The effect of OTFLOW system on temperature distribution in containers specifically for cold steri protocols

Distribution of cold air evenly throughout a shipping container is a problem, with warm spots close to the door and cold spots close to the cooling unit. The pulp temperature of plums undergoing cold treatment for phytosanitary purposes in a shipping container, may not exceed 1.11°C at any point in the stipulated period. In an event of this happening the treatment will be nullified. The OTFLOW system is easy to use and evidently channels more air to the warmer part of the container and less air to the coldest part, thus potentially providing a more even temperature distribution

The 2019 and 2020 trial consisted of four standard 40 ft shipping containers, two with OTFLOW and two without In each container, 3 pallets (near cooling unit, middle and near door) were utilized to measure air temperature, pulp temperature and relative humidity (RH), at 2 positions (bottom and top) per pallet. Containers were loaded with African Delight plums exported at a single-temperature regime at -0.4 °C. In 2019, the loggers were misplaced overseas. In 2020, on arrival of fruit in Rotterdam, containers were opened, pallets off loaded and temperature and RH loggers retrieved. Samples comprising 50 plums each were drawn at each of the six monitoring positions for fruit quality assessment. In 2021, as a consequence

of Covid-19 travel restrictions and past research results, the testing of the OTFLOW system was discontinued. Alternatively, valuable data on air temperature and RH during sea export in standard shipping containers was gathered.

In 2020, all shipping containers exhibited higher fruit pulp temperatures at the door, exceeding 1.11 °C at some stage. Air temperature spikes in one container without OTFLOW were higher and for longer periods, compared to one with the OTFLOW system. Across containers, air temperatures at the top of the pallets near the door were highest, while the top of the pallets near the cooling unit were lowest. All the containers exhibited a build-up of RH over time, probably due to fruit losing moisture because of a vapour pressure deficit (VPD) between fruit and atmosphere. RH was lowest near the door of containers and highest near the cooling unit. The lowest flesh firmness, which suggested faster fruit ripening, occurred in the door positions of the containers fitted with OTFLOW. Data for both containers fitted with OTLOW indicated that a lowering of the set point might provide a better result in lowering the door side temperature without causing freezing damage at the cooling unit side and still comply with the cold-sterilization requirements. Until further results on the effectiveness for use of the OTFLOW system for plums are obtained from the supplier, for the purpose of cold-sterilization, this technology can’t be recommended as an improved system compared to standard practice.

Data from 2021 trials clearly showed a lot of variation in air temperature and RH between different containers and within individual containers, when comparing cooling unit and door positions. Air temperatures near the door side of containers were higher and RH levels lower than at the cooling unit side. The door positions also revealed more excessive spikes in temperature and RH which could further increase risk to fruit quality. One of the containers tested had much lower RH levels (door:74.81% and unit: 82.30%), compared to the others with similar average RH readings at the door between 86.77% and 87.93% and at the cooing unit between 90.69% and 93.18%. This was similar to data from the 2020 season.

Good information on what happens in commercial containers regarding air and pulp temperatures as well as RH were generated. The importance of loading at the correct temperature and possibly also the optimal position to load plums of different maturities in containers, based on cooling efficiencies and RH deficiencies was highlighted. It was apparent that plums located near the door of shipping containers will be subjected to higher air temperatures and lower RH than those near the cooling unit. It is possible that if optimized, the OTFLOW system may reduce energy usage because of less temperature spikes. Closing of vents needs to be investigated establish effects on relative humidity inside shipping containers.

This research can be used to improve and develop systems to reduce temperature differences between cooling unit and door positions in shipping containers, and to increase the relative humidity. This will be crucial if single-use plastics are restricted in certain markets, which will escalate the need for alternatives to reduce moisture loss and shrivel in fruit.