German Research Foundation (DFG) - Project number: 436145430
In the agricultural process chain leading from the harvest to the final consumer, potato tubers undergo a myriad of mechanical loads which frequently entail internal structural and physiological damage. Contrary to external damage, internal damage is particularly worrisome as it is not easily detectable in grading stages.
Typically, internal tissue damage elicits biochemical processes which, over time, entail the inception of the black pigment melanin. This process is commonly referred to as blackspot bruising. On the scale of a whole potato tuber, it has been extensively investigated, mainly in order to assess the influence of adjustable process parameters such as fertilization, harvest temperature, cultivar and tuber maturity on the susceptibility to blackspot bruising.
While many researchers have emphasized the connection between mechanical characteristics of tuber tissue and bruising susceptibility, a detailed physio-mechanical model relating the stress-strain response to blackspot bruising on the tissue scale is as yet absent, to the awareness of the applicants.
In this project, a multifield modeling approach synthesizing inelastic constitutive relations for potato tuber tissue and reaction kinetics for the intensity and progress of blackspot bruising is developed. The constitutive parameters for the stress-strain response and the discoloration kinetics are determined based on tissue scale experiments and, importantly, encompass/absorb the pre-testing history of a potato tuber batch.
The experimental campaign involves compression tests, respirometric and discoloration measurements as well as a quantification of gas diffusion rates in tuber tissue. Conceptually, the resulting continuum mechanical model can be used to emulate and analyze the response of potato tuber tissue to different load patterns and to forecast the physiological bruising response over time.
In order to quantify the load collectives that potato tubers are subjected to in the course of a typical postharvest process, a discrete element model (DEM) for the tuber-tuber and tuber-machinery interaction is advanced. The main persisting challenges for DEM of agricultural produce are related to the construction of reliable contact force–penetration laws and the identification of a representative tuber shape. On a tuber's surface, moreover, impact forces are collated into load collectives according to the blackspots which they contribute to.
By joining these two branches of development, an unprecedented predictive approach for quantifying the fraction of internally damaged potato tubers and the temporal change in damage extent for a given potato tuber batch and handling equipment is provided. Both the projected continuum model and the predictive capabilities of the DEM are validated in a series of dedicated experiments.