Biodiversity of biopolymers produced by thermophilic lactic acid bacteria: effect on the rheology of fermented milks

CECILIA RODRÍGUEZ; JUDITH PIERMARIA; FERNANDA HAMET; LUC DE VUYST; FERNANDA MOZZI; GRACIELA FONT DE VALDEZ; ANALÍA ABRAHAM

Egmond aan Zee, The Netherlands

Simposio; 9th Symposium Lactic Acid Bacteria. Health, Evolution and Systems Biology; 2008

Federation of European Microbiological Societies. The Netherlands Society for Microbiology

The ability of some lactic acid bacteria to produce exopolysaccharides (EPS) in milk during fermentation is a feature of particular importance for the dairy industry because these biopolymers are able to improve the viscosity and texture of fermentd milks, such as yoghurt, and to decrease the susceptibility to syneresis. The final textural characteristics of yoghurt are strongly dependent on the structural properties of the EPS, such as type (capsular, CPS; or ropy, R), the degree of ropiness, sugar composition and degree of branching. The objective of this work was to evaluate the effect on the rheological properties of fermented milks with EPS-producing Streptococcus thermophilus and Lactobacilus delbrueckii subsp. bulgaricus strains. Three S. thermophilus and five L. bulgaricus strains were evaluated for their ability to produce EPS or CPS and the diversity in the characteristics of the EPS (molecular mass, MM and monomer ratio). One non EPS-producing S. thermophilus strain and milk without inoculated were used as comparison. The strains were grown at 2% (v/v) in ultra high temperature low fat milk at 37ºC during 16 h and sample pH was determinate. Haake RheoStress 600 (Karlsruhe, Germany) with parallel-plate sensor was used to determine at 20ºC the flow curves and apparent viscosity (happ) of gels, and the dynamical rheological measurements recorded as a function of frequency. All samples coagulated at 16 h with pH values between 4.3 and 4.5 and exhibited a pseudoplastic thixotropic behaviour with structure type gel of different consistency. No clear correlation between happ and the concentration of exopolisaccharides has been found S. thermophilus CRL 638 (CPS-, R++, high MM-EPS) showed the highest hysteresis loop area, with structures type weak gels, following of L. bulgaricus CRL 553 (CPS-, R-) and CRL 142 (CPS-, R-); those EPS are polygalactan of low MM. L. bulgaricus CRL 861 produced a low amount of low MM-EPS, however the fermented milk with this strain showed strong gels and good viscosity, been the polymer the responsible for such property. The results suggesting a possible application of those EPS-producing strains as a starter cultures for the making of the stirred or firme yogurt.