Physico-chemical and rheological changes of fruit purees during storage

A large number of purees of fruits and vegetables are produced commercially; some are usually consumed directly, while others are used as intermediate products and ingredients. The fruit purees studied were realized with peach, apple and pear. Immediately after the production (t0) and after the storage period (tn) the pH, soluble solids content, titratable acidity, ascorbic acid content and Bostwick consistency of samples were evaluated. Furthermore after the storage period (tn) the moisture content, the water activity and the colour parameters were also determined. Rheological properties were evaluated by using a controlled stress–strain rheometer and a cone and plate geometry sensor. The viscosity (η) against shear rate were evaluated to study the rheological behaviour of all fruit purees. In this study the flow curves were also evaluated and fitted according to the rheological model of Casson. The experimental measurements showed that the viscosity variation against the shear rate of fruit purees is exponential and therefore they are non-Newtonian fluids. The Casson model was found to fit adequately (R2 ≥0.84) over the entire shear-rate range. Moreover significant and interesting correlations between Bostwick consistency and some rheological parameters were found. Unfortunately a scanty production's standardization over the years was observed.