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Fruit ripening is a genetically programmed highly co-ordinated physiological process. The process of fruit ripening is chiefly regulated by a gaseous plant hormone called ethylene. Most fruits have elevated ethylene levels during ripening. Ethylene regulates the expression of several genes involved in fruit ripening so as to modulate the activity of various enzymes involved in the process of ripening. The hydrolytic enzymes bring structural and chemical modifications. Enzymes observed to increase during ripening of fruits include lipase, pectic enzymes, invertase, chlorophyllase and peroxidase. Changes occur during ripening due to the presence of enzymes present in the plant tissue.
The colour changes from green to yellow or orange red in colour. There is breakdown of chlorophyll. Synthesis or exposure of carotenoids and anthocyanin pigments occur.
Softening of the flesh occurs during ripening. Protopectin is converted to pectin and in overripe fruits, pectin is converted to pectic acid. During ripening, hydrolysis of hemicelluloses occurs resulting in softening of the fruit.
There is decrease in acidity, increase in sugar, increase in volatile substances and increase in essential oils. Increase in sugar and decrease in starch causes change in the soluble solids. When harvested, a banana contains about 20 percent starch and only 1 percent sugar. By the time the fruit is ripe, the proportions are reversed. Astringent property decreases. All these changes produce a pleasant flavour.
The optimum temperature and humidity for ripening are about 20oC and 90-95 percent relative humidity. Ripening of fruits can also be done artificially by smoking.
Chemical fruit ripening
Natural process of fruit ripening is accelerated by using certain chemicals. The chemical commonly to ripen fruits commercially is ethephon (2-Chloro ethyl phosphonic acid) which penetrates into the fruit and decomposes to ethylene. Incidentally, chemicals like calcium carbide that produce acetylene, an analogue of ethylene, are also used in some places posing dangers of explosion and carryover of toxic materials to consumers. Higher levels of ethylene and enhanced respiration might contribute to ripening when stored at higher temperatures. Since the amount of carbide needed to ripen the immature fruit is more, it makes the fruit become more tasteless and toxic. Presence of trace amount of arsenic and phosphorus in carbide makes the healthy fruits poisonous. One can distinguish the artificially ripened fruit by the uniform skin colour in fruits like tomato, mango, papaya and in the case of banana, yellow colour fruit with dark green stem.
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