Modelling of Rheological Behaviour of Pulps and Purées from Fresh and Frozen-Thawed Murta (Ugni molinae Turcz) Berries

Abstract

Abstract The rheological behaviour of purées and pulps from fresh and frozen-thawed murta (Ugni molinae Turcz) berries was studied at 30, 45, 60 and 75 °C. The Mitschka method was shown to be applicable to the murta berries, with its products showing non-Newtonian pseudoplastic behaviour. Rheological models as Ostwald-de Waele, Herschel-Bulkley, Bingham, Casson and Mizrahi-Berk fitted the experimental data with high values for correlation coefficients. Effect of temperature was evaluated using an Arrhenius-type equation. Compared to the murta pulp, the fresh or frozen-thawed murta purées had higher apparent viscosity at any shear rate and would undergo a more rapid change in viscosity with temperature. Activation energy for fresh and frozen-thawed purées were 22.335 kJ mol-1 (R2 = 0.9225), and 16.478 kJ mol-1 (R2 = 0.9654) respectively, while for fresh and frozen-thawed pulp Ea values of 8.967 kJ mol-1 (R2 = 0.9618) and 6.943 kJ mol-1 (R2 = 0.9773) were determined respectively.