How Does Fruit Ripen?

I've never really known what happens when a fruit ripens, except that ethylene gas is released and somehow triggers the process. And that bananas give off a lot of ethylene as they ripen, leading to the well-known technique of stimulating ripening by enclosing fruit together with some bananas in a paper bag.

That's not a lot to know about it, so I went to the Google machine (actually, I use Ecosia, a very capable search engine that sends 80% of its surplus income to good causes, primarily tree planting). Up popped a clear and non-technical explanation of the ripening process at a website maintained by Professor Ross Koenig of Eastern Connecticut State University.

It's a complicated world out there, of course, and there are many ways that fruits carry out their miraculous transformations. But typically, as explained by Dr. Koenig, an unripe fruit contains acid (hence the sour taste), starch, pectin (basically the glue that holds the cells together, making the fruit hard), chlorophyll, and various large organic molecules. For many fruits, the ripening process begins with the release of ethylene gas, one of the simplest possible hydrocarbons (C₂H₄). This signal activates the genes that produce a host of enzymes that transform the fruit from inedible to delicious.

The acids are broken down, while enzymes called amylases convert starch into sugar, producing sweetness. Other enzymes break down the pectin, softening the fruit. Still others decompose large organic molecules into smaller volatile ones that are released, often producing a characteristic aroma. Chlorophyll is broken down, and revealed or newly produced pigments give the fruit its ripened color.

Damage such as a wound to the skin of a fruit may trigger the process too early, which explains why my lemon tree drops a few early yellow fruit that usually turn out to be no good inside. And that's why one bad apple really can spoil a whole barrel, by releasing ethylene and triggering undesired ripening of the rest.

As unpicked fruit ripens, a similar process occurs in part of its pedicel--- the stem that attaches the fruit to the tree. This allows it to drop off at the right time for an animal to carry it off and disperse the seeds to new locations. If the fruit is good enough, some social primate might even start cultivating it, propagating it, and spreading it to any place in the world it can grow.