Ethylene is a gas that naturally occurs in fruit and vegetables and acts as a plant growth regulator. Over the years, scientists have successfully synthesised ethylene in laboratories, and it is widely used in the fresh-produce industry to regulate the postharvest ripening of climacteric fruit such as bananas, avocados, plums, kiwifruit and pears. In certain non-climacteric fruit, such as pineapples and citrus, ethylene is applied to de-green the fruit skin postharvest.
Useful as it is, too much ethylene has a negative impact on fruit quality, appearance and shelf life. Studies have shown that even at concentrations as low as and below 0.001 μL L-1, ethylene can cause fresh produce to age prematurely, resulting in discoloration, tissue softening and increased susceptibility to decay.
When and how fresh fruit will react to ethylene, however, is not an exact science, as fruit responses vary widely depending on how much ethylene it contains naturally, the type and developmental stage of the fruit, and storage conditions.
As the international trade in fresh produce developed, so did the ways to control the impact of ethylene. Ethylene absorbers or scrubbers, such as zeolite, palladium, activated charcoal and the oxidizers potassium permanganate (KMnO4) and ozone, have proven to be useful in preventing the accumulation of ethylene in storage facilities. Other ethylene management methods include venting by air, high-temperature catalytic oxidation, hypobaric storage, and the application of bio-filtration.
All these techniques, however, have significant drawbacks: they require a long exposure time and have to be replaced regularly as they get saturated over time. The latter is a severe constraint in controlled atmosphere (CA) storage, in addition to being an expensive exercise in waste disposal.
Given the continued growth in SA's fresh-fruit exports, the time is ripe for the introduction of a more effective ethylene control technique. Heeding this call are Dr Oluwafemi J Caleb (previous project leader) and Dr Zinash A Belay from ARC, Infruitech-Nietvoorbij, who led a three-year study into ethylene management.
Project objectives
In research done elsewhere, new techniques based on advanced oxidation processes, notably photocatalytic oxidation (PCO), showed potential for ethylene removal inside shipping containers. Furthermore, PCO could neutralise the effect that storage variables, such as mixed loading, relative humidity and temperature, seem to have on the efficacy of ethylene scrubbers inside storage facilities.
However, the cost-effective implementation of PCO for ethylene removal does depend on the extension of the catalyst lifetime.
Therefore, the first aim of this project was to determine and to optimise the efficacy of ethylene scrubbers used inside containers, and to investigate the effectiveness of PCO techniques for ethylene removal.
The second aim was to investigate and optimise a vacuum ultraviolet (VUV) photolysis process for removing ethylene from fruit packaging and the storage environment, in order to enhance the postharvest life of fruits.
Work plans and results
- Work plan 1 sets out to optimise the performance of a laboratory-scale PCO reactor. To determine its effectiveness in degrading ethylene, samples of apples were stored inside a storage container with the outlet stream connected to a pump and the inlet stream connected to the reactor outlet. The system was operated continuously in a recycle mode for nine days, and ethylene concentration was measured daily both at the outlet and inlet streams.
The results showed that the PCO reactor removed 85.9% of ethylene from the initial concentration of 53.2 ppm. The system kept ethylene levels in the storage unit between 7 ppm and 9 ppm. Further removal of these low ethylene concentrations can potentially be achieved by combining the PCO reactor with the VUV photolysis reactor.
The second study in this work plan investigated ethylene degradation in a mixed-storage setting, using apples, bananas and pears. Divided into three equal mixed batches, the fruit was first placed in storage chambers at room temperature for six days.
The first batch was stored without any ethylene removal material, while the second included 10% (kg/kg of total fruit) potassium permanganate (KMnO4) in a petri dish hanging in the centre of the container to represent the industry practice. The third batch of fruit was stored in a chamber connected to a VUV photolysis reactor for continuous removal of ethylene. All treatments were replicated in triplicate.
The results indicated that the VUV photolysis reactor removed 87% of ethylene and the potassium permanganate 25%, compared to the ethylene present in the control chamber. These results warrant further study to evaluate the commercial potential of VUV photolysis reactors.
- Work plan 2 studied ethylene removal by photolysis. Thirty apples were placed in photolysis reactor units and stored at 15°C. Each reactor was fitted with an inlet and outlet port to quantify gas concentrations, and ozone-producing VUV lamps. Once the storage chambers were hermetically closed, the reactor was switched on after one hour. The accumulation of ethylene inside the chambers was monitored at regular intervals for 21 days.
To study the physiological responses to ethylene, fruit was placed in storage chambers with and without reactor units for 14 days, after which the units were left open until day 21. To determine the respiration and ethylene production rate, the fruit was analysed in terms of colour, total organic acids and sugars, and firmness.
The results of these two studies confirmed findings from previous studies, namely that:
- More ethylene is removed when the initial concentration is higher, suggesting that the reaction rate increases in the presence of higher initial concentrations. It was also found that the reaction order was first order with respect to initial ethylene concentration.
- Ethylene exposure elevates the respiration rate of many fruits.
- Ethylene inside the storage chamber enhances ethylene production in apples.
- VUV radiation maintains a low ethylene production and respiration rate, indicating that this is an efficient technology for prolonging the shelf life of apples.
- There are no significant changes in TSS and TA of treated apples.
- The continuous exposure of apples to direct VUV radiation damages their skins and reduces firmness.
The overall conclusion drawn from these studies was that VUV photolysis is a promising technique for ethylene removal in the storage environment, and could offer a better solution to maintaining the postharvest quality of fruit. However, further study is required.
Conclusions and the way forward
The ethylene degrading or oxidising reactor units developed in this study have proven their ability to oxidise ethylene during storage. Not only did the units keep ethylene concentrations at very low levels, they also outperformed potassium permanganate in terms of efficacy in a laboratory setting. However, much more work is needed before the technology will be ready for commercial use.
Commercially, the project offers the opportunity to develop a detailed protocol that will ensure the optimal use of ethylene scrubbers, resulting in significant cost savings and preventing breaks in the cold chain.
The development of new ethylene management systems for the fresh fruit industry could add tremendous value throughout the export value chain.
BOX 1
The project's economic impact
The management of ethylene when implemented correctly in packhouse cold storage facilities, will significantly reduce postharvest losses and maintain quality. The estimated economic value of controlling ethylene across different commodities in the South African pome industry would be more than R200 million per year. Currently, we are working on the economic analysis of VUV lamps, using different sources of energy to establish a proven background about its economic value.
BOX 2
- Project title: Integrated post-harvest ethylene management along the value chain
- Principal investigator: Zinash Belay
- Duration: 01/01/2019 – 31/12/2021
- PHI Programme and Industry Contributions: R218 662.50 and R218 662.50
- Lead institutions: ARC, Infruitec-Nietvoorbij and Post-Harvest and Agro-Processing Technologies
- Beneficiary: The South African pome industry
- Human resource development: 1 internship, 1 MSc, 2 PhD students
- Focus area: Ethylene management and post-harvest quality
