Abstract
Need for postharvest treatment: Fruit and vegetables are unique among food groups in being living biological systems throughout the postharvest chain from farm to consumer. Since harvested produce are removed from the source of nutrients and moisture supplied by the parent plant, they can only maintain full metabolic activity from existing reserves of metabolites. Over time, reduced availability of key metabolites will result in an imbalance in metabolism which often manifests as loss of cellular integrity. A consequence of this loss of integrity is increased leakage of metabolites from cellular compartments. The mixing of previously separated metabolites and enzymes can enhance the production of compounds that further accelerate cellular disruption. The overall process is known as senescence, which is analogous to the ageing process. In addition, the leakage of nutrients and reduced cellular integrity provide ideal conditions for microbial spores present on the surface of produce to germinate and invade internal tissues. This microbial growth, primarily of fungi but also some bacteria, can rapidly manifest as a rot which renders produce unsaleable. Fruit and vegetables used to be marketed soon after harvest and the senescence process was not a great concern. With growing affluence and greater ease of transportation, the consumer has come to expect produce to be available well beyond the normal harvest period, and to purchase commodities that are grown in distant countries or regions within a country. Many produce are now available 12 months of the year. To meet these expectations the quality of many produce needs to be maintained for long periods. Due to the limited supply of metabolites, extended storage is only possible if the rate of metabolism, and hence the rate of utilisation of reserves, is reduced. A range of postharvest technologies that minimise the rate of metabolism of fruit and vegetables and hence extend storage life have been developed. However, some understanding of factors involved in the underlying physiology and biochemistry is needed to appreciate how the developed technologies are effective. Respiration and ethylene: The respiration rate is a good indicator of metabolic activity of fruit and vegetables and is a useful guide to the potential storage life of produce - a higher respiration rate indicates a lower storage life. Respiration involves the oxidative breakdown of the more complex materials such as starch, sugars and organic acids into simpler molecules such as carbon dioxide and water, with the concurrent production of energy and other molecules which are used by the cell for synthetic reactions. The respiration rate is measured as either oxygen consumed or carbon dioxide evolved. It is axiomatic that any technology that reduces the respiration rate will extend storage life. Ethylene is produced by all plants and is involved in the regulation of all stages of plant growth from seed germination to plant senescence. Postharvest fruit and vegetables are sensitive to very low concentrations of ethylene (nL/L range) which accelerate ripening and enhance senescence. Exogenous ethylene generated by other produce in the same storage container or from the exhaust of internal combustion motors is readily absorbed into produce. Hence, it is the absolute concentration of ethylene in the atmosphere around produce that determines the metabolic impact on it. Climacteric and non-climacteric produce: An important classification of fruit and vegetables is those that exhibit either a climacteric or non-climacteric pattern of physiology and biochemistry. Climacteric produce are fruits that exhibit a marked increase in respiration coincident with the ripening process. The increase in respiration rate reaches a maximum value at some time during ripening and the peak value is called the respiratory climacteric. It is during the climacteric period that all the other changes characteristic of fruit ripening such as colour, taste and texture occur. Climacteric fruits include those that undergo marked changes in colour, texture and taste such as mango, banana, apple, pawpaw and kiwifruit, as well as produce such as tomato which are consumed as a vegetable but are botanically fruits. As might be expected, fruits that do not exhibit a respiratory climacteric are known as non-climacteric produce - this group includes citrus fruits, pineapple and strawberry. They, however, do undergo a ripening phase, albeit more slowly than the climacteric fruit. All vegetables have a non-climacteric type of respiratory pattern. Climacteric and non-climacteric fruits are also differentiated by their response to ethylene. During ripening, climacteric fruits produce much larger amounts of ethylene than non-climacteric fruits. They also show a differing response to applied ethylene. Ethylene applied at a low concentration (e.g. 0.1 μL/L) for one day is sufficient to initiate full ripening of climacteric fruit, whereas applied ethylene only causes a transient increase in the respiration of non-climacteric produce. Moreover, the rise in respiration in response to ethylene may occur more than once in non-climacteric produce in contrast to a single respiration increase in climacteric fruits.
535 The Potential Benefits of 1-Methylcyclopropene (1-MCP) for Regulating the Ripening and Extending the Storage Life of Avocados
