Watermelon ripeness sensor promises savings worldwide.

Forget thumping watermelons to check for ripeness. Green watermelons, a serious economic threat to farmers and a disappointment for consumers, may be quickly rejected by a new sensor developed by four University of Delaware engineering students (Department of Bioresources Engineering, 71 Townsend Hall, Newark, DE 19716).

Worldwide, the industry is believed to annually generate more than 1.5 trillion lb of fruit for sale in 90 countries on five continents. A prototype version of the device cranks out a ripeness reading in just 12 seconds. It's also durable, easy to use, weighs about 18 lb and costs less than $1100. This is the first generation of a very promising new machine. Down the road, scientists envision a hand-held microprocessor to replace the laptop computer currently used with the device.

Such a device could prove essential for farmers, and it might be handy for consumers, too. Watermelon growers need an automatic ripeness sensor because it's not at all unusual for a 40,000-lb truckload of watermelons to be rejected at the marketplace. An entire load can be rejected if 10 melons are green. The watermelon ripeness sensor was developed as part of a senior design class focusing on real industry problems and customers.

The device's central feature is a platform on which the watermelon rests. Sandwiched between the platform and the melon, a piece of foam rubber holds the fruit steady. A mallet attached to a metal arm protrudes from the right side of the machine, while a microphone sits close to the melon, on the left. When the metal arm swings, the mallet strikes the melon. The microphone picks up the sound and transfers it via an electric signal to a laptop computer. The voltage signal is then converted into digital information, which is analyzed.

Because the hollow thunk of a ripe melon echoes, it produces an acoustical signal that shows up as a peak on the computer screen, which then dies down gradually. On melons tested thus far, the frequency of the signal, when normalized using volume, has shown a promising correlation to the actual sugar content of the melon, according to researchers. The size of the watermelon influences the frequency of its signal and therefore is taken into account during analysis.

Frequencies have ranged from 100 hertz to 250 hertz, corresponding to the desired sugar content of 8% to 12%. These findings will be put to the test this summer, as researchers investigate more melons. The work remains preliminary, but results so far are promising.

Further information. James Glancey; phone: [phone]; fax: [phone]; email: [e-mail].