- It should not be necessary to emphasize that fruits are still alive after harvest. However, a surprising number of people who make their livelihoods growing, packing, shipping, and selling fruit do not realize that they are handling living, breathing creatures, subject to specific diseases and the ravages of senescence. Moreover, effective postharvest handling is not merely a matter of maintaining the state of fruit quality at the time of picking. Properly handled, many fruits improve in eating quality after harvest. Others degenerate rapidly or slowly, depending on their innate physiology and the postharvest conditions to which they are subjected.
Climacteric Versus Nonclimacteric Fruits
- The first step in proper postharvest handling of a given type of fruit lies in understanding its type of life cycle. The climacteric rise in respiration of fruits such as apple, pear, avocado, mango, and banana represents a rapid depletion of potential postharvest life. For fruits such as pear, banana, and avocado, experiencing the climacteric is essential to the ripening that makes them truly edible. But it should be delayed as much as possible until the consumer is ready to eat that piece of fruit. Very prompt refrigeration is essential for orderly marketing of climacteric-type fruits, to delay or suppress the evolution of endogenous ethylene that initiates the climacteric rise. As the height of the climacteric is reduced, its duration is extended proportionately. Immediate temperature and humidity control is the first line of defense against expensive wastage. Humidity control is important if for no other reason than that a shriveled fruit ceases to be marketable. However, there are other physiological benefits also. Even within a specific variety, response to such storage techniques as controlled atmosphere storage can be sharply influenced by cultural and climatic factors. When the peak of the climacteric rise is past, the fruit becomes senescent. Although adequate reserves of respiratory substrate may be available, cellular organization breaks down, the cell membranes lose their integrity, and the fruit dies of old age. Thus the challenge with climacteric-type fruits is to suppress and extend the respiratory rise. Apples and pears are examples of climacteric-type fruits that have to be harvested within a very brief period but marketed for as long a period as correct storage procedures permit. Long-storing varieties have ample reserves of respiratory substrate and resilient respiratory systems. Under near-optimum conditions, late varieties such as Winesap can be kept year-round. Some, such as Northern Spy and Winter Banana, improve in eating quality during the first few months of storage. The avocado (Persea americana) is an interesting climacteric-type fruit. Although strongly climacteric, the characteristic respiratory rise will not start until the avocado is picked. For many years research workers were convinced that when their instrumentation improved sufficiently, they would be able to identify a preharvest “climacteric inhibitor.” Even with modern equipment, it has been impossible to identify any such inhibitor.
- Most varieties of pears (Pyrus communis) do not ripen to acceptable eating quality on the tree. Once picked, pears have to be either ripened for immediate use (preferably at 20 to 25°C) or held in cold storage at only a degree or two above their freezing point. Pears, particularly the popular Bartlett variety, will neither ripen nor store at intermediate temperatures, particularly in the range 8 to 12°C. Instead, they become rubbery in texture and virtually inedible. This is necessarily an abbreviated and simplified account of the complex physiology of climacterictype fruits. The extraordinary development of nonchemical analytical equipment has stimulated much postharvest research. Some surprising results are being encountered, such as a newly developed thornless blackberry being strongly climacteric. Handling of nonclimacteric fruits is very much simpler. There are no significant physiological changes involved in separation from the tree and no postharvest ripening cycle. With no climacteric rise to suppress, nonclimacteric fruits such as citrus of various types, grapes, and certain vegetables that are botanically fruits do not benefit nearly as much from prompt refrigeration as do climacteric-type fruits. Indeed, for fruits susceptible to chilling injury, delayed storage may be beneficial by enabling the fruit to adapt to lower storage temperatures. Sooner or later, of course, any fruit can be expected to abscise if left on the tree long enough.
- Modern research shows this to be a surprisingly complicated biochemical and histological process. Such abscission is always due to trace amounts of ethylene at the abscission zone. Typically, this is induced by ABA (abscisic acid), the growth regulator produced in response to such environmental stresses as low temperature or drought. Deciduous fruit trees have deciduous fruits that fall when fully mature. Such natural abscission can be delayed with “stop drop” sprays, but at a loss of some postharvest shelf life. Citrus fruits, typical fruits of evergreen trees, have no such programmed abscission, making harvesting much more onerous than for deciduous fruits. [Typically, a Valencia orange must be removed with a pull force of 18 to 22 pounds (8 to 10 kg) as compared to ca. 4 to 5 pounds (1.8 to 2.5 kg) for a McIntosh apple.] Abscission-causing ethylene in citrus fruits can also be triggered by endogenous indole acetic acid (Okuda H, 1999).
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