The determination of the first normal stress coefficient of an exopolysaccharide solution by rheo-optical measurements

Abstract This paper illustrates how rheo-optical techniques may be utilized to determine the first normal stress coefficient for an exopolysaccharide (EPS) produced by the Lactobacillus delbrueckii ssp. bulgaricus LY03, which is widely used in yoghurt production. In this technique both shear stress, optical birefringence and extinction angle are measured simultaneously as a function of shear rate. From the stress optical rule the first normal stress difference can be determined directly without relying on any specific rheological model. Of special interest is the point where the first normal stress difference become comparable in magnitude to the shear stress. Here we expect to find a notable influence of the first normal stress difference on mouthfeel. Keywords: Flow birefringence - First normal stress coefficient - Exopolysaccharide - EPS - Viscoelasticity