Modelling fruit bruise damage from the macro- to the microscale using the Discrete Element Method

Mechanical bruise damage in fruit is the consequence of the progression of cell ruptures and inter-cellular separation. It leads to significant economic losses and food waste. The objective of this PhD project was the development of generic methods and models that can help predict fruit bruise damage and advance insights in mechanical bruise damage.

In order to develop bruise prediction models, it is valuable to have a non-destructive technique that allows for accurate bruise volume measurements. Such a technique was developed in this thesis using X-ray CT images of bruised fruit. It was shown that bruises can be highly irregularly shaped, which implies that the bruise volume estimates based on destructive measurements in combination with simple geometric assumptions will be inaccurate.

A novel visco-elastoplasic contact force model was introduced that can predict impact forces on fruit more accurately than previously existing models. The model is applicable in Discrete Element Method (DEM) simulations with arbitrary rounded shapes, which is an important feature for dealing with the non-perfectly-spherical shape of fruit. The model has been experimentally validated on ‘Jonagold’, ‘Joly Red’ and ‘Nicoter’ apples using both quasi-static and dynamic mechanical measurements. The model is applicable to other fruits and can help to predict and understand bruise damage. In future research, the novel visco-elastoplasic model may be used to optimise picking robots, sorting lines etc. such that less fruit gets damaged before reaching the consumer.

Next, empirical bruise damage prediction models are presented that are based on impact experiments on apples. ‘Nicoter’ apples were found the least bruise susceptible compared to ‘Jonagold’ and ‘Joly Red’. Impact experiments in which multiple impacts per apple were applied on the same position, showed that the number of impacts has an important effect on the bruise size and the impact profiles. This is an important observation, since fruit can undergo multiple impacts at the same location during fruit handling, while in most research on bruise damage single impact experiments have been performed. This effect of multiple impacts on the mechanical behaviour of fruit can probably be explained by the progression of damage at the cellular scale, which highlights the importance of the micro-mechanics of fruit tissue in bruising.

To gain a better understanding of the micro-mechanics of plant tissue, a model was developed to describe plant tissue as a packing of adhering deformable visco-elastoplastic shells under turgor pressure wherein inter-cellular separation is irreversible. Realistic 3D virtual structures of tomato mesocarp tissue were generated using a novel method. The generated tissue structures compared well with micro-CT images of real tomato mesocarp tissue. Compression and tensile test simulations up to tissue failure were performed and provided more insight in the micro-mechanical behaviour of the tissue. The model was used to investigate how the inter-cellular bonding energy and tissue porosity influence the tissue failure characteristics. In future research, it may be used to examine the effect of other cellular properties (like cell size, turgor pressure, etc.) on the overall mechanical behaviour of fruit tissue.