Forget thumping watermelons to check for ripeness. Unripened watermelons-a serious economic threat to farmers and a disappointment for consumers-may be quickly and automatically rejected by a new machine invented by four former University of Delaware engineering students.

The UD watermelon ripeness sensor was developed by Matt Behr of Towson, Md., Dave Bartoski of Camp Hill, Md., Allan Cohen of Wyckoff, N.J., and Jason Firko of Claymont, Del., as part of a senior design class focusing on real industry problems and customers. All are members of the class of '99.

But, the technology isn't simply a student project, says UD faculty member James Glancey, who helps supervise student design teams, with colleague Michael Keefe and class coordinator Dick Wilkins.

"These students have come up with a technology that's absolutely viable," says Glancey, an associate professor of bioresources engineering and mechanical engineering at UD. "It would be very useful to growers."

The computer-controlled ripeness sensor ultimately could result in huge savings for the global watermelon industry, according to Ed Kee, a UD extension specialist, and William J. Watson, executive director of the National Watermelon Association, based in Orlando, Fla.

A prototype version of the UD device cranks out a ripeness reading in just 12 seconds. It's also durable, easy to use, weighs about 18 pounds and costs less than $1,100.

"This is the first generation of a very promising new machine," Kee says. "Down the road, we envision a hand-held microprocessor to replace the laptop computer we're currently using with the device." Such a device could prove essential for farmers, he says, and it might be handy for consumers, too.

Watermelon growers need an automatic ripeness sensor, Kee says, because "it's not at all unusual for a 40,000-pound truckload of watermelons to be rejected at the marketplace." An entire load can be rejected if 10 melons are green, he explains.

How does the UD machine work? Its central feature is a platform where 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 Behr swings the metal arm, the mallet strikes the melon. The microphone picks up the sound and transfers it via 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 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 Kee. (The size of the watermelon influences the frequency of its signal and, therefore, is taken into account during analysis, Behr notes.)

Melon characteristic frequencies have ranged from 100 to 250 hertz, corresponding to the desired sugar content of 8 to 12 percent. These findings were put to the test this summer, as researchers investigated more melons.

Delaware's watermelon harvest for 1997 came to 64.6 million pounds, making it the nation's 12th largest watermelon-growing state, just behind Maryland, U.S. Department of Agriculture statistics show.

Other top watermelon states include California, Florida, Georgia, Texas, Arizona, Indiana, North Carolina, South Carolina, Missouri and Oklahoma. Worldwide, the United States ranks fourth in global watermelon production, behind China, Turkey and Iran.

-Ginger Pinholster