Mircostructure and rheological properties of concentrated tomato suspensions during processing

Abstract: Food processing comprises operations such as dilution (changing the concentration), homogenisation (changing the particle size), and subsequent pumping (shearing), among others. It is thus of great interest to gain a better understanding of the mechanisms governing the creation and disruption of structures during these engineering operations, and the way in which they are related to the textural and rheological properties of the material. The influence of processing on the microstructure and the rheological properties of tomato paste suspensions has been studied. The microstructure was characterised using light microscopy and particle size distribution analysis. The way in which particles of varying size are packed in a specified volume at different concentrations was estimated in terms of the compressive volume fraction. The rheological properties were studied using small-amplitude oscillatory tests, giving the elastic (G') and viscous (G'') moduli, as well as steady shear measurements, giving the viscosity (etha). In the latter case both a rotational and a tube viscometer were used. The results indicate that tomato suspensions consist of a collection of whole cells and cell wall material forming a network (G'>G''). During the process of homogenisation, the particles are broken down, resulting in a smoother and more evenly distributed network of finer particles. The effectiveness of homogenisation in decreasing particle size seemed to be governed by the inherent susceptibility of the particles to breakage (i.e. the type of paste), the viscosity of the suspending medium, and the concentration of particles. Higher viscosities and concentrations were found to prevent breakage to some extent. The presence of larger amounts of fine particles in the homogenised suspensions had a considerable effect on the rheological properties. The yield stress was found to increase, and time-dependent effects became more apparent. At low deformations (gamma < 20), the system consisting of finer particles exhibited rheopectic behaviour (increasing viscosity with time), which was suggested to be caused partly by the rotation of the particles induced by the flow, and partly by the remaining elastic behaviour at stresses close to the yield stress. At larger deformations (gamma<1000), the non-homogenised system exhibited steady-state viscosity, while in the homogenised system it continued to decrease. The unstable behaviour observed in homogenised systems at large deformations gave an indication of particle rearrangement under flow conditions. Micrographs of homogenised suspensions subjected to shearing showed the formation of flocs consisting of densely packed particles that could easily orient in the shearing direction. At high concentrations, the changes in the microstructure caused by homogenisation and shearing were better reflected by the compressive volume fraction than by the elastic modulus. Tube viscometer measurements showed the presence of wall slip in highly concentrated tomato suspensions, which tended to disappear at lower concentrations. The wall slip, which could be as high as 70% of the flow rate, was estimated using both the classical Mooney approach and an inverse numerical method, and the performance of these two methods was compared. The performance of the methods was complicated by the relatively poor reproducibility of the data. Steady shear rheological measurements obtained using a rotational rheometer with different geometries (concentric cylinders, vane, vane-vane) and tube viscometer measurements agreed when no slip was present, and the vane and vane-vane geometries were found to be free of wall slip effects. Finally, the applicability of the Cox-Merz rule (superposition of oscillatory and steady shear data) seemed to be limited to systems that do not form a network (G'<G''), and did not apply to structured systems having a yield value and G'>G''. However, the dynamic and steady shear data obtained for tomato suspensions coincided when using a shifting factor of about 0.1 on the frequency , which was fairly constant for a large range of tomato paste concentrations (from 100 to 30%, all with G'>G'').