Tomato concentrates yield stress

Yoo, B. and Rao, M. A. 1995. Yield stress and relative viscosity of tomato concentrates effect of total solids and finisher screen size. J. Food Sci. 60 777-779,785. 

Figure 5-19 Texture Map of Structured Foods in which Values of the Static (S) and Dynamic (D) Yield Stresses Determined Using the Vane Method at Different Shear Rates were Plotted Against the Corresponding Values of the Angular Deformations. Products studied apple sauce—AS, ketchup—KH, mustard—MF, and tomato concentrate— TD. Based on unpublished data Genovese and Rao (2005).

Plant food dispersions such as tomato concentrates and concentrated orange juice are important items of commerce. The viscosity function and the yield stress are two important rheological properties that have received considerable attention. Corrections for slip, due to the formation of a thin layer of fluid next to solid surfaces, in a concentric cylinder viscometer depended on the magnitudes of applied torque and on the shear-thinning characteristics of the dispersion. Mixer viscometers were used for obtaining shear rate-shear stress and yield stress data, but the latter were higher in magnitude than those obtained by extrapolation of flow data. 

Finisher screen openings and concentration methods, were found to affect the rheological properties of tomato concentrates. The apparent viscosity of 65 °Brix concentrated orange juice was strongly influenced by temperature. The dispersed phase plays a major role in the magnitude of apparent viscosity and yield stress, and functional relationships for predicting the properties were developed. 

Yield stresses of tomato concentrates from the juice evaporation process using four FSO were determined (AS.) by the stress relaxation method (18.) over the concentration range of 9 to 14% T.S. The magnitudes of yield stress increased not only with total solids content but also with the FSO. The use of larger finisher screens resulted in concentrates with higher yield stress. 

Yield stresses can also be obtained by extrapolation of shear rate-shear stress data to zero shear rate according to one of several flow models. The application of several models was studied by Rao et al. (AS.) and Rao and Cooley (Al) The logarithm of the yield stresses predicted by each model and the total solids (TS) of the concentrates were related by quadratic equations. The equations for the yield stresses predicted by the Herschel-Bulkley model (Equation 4) which described very well the flow data of Nova and New Yorker tomato cultivars were ... 

Curves A and B of figure 3.3 illustrate visco-plastic behaviour. Fluids of this type will not flow until a critical yield stress is exceeded. Concentrated tomato ketchup is a good example. Although less... 

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