Microstructure of highly concentrated tomato suspensions on homogenisation and subsequent shearing

The changes in the microstructure of tomato paste suspensions have been investigated during homogenisation and subsequent shearing in suspensions with similar composition, at three tomato paste concentrations 10, 30 and 40%. The suspensions were characterised by the particle size distribution (PSD), volume fraction (empty set), and dynamic theological properties (G`, G ''). All suspensions exhibit a solid-like behaviour with G`> G ''. Micrographs indicate that the process of homogenisation creates a smooth network of finer particles, that is easily disrupted by prolonged shearing, giving rise to the formation of densely packed flocs that become clearly oriented in the direction of the shearing. At high concentrations, these changes in the microstructure on homogenisation and subsequent shearing were better reflected by differences in empty set than in G`. The rheological behaviour of the suspensions exhibits a power-law dependence on empty set, over a large range of PSD and for 0.05 < empty set < 0.55. Finally, an experimental equation, including empty set and the size of the coarse particles in the surface-weighted PSD, is found to accurately estimate G` (R-2 > 99.3%, p < 0.001). (c) 2011 Elsevier Ltd. All rights reserved.