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Food Structure

Abstract

Rheological and microstructural changes that occurred in ropy yogurt during shearing were observed. Yogurt made with an exopolymer-producing (ropy) strain of Lactobacillus delbrueckii subsp. bulgaricus and non-ropy strain of Streptococcus thermophilus was subjected to an increasing shear rate from 0-833 s-1 using a Haake Rotovisco RV2. Shear stress niticeably increased to a peak value and then decreased to a plateau value as the shear rate continued to increase. Samples taken at eight different shear rates were examined by scanning electron microscopy (SEM). At low shear rates, the exopolysaccharide (EPS) existed as a filamentous network attached to the lactobacilli and casein matrix. At the shear rate where the highest shear stress was recorded, the EPS/bacteria bonds were broken.

SEM micrographs and shear stress curves were used to determine a "bond-strength" of the EPS/lactobacilli interaction. After the interaction was disrupted, the EPS was still incorporated with the casein, where it continued to influence viscosity.

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