Selection of Streptococcus thermophilus strains able to produce exopolysaccharides in milk

A collection of 139 Streptococcus thermophilus strains was genetically screened by PCR targeting some genes involved in exopolysaccharide (EPS) biosynthesis. Twenty‐nine strains were found to be PCR positive and were grown in skim milk to highlight their ropy character, that is the ability to efficiently thicken the milk cultures and minimise syneresis. Three strains producing a skim milk culture with apparent viscosities of 275 ± 14, 340 ± 21 and 510 ± 42 mPa.s, significantly higher than the average value (135 ± 87.3 mPa.s), were selected. EPS from milk cultures of these three strains was extracted and quantified from 84.4 to 209.2 mg/L.