Viscoelasticity and compaction behaviour of the foam-like pomelo (Citrus maxima) peel

The pomelo (Citrus maxima) is the largest and heaviest fruit of the genus Citrus and can acquire considerable potential energy as it ripens hanging up to 15 m height. Its thick foam-like structured peel presumably acts inter alia as a shock absorbing layer, protecting the fruit as it impacts on the ground upon being shed. Thereby the peel dissipates kinetic energy by being compacted. In order to elucidate the compaction mechanism of the highly heterogeneous pomelo peel, we conducted incremental stress relaxation tests. Two different models describing the stress relaxation curves, namely, the well-known Maxwell model and the Peleg model were compared and found to be suitable to describe the stress relaxation. As the Peleg model involves only two constants describing the relaxation curves it was the method of choice for interpreting the compaction of the peel samples. The inverse of k ₁ reflects the initial decay rate of the relaxation process and k ₂ is a measure of the samples’ solidity. The behaviour of these constants with increasing strain indicates the strong influence of the peel samples’ geometry and composition which is attributed to the fruit shape, a gradual changing density of the peel, which can be considered as a stacked array of foam layers differing in density, and the turgescence of the biological cells.