Kitchen chemistry: Ethylene and ripening fruit

My previous kitchen chemistry post discussed the combustion of propane. Propane is a hydrocarbon – propane molecules are made up only out of carbon and hydrogen atoms.

Another hydrocarbon gas that can be found in the kitchen is ethylene (ethene). Ethylene molecules consist of two carbon atoms and four hydrogen atoms (with a double bond between the carbon atoms, so that each carbon atom still has four connections). Here are two views of an ethylene molecule:

               

Like propane, ethylene burns (C₂H₄ + 3 O₂ → 2 CO₂ + 2 H₂O). Ethylene molecules can also be combined together to give the plastic polyethylene (polythene).

Perhaps the most important function of ethylene in the kitchen, however, is that it is produced by ripening fruit (as was discovered in the 1930s). What’s more, ethylene gas causes other fruit (particularly apples, pears, and bananas) to become first ripe, and then over-ripe. As the saying goes, “one bad apple spoils the whole bunch,” because the ethylene gas produced by one ripe, over-ripe, or damaged apple affects all the others in the barrel (see this home experiment on the subject). Vegetables are affected as well. To stop this from happening, packets of ethylene-absorbing material can be placed in the refrigerator.