Quick Answer?
As the tomato ripens, its colour starts to change from green to yellow and then eventually to red. This is due to the breakdown of chlorophyll, which in turn synthesises a red carotenoid (another pigment group), lycopene. When ripe, the carotenoid can easily be seen as the dominant colour of the tomato, i.e red
Break it down a little more?
Tomatoes contain two pigments for photosynthesis—chlorophyll, which is green, and lycopene, which is red. When tomatoes start to grow, they contain much less lycopene than chlorophyll, which gives them their green color. But when harvest season arrives, the days shorten and temperatures drop, causing chlorophyll to dissolve and lycopene to take over the shade of the fruit. During this time, sugar levels rise, acid levels drop, and the tomato softens. It becomes ready to eat.
The trick is that this final stage of a tomato’s life takes place in a relatively short period of time—and that poses a big problem for farmers trying to get ripe produce to grocery stores before it rots. Most farmers begin picking tomatoes while they’re still green on the vine, and then they treat them with a ripening agent called ethylene gas to induce the red color. Far from being a synthetic compound, ethylene gas is produced naturally by other fruits and vegetables as they ripen. In fact, bananas release ethylene gas directly into the air. If you place a ripe banana next to a green tomato, the tomato will ripen, too.
Temperature also plays a role in the ripening process. Lycopene will not develop easily in either very cold or very hot temperatures. For tomatoes to ripen properly, the temperature needs to stay approximately 50-85º F.
If you grow tomatoes at home, you will occasionally find a tomato that has fallen off the vine while it’s still green. Likewise, you might buy a tomato at the market that’s still mainly green. Should you throw away these tomatoes?
No! If you have an unripe tomato, simply place it in a paper bag and give it a little time. As long as the tomato has reached the mature green stage, the paper bag will trap ethylene gas as it’s produced by the tomato. The trapped ethylene gas will help to speed the tomato along to full ripeness in a matter of days!
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