produce into refrigerated containers for cooling during marine
transport. This cold-chain protocol differs in many ways from FAC
due to the vertical airflow, low airflow rates and limited installed
cooling capacity within the reefer container. By evaluating higher
flow rates up to those characteristic for FAC, this study also covers
vertical cooling in general. The main perspective was to gain
insight in cooling rates and cooling uniformity between individual
fruit in a box and between boxes at different heights on a pallet.
The main conclusions were the following:
At low airflow rates (volumetric rate of air movement per mass of
product, L s1 kg1), typical for refrigerated containers, the
cooling rate was obviously lower but also the heterogeneity in
cooling between different layers of boxes (in height) and
between individual fruit in a single box was larger, compared
to vertical cooling at FAC airflow rates.
At similar airflow rates, the fruit cooling rates for horizontal and
vertical airflow agreed quite well, which was attributed to the
box design since it also included vent holes that promoted
vertical airflow pathways. Note, however, that the (vertical)
stacking of boxes on the pallet in the computational was more
idealised compared to practice.
The presence of gaps between the pallets invoked significant
airflow short-circuiting, which reduced the cooling rate at all box
heights on the pallet, but it improved cooling uniformity
between the boxes to some extent.
From the (idealised) computational modelling, a seven-eighths
cooling time (SECT) below about 3 days could be obtained at
airflow rates typical for refrigerated containers. Such SECTs
however cannot be achieved in practice with the (low) installed
cooling capacity in a reefer container, for realistic stacking
configurations and with the presence of gaps between pallets.
In this study, an important step was taken towards a more
systematic evaluation of this cold-chain protocol for further
optimisation, industrial implementation and application for other
citrus cultivars or horticultural products. Strategies for future
improvement of the ambient loading protocol include optimising
box design and stacking on the pallet specifically for vertical
airflow and reducing the airflow short-circuits between the pallets.
Although CFD was successfully shown to be a suitable tool to assess
fruit cooling behaviour, a synergy between numerical modelling
and full-scale experiments is a key focus for further model
development.
Acknowledgements
We would like to thank Gerard Grinwis and Johan-Carel
Bossman of Maersk Container Industry AS and Richard Lawton
from Cambridge Refrigeration Technology for providing us with
detailed information on the specifications of refrigerated containers.
We would also like to thank Sundays River Citrus Company
(Andre Mouton and John Perrold) and Capespan (Keith Roxbury)
for fruit and logistical support. U.L. Opara’s contribution was
supported by the South African Research Chairs Initiative of the
Department of Science and Technology and the National Research
Foundation. We also want to acknowledge the support of the
World Food System Center (WFSC) of ETH Zürich (www.
worldfoodsystem.ethz.ch). This research was carried out in the
context of the European COST Action FA1106 (‘QualiFruit’).