EFFECTS OF STARTER CULTURES ON LINEAR VISCOELASTIC AND PHYSICAL-PROPERTIES OF YOGURT GELS

Set-style yogurts were produced from cow's milk under standard conditi ons by using 11 commercially available starter cultures. Samples were prepared in control beakers and within the Couette device of a dynamic rheometer. Repeatability of rheometer yogurt fermentations proved to be satisfactory. Within the linear viscoelastic region a significant i nverse relationship between storage modulus and tan delta of yogurt sa mples was found. Specific properties of starter cultures clearly affec ted response in dynamic frequency sweeps and in stress relaxation expe riments. Dynamic and relaxation data showed good agreement. Relaxation spectra H(tau) were obtained from dynamic measurements and stress rel axation by approximation methods. Both the shape of H-tau curves and t he magnitude of H was affected by starter cultures. As compared to lit erature data on model casein gels and microstructural findings on yogu rt the results imply that (a) a permanent network exists in yogurt, (b ) interactions with long relaxation times contribute less to viscoelas ticity in the case of yogurt produced with viscous starter cultures, a nd (c) attachment of mucogenic bacteria to the protein matrix via exog enic polysaccharides decreases yogurt firmness.