International Dairy Journal 14 (2004) 857–864 Streptococcus thermophilus ST 111 produces a stable high-molecular-mass exopolysaccharide in milk-based medium Frederik Vaningelgem a , Roel Van der Meulen a , Medana Zamfir a , Tom Adriany a , Andrew P. Laws b , Luc De Vuyst a, * a Research Group of Industrial Microbiology, Fermentation Technology and Downstream Processing (IMDO), Department of Applied Biological Sciences, Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium b Department of Chemical and Biological Sciences, University of Huddersfield, Queensgate, HDI 3DH Huddersfield, UK Received 8 December 2003; accepted 12 March 2004 Abstract Streptococcus thermophilus ST 111, grown in milk medium supplemented with whey protein hydrolysate, produced an exopolysaccharide (EPS) composed of galactose and rhamnose in a molar ratio of 2.5:1, and with a molecular mass of more than 5000kDa. Fermentations in milk medium supplemented with whey protein hydrolysate further demonstrated the stability of this high-molecular-mass EPS, as its molecular mass was affected neither by the pH of the medium nor the fermentation time, indicating no enzymatic degradation. On the other hand, drying of the isolated EPS resulted in a decrease of the molecular mass. The stability of an EPS produced in milk-based media is an important asset for the production of fermented milk and yoghurt products. r 2004 Elsevier Ltd. All rights reserved. Keywords: Exopolysaccharide; Streptococcus thermophilus; Molecular mass; Structure; Stability 1. Introduction Exopolysaccharides (EPS) from lactic acid bacteria (LAB) can be subdivided into two major groups: homopolysaccharides and heteropolysaccharides (De Vuyst & Degeest, 1999; Monsan et al., 2001; De Vuyst & Vaningelgem, 2003). Based on their structure, four groups of homopolysaccharides can be distinguished: a-d-glucans (dextrans, mutans, alternan), b-d-glucans, b-d-fructans (levans, inulin-type fructans), and others like polygalactan. Heteropolysaccharides are produced by LAB in a greater variety concerning chemical composition, monomer ratio, and molecular structure of the repeating unit, as well as the molecular mass of the polymer (De Vuyst & Vaningelgem, 2003). The repeating units of heteropolysaccharides most often contain a combination of d-glucose, d-galactose, and l-rhamnose in different ratios and, in a few cases, fucose, acetylated amino sugars, glucuronic acid, and nononic acid; as well, non-carbohydrate constituents such as glycerol, phosphate, acetyl, and pyruvyl groups may be present (De Vuyst, de Vin, Vaningelgem, & Degeest, 2001; De Vuyst & Vaningelgem, 2003). Furthermore, within one repeating unit, different types of linkages can occur. In the backbone of heteropoly- saccharides from LAB, a(1-3) linkages are more abundant than b(1-4) ones, which in turn occur more frequently than a(1-2), b(1-3), and b(1-6) linkages (De Vuyst & Vaningelgem, 2003). In general, the types of linkages within an EPS polymer determine the rigidity (Laws & Marshall, 2001) and hence the intrinsic viscosity of the polymer (Ruas-Madiedo, Tuinier, Kanning, & Zoon, 2002b). The apparent viscosity of an EPS solution is primarily determined by the molecular mass of the polymer. The molecular mass of heteropolysaccharides varies from 1.0  10 4 to 9.0  10 6 Da. Some strains produce high-molecular- mass EPS (Navarini et al., 2001), others produce low-molecular-mass EPS (Almir ! on-Roig, Mulholland, Gasson, & Griffin, 2000), while still others produce both types of EPS (Marshall, Cowie, & Moreton, 1995; Grobben et al., 1997; Degeest & De Vuyst, 1999; Petry et al., 2003). The biosynthesis of the repeating unit of the heteropolysaccharides produced by LAB has been studied on an enzyme level for Streptococcus ARTICLE IN PRESS *Corresponding author. Tel.: +32-2-629-32-45; fax: 32-2-629-27-20. E-mail address: ldvuyst@vub.ac.be (L.De Vuyst). 0958-6946/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2004.03.007