Ripening Fruit

    Kids, is a tomato a vegetable or a fruit?  Tomatoes are a fruit and, in fact, they are more like berries than any other fruit. Like all berries, they are wonderful when in season but mediocre when not.  The problems with tomatoes are that their season is very short and that they don't like to travel.  The same travel part can be said for bananas.  And avocados, and more.  Tomatoes and many other fruits would never make the trip to the market when ripe.  After all, their job is to rot and deliver seeds.

    Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

    Most commercial tomatoes are picked at the “breaker” stage when they have reached full size but have only a hint of red/tan/pink visible.  So how is it that they're all an appealing red color by the time they all get to the market?  After washing, sorting, sizing and packing, tomatoes are moved to an airtight room where they are exposed to a "ripening" agent.  This agent is ethylene (C2H4) gas.  Ethylene is a hydrocarbon that occurs naturally in many fruits and vegetables.  As some fruits and vegetables mature, they produce their own ethylene, which continues the ripening process.  Without ethylene, some items, such as bananas, would never ripen. Bananas are picked before they are mature enough to produce their own ethylene.  After their journey from Central or South America up to here, they are placed in special rooms, which are then filled with ethylene to trigger the ripening process.  The bananas are then sent to supermarkets, where they continue to ripen themselves by producing their own ethylene gas, going from the unripe green stages to the ready-to-eat yellow stage.

    You can use a similar process to help ripen some fruit at home.  For those that will ripen after harvest (only some, like avocados, can ripen after harvest; others, like pineapples, cannot), try this:  Place a green tomato or hard avocado in a paper bag.  Add a yellow banana or an apple and close.  The banana or apple will give off their own ethylene gas and therefore help ripen the unripe fruit.  Put similar unripe items into a bowl alone for comparison.  These will also eventually ripen because fruits produce their own natural ethylene, but it will take longer.  Wait several days, observing your fruit each day during the process.  Occasionally we observe the detrimental effects of ethylene.  For example, it can wilt flowers!  Bananas and other ethylene-producers should therefore be kept away from fresh-cut flowers.  Of course, you could intentionally try this in another experiment.

    Ethylene producing fruits include apples, apricots, avocados, bananas, cantaloupe, figs, guava, honeydew, kiwi, mango, nectarines, papaya, passion fruit, peaches, pears, plantains, plums, and tomatoes.  The experts say to keep supermarket tomatoes out of direct sunlight, and to never put them in the refrigerator.  Below 50°F a volatile flavor compound called hexenal flips itself off like a chemical switch — permanently.


    Kathleen Carrado Gregar, PhD, Argonne National Labs 
    [email protected]
    June 2004