A systematic approach for developing and manufacturing fruit simulators : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Engineering, Massey University, Palmerston North, New Zealand
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Date
2023
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Massey University
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Abstract
Due to the high cost, variable nature and seasonal availability of fruit, conducting large scale experiments for research purposes is not easy. A fruit simulator is a physical tool that mimics the mechanistic features and properties of the targeted fruit; hence, it can be used as a replacement for the fruit in research experiments. This study focuses on developing simulators for heat transfer experiments, especially in horticultural produce precooling.
A framework for developing the simulator was established based on the importance of each mechanistic feature. Depending on the application's needs, the simulator can mimic different length scale levels of the targeted fruit, such as the individual fruit, the bulk stacking of the fruit or sub-units of the fruit (e.g., a punnet/bag of table grapes). The scale level determines whether certain mechanistic features are important and affects the values of the thermal properties that must be matched. For example, a simulator that mimics a punnet of fruit with enclosed air pockets has an effective thermal conductivity and volumetric heat capacity that includes contributions from the thermal properties of the fruit and air, which provides more room for material selection.
Based on this framework, a systematic approach for the simulator manufacture and material selection was developed. Three different simulators were developed based on the framework: kiwifruit, apple and table grape simulators. The comparison of a simulator and real fruit precooling trials showed good agreement, validating the approach and demonstrating the feasibility of using simulators in postharvest research. The kiwifruit simulator was validated at different experimental scale levels, from individual kiwifruit to multiple kiwifruit boxes containing numerous individual kiwifruit simulators (which reflected pallet scale precooling).
During the simulator development, the concept of a time-scaled approach was identified and was explored. In theory, if the volumetric heat capacity of a simulator becomes smaller while the Bi of the simulator remains the same, the heating/cooling time of the simulator in an experiment will decrease proportionally according to the Fourier number (Fo). This approach was validated via the three simulators developed in this study. The validation of the simulators confirms the feasibility of this time-scaled concept. This approach has a significant advantage in reducing the experimental time and easing the material selection process for the simulator manufacture.
In the table grape simulator development, a process of using CT scans of the bulk packaged system to study the bulk shape and effective properties of the fruit subunits (bags) were developed, where the bulk shape and effective thermal properties of a bag of table grape were determined based on the process. A set of bag shaped fruit simulators was then manufactured with equivalent bulk thermal conductivity and used to validate the bulk simulator approach by comparison of cooling rates with real fruit.
Overall, this study has successfully developed a generalised heat transfer simulator development framework. In addition, this study validated the feasibility and applicability of the time-scaling approach, which could be helpful for any future experiments. Furthermore, this study has developed a process to use CT scanning to determine a bulk object's bulk shape and effective property.
The outcomes of the work pave the way for carrying out postharvest and packaging optimisation experimental trials with reduced variability, greater ease and without seasonal constraints. The simulator development framework provides a basis for further expansion of these concepts into other applications beyond the heat transfer focus that they were developed for in this work.
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Keywords
Fruit, Simulation methods, Precooling, Heat, Transmission