Plant Cell Wall Modification During Tomato Processing and Its Effects on the Physical and Rheological Properties of End Products
Understanding the relationship between structure and functional properties in plant-cell-wall-derived foods has become a growing interest to both academia and industry. Tomato is one of the most cultivated vegetable crops and mostly is consumed as processed products in the form of suspensions. Rheological properties of tomato product, a key functional attribute, depends on both the serum and particle phases of these products. This research systematically evaluated the contributions of soluble pectin and particle phase on the rheology of tomato suspensions, and identified that the particle structure and its physical properties are crucial in determining the rheology of such systems. Alteration of these properties either by processing conditions or by internal enzymatic activity could cause a significant change in the rheology of tomato products. The serum phase of the suspensions displayed a Newtonian behavior with a low viscosity. The contribution of soluble pectin to the overall viscosity of the suspensions was found to have a little influence despite that reconstituted suspensions were prepared either with large pectin concentrations or with pectin having a high degree of methylation. However, the presence of pectin was important because its role on stabilization of the suspension systems by increasing the interaction between particles. When pectin concentration was low, wall slippage during measurements was observed due to phase separation by using cone-plate geometry. A vane geometry was able to alleviate the slippage artifact and a good correlation was found between the empirical Bostwick consistometer method and fundamental measurements performed employing the vane geometry. Hence, the vane geometry was recommended in the viscosity measurements of cell-wall-derived suspensions. The particle structure and its physical properties, and the associated particle interaction controlled the rheological properties of the cell-wall-derived suspensions. Changes in the particle phase were achieved in this study by two means: external processing with various conditions and molecular biological modification by reduced pectin methylesterase activity. The focus was on effects that this process has on the properties of the particles and the rheology of the suspensions when they are reconstituted from the paste to juices. Although both the ultrasound and high shear treatments reduced significantly the particle size of the treated tomato suspensions, the former led to an increase in their rheological properties whereas the latter caused a significant decrease. It could be explained by formation of particles with structural differences provoked by these two treatments. Ultrasound treated suspensions contained more intact particles, and with large strength, which was evaluated by a compression test on a limited number of particles. Conversely, high shear treated suspensions resulted in mostly ruptured particles that lost mechanical strength. The water-soluble pectin fraction increased after ultrasound and shear treatments. However, soluble pectin is not the direct cause for the changes in the suspension rheology; it is an indicator or consequence of the changes in particle properties. The last part of this research investigated the flow behaviors of industrially processed hot-break and cold-break tomato suspensions under steady-state and dynamic oscillatory shear conditions. The HB suspensions exhibited considerably higher viscosity and viscoelastic properties than CB suspensions because their particles had a structure that was able to retain better water and higher mechanical strength. Both industrially processed samples exhibited temperature-dependent and time-dependent rheological behaviors. The consistency coefficient as a function of temperature could be modeled by an Arrhenius-like equation. The activation energy of the HB sample was higher than that of the CB sample, indicating a more integral structure resisting changes in temperatures. The thixotropic behavior of HB and CB suspensions was described by the Stretch Exponential equation. A characteristic time used in the Stretch Exponential equation increased with temperature for the HB sample whereas it showed the opposite trend for the CB sample. Particle interactions showed great impact on the rheological properties. When particle concentration was low, both HB and CB samples almost had the same apparent viscosity due to a limited contact between particles. However, when the particle phase was high, the particle-particle contact significantly increased, and the HB sample demonstrated a considerably higher viscosity and viscoelasticity. Results indicated that the HB system has larger particle elasticity and stronger particle interaction than the CB system. Furthermore, the local Young’s modulus distributions of individual HB and CB particles investigated by Atomic Force Microscopy were in good agreement with the bulk rheology data. It can be concluded that the differences in rheological properties of tomato products are originated from differences in their particle phases. (Abstract shortened by ProQuest.)
Reuhs, Purdue University.
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