Festive gift trays of sweet, sun-ripened fruits often include delicious dried plums - also known as prunes. Agricultural Research Service (ARS) scientists in California have invented an inexpensive approach that dried plum processors can use to help ensure no large pieces of a plum's pit remain inside the fruit.

Agricultural engineers Eric Jackson, Ronald Haff and Thomas Pearson developed and tested the technology for about 1-1/2 years before deciding it was ready for processors to try. The researchers put thousands of tender, moist dried plums to the test in their laboratory experiments at the agency's Western Regional Research Center in Albany, Calif. Co-inventor Pearson is now with the ARS in Manhattan, Kan.

The pit detector could be used as an inexpensive addition to processing lines already equipped with other detectors to find hidden pits or pit pieces.

Dried plums processed with the new device would be moved on a conveyor belt to a roller that gently presses them against the belt. A device known as a force transducer, mounted underneath the conveyor belt and in line with the roller, detects the amount of resistance that the roller encounters. The transducer's reading is sent to a signal processor that is linked to a computer.

Using a mathematical formula or algorithm that the scientists wrote, the computer determines whether the transducer's reading likely indicates the presence of a pit or pit piece. If that is the case, the signal processor instructs a sorter to remove the prune from the processing line, so it can be retested, hand-sorted, or simply rejected.

The accuracy rate is impressive: false positives occur less than one percent of the time.

Though so far tested primarily at laboratory speeds, the device could easily be ramped up to processing plant rates. And, it could likely be used to check other dried stone fruits such as apricots, cherries and peaches.

The scientists received a patent for their invention earlier this year. Some dried fruit processors have already shown interest in it.