Understanding kiwifruit postharvest physiology and quality changes in tropical retail market conditions : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, New Zealand
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Date
2022
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Massey University
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Abstract
Kiwifruit is New Zealand’s major horticulture crop. A challenge that exists for the New Zealand kiwifruit industry in emerging tropical markets is maintaining fruit quality during marketing. In Asia and the Middle East, high ambient temperatures (> 30 °C) are often prevalent. The domination of traditional fruit marketing practices in these regions results in fruit exposure to these conditions. Understanding of fruit postharvest responses to high ambient temperatures is limited.
In this thesis, a supply chain survey was conducted in India and Singapore, where kiwifruit at edible ripeness were found to be exposed to temperatures > 30 °C and ethylene concentrations as high as 150 nL L-1 during retail. Back in the laboratory, kiwifruit were exposed to simulated tropical conditions, where 12 h of exposure to 40 °C resulted in significant changes in fruit physiology as evidenced by rapid respiratory decline. The findings from this initial study indicated that 40 °C exposure may have a lethal impact on kiwifruit whilst 33 °C may not.
The role of ethylene in influencing kiwifruit physiology under tropical conditions was investigated by treating kiwifruit with 1-MCP to prevent ethylene responses. Consequently, 1-MCP treated fruit exposed to 33 °C retained firmness both in the presence or absence of exogenous ethylene whilst untreated fruit softened rapidly, indicating that the possible ethylene exposure in tropical conditions advances kiwifruit softening.
A further study aimed to identify exposure temperature and time combinations that trigger detrimental outcomes for kiwifruit including after a period returned to coolstorage. Like previous, kiwifruit at 40 °C exhibited rapid respiratory decline, while at 33-38 °C, no such response was evident. This confirmed that 40 °C exposure for 12 h could have a lethal impact on kiwifruit physiology. Fruit that were exposed to high temperatures (33-40 °C) beyond 24 h and later coolstored developed internal breakdown symptoms indicating that irrecoverable degradative processes are induced.
A plausible mechanism as a result of heat exposure is the occurrence of anaerobic respiratory metabolism. At 33-40 °C, Respiratory Quotient (RQ) in kiwifruit remained ≥ 1 reaching 3 at 40 °C whilst at 20 °C, RQ remained close to 1. Contrastingly, ethanol accumulation increased at high temperatures but minimal ethanol content changes occurred at 20 °C. At some high temperatures, ethanol content increased with time but no symptoms of heat injury were evident, suggesting that heat injury may not be exclusively caused by the accumulation of anaerobic metabolites. The increase in ethanol content measured in kiwifruit at high temperatures indicates the potential risk of off-flavour development at retail conditions in tropical markets.
Overall, this research contributes to kiwifruit quality maintenance in tropical markets by elucidating safe exposure durations for kiwifruit and identifying a postharvest treatment of 1-MCP as a potential quality maintenance tool. Future research requirements include identifying molecular mechanisms that control physiological changes in kiwifruit at high temperatures, identifying causes for the differential responses observed between ‘Hayward’ and ‘SunGold™’ and investigating the efficacy of 1-MCP at wider ranges of high temperature conditions.
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Figures 2.3 (=Atkinson et al, 2011) and 8.2 (=Tongonya et al, 2021) have been removed for copyright reasons. Two published papers in the Appendix were also removed for copyright reasons.
Keywords
Kiwifruit, Postharvest physiology, Tropics, Effect of temperature on, Ethylene, Physiological effect