Beef quality: ultrasound makes the grade.

Meat inspectors today grade carcasses on the basis of how much intramuscular fat, or marbling, they see. However, any two inspectors may eye things differently, especially when distinguishing between "select" cuts (marbling with just 4 percent fat) and "choice" meats (having 5 percent fat). Such differences have important economic consequences: Retailers and consumers pay a premium for more marbled -- and presumably tastier, tenderer -- cuts of meat.

Now, two researchers at Iowa State University in Ames believe they have a sound means of reducing the subjectivity in grading: acoustical scanning.

Using a hand-held ultrasound pulse generator coupled to a laptop computer, Doyle Wilson and Gene Rouse have scanned the rib-muscle tissue of some 1,200 head of cattle prior to slaughter over the past five years. Then the animal scientists correlated patterns in the resulting black-and-white images with a precise measurement of intramuscular fat that was chemically extracted from a one-quarter-inch-thick rib-eye portion of each carcass. With these data, the researchers developed a computerized image-processing system that can quantify marbling from gray-scale patterns in rib-tissue ultrasound images.

Last year, the Iowa State researchers modified the computer algorithms to evaluate marbling based on ultrasound pictures of about 500 carcasses. When they tested the grading system in an Iowa meat-packing plant recently, they found that 75 percent of the time, human inspectors assigned meat a lower marbling grade than the ultrasound scanning system.

Because of its greater accuracy, "[ultrasound] and other objective instrumentation will eventually replace the current meat-grading system," Wilson predicts. Indeed, later this winter, one Iowa meat packer expects to test such on-line grading of meat. In preliminary tests, "we kept up with a packing line running at 100 carcasses per hour," Wilson says. However, he adds, the device could keep up with double that pace if necessary.

The system, however, may pay more immediate dividends to livestock breeders seeking to develop lean animals with well-marbled muscle. "There's no way you can assess [marbling] by looking at a live animal," Wilson points out. But with ultrasound scanning, he says, breeders can begin reliably identifying those live animals that preferentially lay down desirable amounts of intramuscular fat.