Posted 4-3-00

The answer, in a word, is "ethylene." Ethylene is a natural hormone, or growth-promoting substance, that tomatoes and certain other fruits naturally produce. You can't smell or see ethylene, except for what it does to tomatoes. Like tipping the first domino in a long chain of dominoes, ethylene sets in motion natural ripening processes that cause firm, green tomatoes to become soft, red, flavorful and juicy.

Packing tomatoes in a box, or wrapping them up, concentrates ethylene, which helps speed ripening processes like sugar production. Concentrating ethylene can also cause tomatoes to hit their peak too soon, so that they start to rot or spoil. That's why tomato growers pick the tomatoes while they're still firm and green. This gives the growers enough time to ship their harvest to the grocery store, for example. So, by the time consumers come along with their grocery cart or hand basket, the tomatoes are ready to be eaten.

According to vegetable scientist Ken Gross (shown here) ...tomato fruit also "breathe" in oxygen and give off carbon dioxide (the opposite is true of the leaves). By wrapping tomatoes in plastic or newspaper, you also control how much oxygen gets to them, says Gross. This, in turn, controls the rate at which carbon dioxide is released and escapes outside the wrapping. Depending on which occurs, then, ripening can happen faster, or slower. High temperature also can speedup ripening. Though ethylene is a key factor, "these other factors are definitely involved in ripening," Gross says.

-- Sci4Kids Staff, and Ken Gross, research leader, USDA-ARS Vegetable Lab, Beltsville, Md.

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