If watermelons could ripen off the vine, watermelon growers around the world would rejoice. Unfortunately, they don't. Once you pick a watermelon, it stops ripening. To understand why watermelons don't continue to ripen like bananas and cantaloupe, you need to understand the difference between climacteric and non-climacteric fruit. It's also helpful to learn how to identify a ripe watermelon.
For some fruit, ripening is caused by ethylene and carbon dioxide (CO2) gases. As fruits ripen, they produce more ethylene and CO2. The more ethylene they produce, the faster they ripen. Heat increases ethylene production, which is why fruit ripens faster when the weather is warmer. These fruits continue to produce ethylene after they is picked because they are still producing ethylene. Fruits that continue to ripen are called climacteric fruit and include apples, apricots, bananas, cantaloupes, kiwis, peaches, pears, nectarines, plums and tomatoes.
Watermelon are considered non-climacteric, which means they retain the quality they had when they were picked. Non-climacteric fruit does not produce large amounts of ethylene or CO2. Scientists don't know much about the ripening of non-climacteric fruit and didn't think ethylene was involved. However, in 2009 they discovered a burst of ethylene production during the white-fruit stage, but lesser amounts as watermelon matured. Other non-climacteric fruit includes cherries, grapefruit, grapes, oranges, pineapple, strawberries and tangerines.
Depending on the variety, a watermelon may ripen anywhere between 65 to 85 days after it is planted. Indications of a ripe watermelon include: the light green, curly tendrils on the stem near the melon turns brown and dries up; the underside of the melon touching the ground turns creamy or buttery yellow; dull surface color; skin is rough to the touch and difficult to penetrate with your fingernail.
Thumping a watermelon with your knuckle is often used to test a melon. The theory is that watermelons emit a dull thud when they are ripe. Unfortunately, not all ripe watermelons emit this thud. A dull thud can also indicate a mushy, over-ripe watermelon. Despite this, in 1999 students at the University of Delaware attempted to create a thumping machine. The device thumped watermelon and then measured the resonance of the sound. The belief was that ripe melons would have a sound frequency between 100 to 200 hertz. For unknown reasons, the machine never made it past the testing stages.