Structure Breakdown of Stirred Yoghurt in a Circular Pipe as Affected by Casein and Fat Content

Abstract

In this study, the rheological and flow properties of stirred yoghurt in a circular pipe as affected by casein and fat content were investigated. The structure breakdown of the stirred yoghurt was evaluated from the rheological measurements and compared with that estimated from pipe flow data through estimating the dimensionless structure number (Se) and Deborah number (De). The rheological tests revealed that increasing the casein and fat content led to increase the apparent viscosity of the stirred yoghurt and its dependence on shearing time. Both, the rheological and pipe flow tests revealed that the thixotropic behaviour of stirred yoghurt increased with casein and fat content, where the rate of structure breakdown of yoghurt in pipe flow was greater than the build-up rate. On the other hand, increasing the casein and fat content in stirred yoghurt resulted in an increase in the Deborah number (De), which means an increase in the elasticity of the yoghurt. It was shown in this work that the simple rheological tests can be used to estimate the structure breakdown rate of stirred yoghurt in pipe flow.