Microscale Modeling of Water Transport in Fruit Tissue

During long term storage of fruits their commercial value may degrade due to loss of crispiness and shrivel that are associated with water loss. Water loss also equates to decrease of saleable weight, and thus causes a direct economic loss. Measures that minimize water loss after harvest will usually enhance profitability. For an improved operation of cool rooms a more fundamental insight in water transport processes inside fruit is necessary. The main objective of this dissertation was to study water transport at the microscale so that it will then become feasible to evaluate measures to reduce water loss of fruit during storage and distribution using the this model in a multiscale modeling framework. Pear (Pyrus communis L. cv. Conference) was chosen as a model system.Different artificial cell wall materials (bacterial cellulose (BC), bacterial cellulose with pectin (BCP), bacterial cellulose with pectin and xyloglucan (BCPX)) were produced using bacterial fermentations with Gluconacetobacter xylinus. The microscopic structure of these three artificial cell walls was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface images showed clear differences in the diameter of the microfibrils and the density of the fibril network. The degree of compactness was highest for BCPX, then BCP and BC. From the desorption isotherm it was found that the water retention capacity at the same water activity was higher for BC than for the other two artificial cell walls; BCP had higher water retention capacity than BCPX. The lowest value of water conductivity was found for BCPX while the highest value was found for the pure BC. Adding pectin had a strong effect on the water conductivity, while xyloglucan did not have any appreciable additional effect. The structural properties were related to the water transport properties.A model was developed to describe water transport in fruit tissue, taking into account the microstructural architecture of the cell assemblies in the tissue, which leads to a better understanding of the underlying phenomena causing water loss. The fruit tissue architecture was generated by means of a cell growth model. The transport of water in the intercellular space, the cell wall network and cytoplasm was predicted using transport laws using the chemical potential as the driving force for water exchange between different microstructural compartments. The model equations were discretized over the pear cortex tissue geometry using the finite element method. The different water transport properties of the microstructural components were obtained experimentally or from literature. The effective water conductivity of pear cortex tissue was calculated based on the microscale simulations. The values, 6.10 ±0.14 ´ 10-15 kg.m-1.s-1.Pa-1, corresponded well with measured values of tissue water transport parameters. The model helped to explain the relative importance of the different microstructural features (intercellular space, cell wall, membrane and cytoplasm) for water transport. The cell membrane was shown to have the largest effect on the apparent macroscopic water conductivity. An equivalent micro scale model that incorporates the dynamics of mechanical deformation of the cellular structure was implemented. The model predicted the effective tissue conductivity of pear cortex tissue, 9.42 ± 0.40 ´ 10-15 kg.m-1.s-1.Pa-1, in the same range as those measured experimentally. The largest gradients in water content were observed across the cell walls and cell membranes. Sensitivity analysis of membrane permeability and elastic modulus of the wall on the water transport properties and deformation shows that the membrane permeability had the largest influence.

Jaar van publicatie:2013

Toegankelijkheid:Closed