How do raw mangoes and bananas become ripe when treated with chemicals?

Ripening of Fruits

ANSWER I: The process of fruit ripening is chiefly regulated by a gaseous plant hormone called ethylene. Most fruits have elevated ethylene levels during ripening and sometimes just a peak in ethylene levels, just before the process of ripening begins. 

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. These enzymes act to soften the `skin' of the fruit and also convert complex polysaccharides into simple sugars. 

The chemical commonly used to ripen fruits commercially is ethephon (2-chloroethylphosphonic acid), which penetrates into the fruit and decomposes to ethylene. Incidentally, chemicals (e.g. 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. 

Ethylene is induced by several cues such as higher temperature, wounding, disease etc. Higher levels of ethylene and enhanced respiration might contribute to ripening when stored at higher temperatures. 

ANSWER II: The ripening signal of a fruit comes form a hormone ethylene. Production of ethylene turns on some genes that are transcribed and translated to produce other enzymes. These enzymes are responsible for the conversion of starch into simple sugar, degradation of chlorophyll and appearance of other new pigments like carotenoids, change in the skin colour and the breakdown of acid, making the fruit taste neutral. 

Hardy nature of the skin loosens when pectin is broken-down by an enzyme pectinase. Conversion of larger molecules into smaller volatile substances causes an aromatic odour. 

Natural process of fruit ripening is accelerated by using certain chemicals. Here, calcium carbide is used. When carbide is dissolved in water it produces acetylene, an analogue of ethylene, a natural fruit-ripening agent. 

The ripening process is accelerated since acetylene imitates ethylene. 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 phosphorous in carbide makes the healthy fruits poisonous. 

One can distinguish the artificially ripenened fruit by the uniform skin colour in fruits like tomato, mango, papaws, etc and in the case of banana, yellow colour fruit with dark green stem.