Exopolysaccharide production and texture-promoting abilities of mixed-strain starter cultures in yogurt production

An exopolysaccharide-producing strain of Lactobacillus delbrueckii ssp. bulgaricus (CNRZ 1187) and two colonial white and pink variants were grown in skim milk in association with a strain of Streptococcus thermophilus (CNRZ 389) that did not produce exopolysaccharide to assess the effects of associative growth on exopolysaccharide formation. After 10 h of fermentation, viscosity (380 mPa.s) was highest with the parental strain CNRZ 1187 in the mixed-strain starter, the lowest (220 mPa.s) with the mixed-strain starter containing the white variant and S. thermophilus CNRZ 389, and intermediate (290 mPa.s) with the mixed-strain starter containing the pink variant and S. thermophilus CNRZ 389. Viscosities of the milks with exopolysaccharide-producing strains decreased after mechanical stirring, but values remained higher (170 to 230 mPa.s) than those obtained with nonproducing strains (70 mPa.s). The mixed-strain starter composed of the white colonies of L. delbrueckii spp. bulgaricus and S. thermophilus produced the highest amount of exopolysaccharide (240 mg/L), and the mixed-strain starter containing the parental strain CNRZ 1187 produced the lowest amount (110 mg/L) of exopolysaccharide. The exopolysaccharide production from the mixed-strain starter containing the white and pink L. delbrueckii ssp. bulgaricus colonies began earlier and was higher after 3 h of fermentation than that from the starter containing the parental L. delbrueckii ssp. bulgaricus strain. Production of exopolysaccharide from the latter approached the maximum in the beginning of the stationary growth phase; production from the mixed-strain starter with the pink and white variants continued during the early stationary growth phase. The exopolysaccharide contained mainly galactose with small amounts of glucose and rhamnose. The monosaccharide composition of the exopolysaccharide changed during the exponential growth phase (6 h) and remained stable thereafter.