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Post-harvest studies of a radiant energy-induced disorder of tomato fruits Adegoroye, Adegoke Samuel


Laboratory experiments and some field and greenhouse studies were carried out to determine the mode of action of radiant energy in the development of a post-harvest sunscald disorder of tomato (Lycopersicon esculentum Mill c.v.Vendor) fruits. The effects of the disorder on post-harvest fruit quality and physiology were evaluated by the physical determination of textural characteristics, the analysis of some chemical constituents and the measurement of fruit gas exchange. The ability of various storage temperatures, photoperiods and oxygen concentrations to alleviate injury was also examined. The effectiveness of radiant energy in causing sunscald injury was found to depend on the amount of radiant energy in the infra-red waveband. The action of infra-red radiation was thermal rather than photochemical. The use of high air temperatures to heat fruits produced symptoms similar to sunscald except that the colour of the fruit surface became greyish rather than white. Injury can develop at all stages of ripeness but symptoms vary with stage of ripeness. Injury also develops in light in the absence of oxygen or in darkness. Photooxidation does not appear to be a prerequisite for injury development. Injurious radiation treatments caused a loss of green colour and injured fruits failed to synthesize the red pigment; lycopene, in storage. Injury did not result in any appreciable loss in lycopene content once the pigment had formed. The mode of action of intense electromagnetic radiation on fruit colour therefore appears to be by destruction of the mechanism of carotenoid synthesis rather than by increased carotenoid degradation. Injury prevented protopectin solubilization during subsequent storage. It also led to a large reduction in ascorbic acid content and an increase in pH. The changes in ascorbic acid content and pH occurred progressively during the development of injury. Changes in the textural attributes of tomato fruits were assessed by measurements of six force-deformation characteristics: Deformation, Pericarp strength, Mesocarp resistance, Compliance, Firmness and Toughness. Toughness did not significantly change during ripeness. Although the effect of injury on Mesocarp resistance was not significant, the interaction of injury with storage was significant for the characteristic. Four characteristics: Pericarp strength, Mesocarp resistance, Compliance and Firmness were highly correlated with protopectin content. Of the textural indices, Compliance was the best predictor (R² = 90.4%; SŶ = 1.97) of protopectin content at different levels of injury, irrespective of ripeness or storage. It is therefore recommended that Compliance, rather than Firmness be used for detecting changes in the "softness" of tomato fruits. "Induction" and "incipient" types of injury accelerated the onset of the respiratory climacteric. "Advanced" type of injury reduced rates of respiration during fruit ripening. The failure of injured fruits to ripen normally was not due to lack of ethylene production since rates of ethylene production were high in injured fruits. The relationship between the respiratory climacteric and ethylene production was shifted by injury. Storage treatments involving different temperatures, photoperiods and oxygen concentrations did not alleviate injury. It is suggested that efforts aimed at controlling injury should emphasize prevention of exposure to radiation rather than attempt to cure injury in exposed fruit.

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