Changes in the Rheology and Microstructure of Ropy Yogurt During Shearing

Rheological and microstructural changes that occurred in ropy yogurt during shearing were ob served. Yogurt made with an exopolymer-producing (ropy) strain of Lactobacillus delbrueckii subsp. bulgaricfui and non ropy s tram of Streptococcus thermop us was subjected to an increas1ng shear rate from 0-833 s 1 using a Haake Rotavisco RV2. Shear stress noticeably increased to a peak value and then decreased t o a plateau value as the shear rate continued to increase. Samples taken at eight dif ferent shear rates were examined by scanning elec tron microscopy (SEM). At low shear rates, the exopolysaccharide (EPS) existed as a filamentous net work attached to the lactobacilli and ~asein matrix. At the shear rate where the highest shear stress was recorded, the E PS /bacteria bonds were broken. SEM micrographs and shear stress curves were used to determine a "bond -strength " of the EPS/lac tobacilli interaction. After the interaction was disrupted, the EPS was s till incorporated with the casein, where it continued to influence viscosity. Initial paper received February 8, 1990 Manuscript received May 11, 1990 Direct inquiries to J .A. Teggatz Telephone number: 612 644 5653