Modelling of mechanical deformation of apple fruit tissue due to hygrostresses

Abstract

During postharvest storage at low relative humidity, water from fruit cells is lost, which is accompanied by hygrostresses. This leads to a decrease of the volume of the cells and thus of the total volume, shape and mass of the fruit. Water loss of fruit during storage has a large impact on fruit quality and shelf life, and is essential to fruit drying processes. The main objective of this thesis is to investigate the dynamics of dehydration of apple tissue during drying. For this purpose, a multiscale water transport and mechanical model was developed to predict the water loss and deformation of apple cortex tissue during dehydration. Maturation and senescence could influence the dehydration process but were not included in this study.At the macroscopic level, a continuum approach was used to construct a coupled water transport and mechanical model. Water transport in the tissue was simulated using a phenomenological approach using Fick s second law of diffusion. Mechanical deformation due to shrinkage was based on a structural mechanics model consisting of two parts: Yeoh strain energy functions to account for non-linearity and Maxwell s rheological model of visco-elasticity. The Mooney-Rivlin and Yeoh hyperelastic potentials with three parameters were effective to reproduce the nonlinear behavior during the loading region. The Maxwell model was successful to quantify the viscoelastic behavior of the tissue during stress relaxation. The sensitivity of different model parameters using the nonlinear viscoelastic model using Yeoh hyperelastic potentials was studied. The model predictions proved to be more sensitive to water transport parameters than to the mechanical parameters. One-dimensional water transport and large deformation of cylindrical samples of apple tissue during dehydration was modeled by coupled mass transfer and mechanics and was first validated by calibrated X-ray CT measurements. The accuracy of the 2D model was further verified with quantitative neutron radiography experiments. Both model simulations and experiments showed that the largest moisture gradients occurred at the air-tissue interface. The corresponding shrinkage behavior was similar. Furthermore, the Biot number was quite large, indicating that the drying kinetics were dominated by the water transport in the tissue rather than by the convective flow at air-tissue interface. In addition, the performance of a 3D model was checked, based on a comparison of the total water loss, the transient water distribution profiles and the mechanical deformation profiles, measured using quantitative neutron tomography. The simulated results showed a good agreement with experimental observations. Although access to facilities which produce neutrons is limited, neutron imaging also showed large potential for studies on fruit dehydration, as accurate quantification of the water content was possible. At the microscopic level, a model which took into account the water exchange between different microscopic structures of the tissue (intercellular space, the cellwall network and cytoplasm) was developed using transport laws, which consider the water potential as the driving force for water exchange between different compartments of tissue. The microscale deformation mechanics were computed using a model where the cells were represented as a closed thin walled structure. The predicted apparent water transport properties of apple cortex tissue from the microscale model showed a good agreement with the experimentally measured values.The multiscale model helped to understand the dynamics of the dehydration process and the importance of the different microstructural compartments (intercellular space, cell wall, membrane and cytoplasm) for water transport and mechanical deformation. The validated model can be employed to better understand postharvest storage and drying processes of apple fruit and thus improve product quality in the cold chain.status: publishe