JOURNAL OF MATERIALS SCIENCE 38 (2 0 0 3 ) 2489 – 2498 Time-dependent aspects of the mechanical properties of plant and vegetative tissues H. X. ZHU ∗ Polymer and Colloids Group, Cavendish Laboratory, Cambridge University, Madingley Road, Cambridge CB3 0HE, UK E-mail: hz207@cam.ac.uk J. R. MELROSE Unilever Research, Colworth House, Sharnbrook, Beds MK44 1LQ, UK Based on a single regular cell structural model, the effects of loading rate on the compressive behaviour of plant and vegetative tissues have been qualitatively investigated. The cell walls were treated as a polymeric composite material with microfibrils embedded in the highly structured cell wall matrix. The rubber elasticity, the turgor permeability and the loading rate were taken into account to qualitatively predict the tissue stiffness, cell wall stress, turgor pressure, cell debonding force, and the percentage of weight loss of the cell fluid. The predicted results are consistant with the related experimental phenomena. C  2003 Kluwer Academic Publishers 1. Introduction Fruit and vegetables are food products. It has long been recognised that freezing and cooking can dramatically change the structural and mechanical properties of plant and vegetable tissues [1–3], and those properties are directly related to oral sensory perception. In raw (un- processed) fruit and vegetables, the fluid is retained by the cell membrane inside the cell walls. Cell membrane is the lipidbilayer containing the cell. Cell wall is cel- lulosic fibre composite. When plant and vegetables are processed by freezing or cooking, the cell membrane can be destroyed in different degrees, and the fluid can express through the cell walls. Hence, the mechanical properties of the processed plant and vegetable tissues are loading-rate dependent. Relatively low loading rates have been studied in raw materials [1, 4–9], while in-mouth deformation rates are usually very high [10–12]. To link the mechanical and sensory properties of a plant or vegetable tissue, a wide range of strain rate should be considered. In processed plant and vegetables, if the loading rate is very low, the fluid in the tissue is relatively free to flow through the cell walls, hence the internal pressure can not be built up in the tissue. If the loading rate is high enough, the fluid in the tissue could be trapped in the cells and the tissue behaves in a similar way to the raw (unprocessed) tissue. Theoretical and experimental results indicate that both the tissue stiffness and the cell wall stress increase with increasing cell turgidity [13–16]. Hence, increasing the loading rate or the turgor pressure decreases the compressive stress at failure of the tissues, such as potato and apple [17, 18]. ∗ Present Address: Micromechanics Centre, Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, UK. Although some experimental work has been carried out to investigate the effects of loading rate on the mechanical properties of plant and vegetable tissues, very little theoretical analysis has been done to pre- dict the mechanical behaviour of plant and vegetable tissue. Using a two dimensional hexagonal cell model and assuming that the cell wall material is linear elas- tic, Pitt and Chen [19] have qualitatively analysed the effects of loading rate on the mechanical response of vegetative tissues. However, the cell walls of plant or vegetables are polymeric composite materials with mi- crofibrils embedded in the highly structured cell wall matrix. The aim of this work is to qualitatively relate fluid diffusion, loading rate, and the cell wall elasticity to the mechanical behaviour of the whole tissue of plant or vegetables. The analysis is based on a three dimen- sional hexagonal cell structural model. The cell walls are treated as a rubber-like (polymeric) material, and the case of cell walls with microfibrils stiffening has been taken into account. The emphasis is focused on the mechnaical properties, hence the extremely com- plex situation in the living cells has been greatly sim- plified in this analysis. However, the analytical results should give qualitative insight into the response of the actual tissue. 2. Model development The model and the analysis developed in this paper are the direct extension of our previous work [16]. As before, we use an array of identical, regular three dimensional hexagonal cells to present the plant or 0022–2461 C  2003 Kluwer Academic Publishers 2489