Mathematical modeling of the viscosity of tomato, broccoli and carrot purees under dynamic conditions

Different viscosity models were developed to describe the viscosity of unprocessed fruit and vegetable purees under dynamic conditions. Temperature hysteresis cycles were carried out for three purees with different structural characteristics (tomato, carrot, and broccoli), with heating and cooling phases from 10 to 80°C with isothermal (holding) phases at 10, 30, 60 or 80°C. The apparent viscosity was measured continuously with a rotational rheometer and the data was analyzed with time-independent and time-dependent models (quantifying rheopexy, thixotropy, or both). The results revealed clear thixotropic behavior in tomato puree, attributed to shearing effects, and rheopectic in broccoli puree, attributed to gel formation at the higher temperatures. Although carrot puree data from the isothermal periods could be quantified satisfactorily with no time dependency, analysis of the nonisothermal periods proved that rheopectic effects also needed to be included.