophilus and Lacto- bacillus delbrueckii subsp. bulgaricus, which collaborate synergisticall into yoghurt are widely used to as the major starter bacteria for the production of yoghurt (Radke-Mitchell & Sandine; 1986; Siitonen et al., 1990). The ability of synthesise EPS as well as the amount of polymer produced depends on both the culture condi- tions and the microorganism. Milk has been commonly used as a culture medium for EPS production (Torino et al., 2000). Some LAB secrete an exopolysaccharide (EPS) (Wel- man & Maddox, 2003). It is considered to be related to cell adhesion and cell protection in different environ- ment e.g. biofilm formation (Jolly et al., 2002). In addition, the bacterial EPSs are thought to play a role in the protection of microbial cell against phagocytosis, phage attract, antibiotics, toxic compounds (e.g. toxic metal ions, sulphur dioxide and ethanol), osmotic stress and nisin (Ruas-Madiedo et al., 2001; Sutherland, 1999; De Vuyst & Degeest, 1999). Exopolysaccharides also play a major industrial role in the production of fermented dairy products in particular for the production of yoghurt, drinking yoghurt, cheese, fermented cream and milk-based desserts (Jolly et al., 2002; De Vuyst & Degeest, 1999). Their presence significantly contributes to texture, rheology, mount feel, taste perception and stability of the final products (Welman & Maddox, 2003; Duboc & Mollet, 2001). EPS interaction with milk proteins also helps determine the viscosity of a fermented product (Broadbent et al., 2003). Other physiological benefit is that EPSs remain for longer in the gastrointestinal track, so that colonisation of *Correspondent: Fax: +903122122279; e-mail: baslim@gazi.edu.tr International Journal of Food Science and Technology 2006, 41, 973–979 973 doi:10.1111/j.1365-2621.2005.01155.x Ó 2006 Institute of Food Science and Technology Trust Fund