Optimising ventilated package design for postharvest handling of pomegranate fruit in the cold chain

Packaging is an indispensable unit operation in handling and distribution of fresh fruit. Studies on postharvest handling of a number of horticultural products highlighted the importance of package design and knowledge of fruit and package thermophysical properties to effectively accomplish the precooling, cold storage, and refrigerated transport processes. However, the thermal properties of pomegranate fruit and its parts are unknown, and packages for postharvest handling of pomegranates have not been properly investigated. The aim of this study was to address the multi-parameter design requirements of ventilated packages for handling pomegranate fruit to ensure efficient cooling, high precooling throughput, reduction in packaging material used, and improved space utilization during cold storage and refrigerated transport. Firstly, the thermal properties of whole fruit and the parts (epicarp, mesocarp, and arils) of early (‘Acco’) and late (‘Wonderful’) commercial pomegranate cultivars were determined experimentally using a transient heating probe. The values of thermal conductivity and diffusivity of both cultivars increased significantly with an increase in tissue temperature. The aril part was observed to have the highest thermal conductivity and specific heat capacity, respectively. For example, at 7 °C, the thermal conductivity (W m-1 K-1) of ‘Acco’ was 0.419 ± 0.047, 0.352 ± 0.040, and 0.389 ± 0.030 for arils, mesocarp, and epicarp, respectively. Next, a survey of the packaging used for pomegranate fruit in South Africa was conducted. Over 10 different corrugated fibreboard carton designs, with largely open tops, were found with different ventilations, ranging from 0.74–4.66% on bottom, to 0.71–5.33% on short (width), and 4.60–13.82% on the long (length) faces. The cartons were largely poorly ventilated on the short faces that leads to vent-hole misalignment and vent-hole blockage on pallet stacking which increases fruit cooling time and energy requirements. Then, a virtual prototype approach based on computational fluid dynamics (CFD) was used to redesign the ventilation of one of the most commonly used pomegranate fruit cartons with intent to improved cooling performance. Fruit cooled in the new design had more uniform temperature distribution and significantly cooled faster (1.6 hours faster in fruit in polyliner) compared to fruit in the commercial design. This result highlights the need of proper carton vent design and vent-hole alignment in stacks. Furthermore, a virtual prototype approach, based on CFD and computational solid dynamics (CSD) was used to design new ventilated corrugated paperboard cartons that hold pomegranate fruit in multilayers. Running virtual airflow and strength measurements enabled selecting the best alternatives, the ‘Edgevent’, and ‘Midvent’, which were then manufactured and evaluated for cold chain performance. The new designs improved fruit throughput by over 1.8 tonnes more fruit in a reefer compared to commercial single layer designs. For similar volume of fruit contained, the new designs saved over 31% cardboard material and an estimated equivalent of 11 trees per fully loaded 40-ft refrigerated container. Overall, the ‘Midvent’ performed best under cold chain conditions in terms of cooling efficiency and mechanical strength requirements. This warrants its commercialisation. Lastly, the quality of fruit stored in ‘Midvent’ for 12 weeks under cold chain condition (7 ± 1 °C, 90% RH) and an additional 2 weeks at ambient (shelf life) condition (20 ± 1 °C, 65% RH) was compared with fruit in commercial carton under similar conditions. Fruit respiration followed a similar pattern in both carton designs marked by a 64% reduction after precooling. At the end of the shelf life period, fruit weight loss was 5.7% and 8.9% in the ‘Midvent’ and commercial design, respectively. Sensory attributes, decay incidence and colour changes were similar in new and commercial carton designs over the storage period. Overall, research reported in this thesis has provided new data on thermophysical pomegranate fruit and has applied the virtual prototyping tool for horticultural packaging design. The new ‘Midvent’ carton design provides additional benefits in savings in packaging material, energy for fruit cooling, and bioresources efficiency. Future research should focus on performance test of this carton design in the commercial chain. New data on the thermal properties of pomegranate fruit provide needed input towards the modelling and prediction of fruit internal temperature profile during cooling processes.