Rheological modelling of the viscoelastic behaviour of the St. Julian mango (Manifera Indica L, var. Julie)

Abstract

Measurements of the depths of penetration or deformation values were obtained for the St. Julian mango under constant load test for four hours duration. During this time the mango exhibited creep. On removal of the load the change in deformation was measured over an hour and a half and creep recovery behaviour was observed. These tests were performed over seven days and the rate of respiration was monitored. Four rheological models were examined and tested to mathematically model the physical behaviour of the St. Julian mango under these tests. The Burgers model was selected as the best fit as over 95% of the samples fitted the model and R² values of greater than 0.90 were obtained. It was also observed that there was a significant increase in deformation on the onset of climacteric. This could be attributed to the onset of the softening of the fruit. Creep recovery measurements indicated a permanent indentation of 2.7 mm on the fruit’s skin after 7 days of harvest. Investigations were also performed on changes in creep recovery, rate of change in creep recovery and recovery strain. Both changes in creep recovery and rate of change in creep recovery showed an exponential relationship with time. A new term called recovery time (l) which was the minimum time required for permanent deformation to set in was developed. Such a value is useful for handling and packing of fruits. A linear relationship was developed between the change in creep recovery and days after harvest. This relationship would have use in determining the minimum acceptable change in permanent deformation during handling and packing processes.