| Summary: | 碩士 === 國立中興大學 === 園藝學系所 === 98 === The maturity of papaya fruit when harvested affects postharvest quality and storage capacity. For example, if the papaya fruit softens quickly, there is serious loss of product during transportation. On the other hand, when ‘Tainung No.2’ papaya fruit pulp does not soften, it known as ‘rubber papaya’. This research will discuss the changes in papaya fruit as it ripens; suggest an index of ripening stage and what factors affect softening; examine the effect of temperature on papaya fruit ripening and if high temperature causes rubber papaya.
‘Tainung No.2’ papaya fruit respiration rate is similar to its internal carbon dioxide concentration, while ethylene production rate is similar to its internal ethylene concentrations. Therefore, by measuring the atmospHeric composition of fruit cavity, the pHysiological state of fruit can be known. In general, the best export maturity (25% of skin turning color) had a respiration rate of 20.69 ml CO2 kg-1 hr-1; ethylene produce rate of 0.036μl C2H4 kg-1 hr-1; internal carbon dioxide concentration of 2.17 %; and an internal ethylene concentration of 0.032ppm. When fruit ripened to the stage of edibility, it had a internal respiration rate of up to 47.6-50.7 ml CO2 kg-1 hr-1; internal ethylene concentration of 5.18μl C2H4 kg-1 hr-1; internal carbon dioxide concentration of 4.6-5.7%; and an internal ethylene concentration of 6.2-9.9ppm. During fruit ripening, fruit skin changed from green to yellow in a hue angle expressed as 120.6° to 65.9°. ChloropHyll fluorescence can also be used to support the determination of the ripening state of fruit. When the ‘Tainung No.2’ papaya fruit ripened and softened, respiration rate and ethylene production rate increased, an indication of the ripening process. Weight, starch, insoluble pectin content, and total calcium content were significantly decreased, while soluble sugar and soluble pectin content increased, all of which are chemical reactions carried out in fruits ripening. Water loss may be the reason for the papaya softening, because the decline in cellular turgor, affects fruit firmness. Two days after ripening, the firmness decreased but then there was no other significant changes. However, the weight loss rate continued to decrease, so turgor was more related to early stage softening. The decrease in starch content also caused firmness to decrease, but the starch content of papaya fruit is very low, thus the effect of starch may be relatively small. Firmness changed at the same rate at which pectin dissolved. Additionally, an increase in the ratio of water-soluble calcium resulted in a loosening in the pectin structure. Increased β-glucosidase activity caused the degradation of cell wall polysaccharides. The ripening and softening of papaya fruit may be affected by the complementary reaction of ethylene response and degradation of cell wall material.
High temperature treatment caused ‘Tainung No.2’ papaya fruit ethylene production rate to decrease, mainly because of ACC oxidase’s sensitivity to high temperatures, which resulted in decrease in ACC oxidase activity. Heat treatment also caused poor color change and inhibited the softening process, and accumulated thermal damage resulted in hard lumps. For example, even after 6 days at 40℃ and 3 days ripening with calcium carbide at 30℃, some pulp still had not softened. High temperatures caused inhibition of respiration and ethylene biosynthesis which blocked fruit ripening. This further resulted in peel and pulp color changing poorly; the inhibition of β-galactosidase and β-glucosidase activity; and the slowdown of the pectin dissolution rate. In conclusion, 40℃ temperature may induce the occurrence of papaya rubber pulp because of the decrease in the ethylene rate and the slow dissolution of pectin.
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