Worms that make their homes in apples may be facing a new enemy. Physicists have developed a spectroscopy-based instrument to characterize the inside of the fruit, identifying apples with worm tunnels, for example, for easy sorting of low-quality fruit from large batches. Researchers at the Politecnico di Milano's Istituto Nazionale di Fisica della Materia developed the instrument as part of the European Community Diffruit Project. The team -- headed by Rinaldo Cubeddu, a physics professor at the college -- has exploited optics expertise in an unconventional way, relying on the analysis of photon migration in turbid media. Firing a very short laser pulse into an apple, the researchers collect relevant information about the absorption and scattering properties of the fruit's pulp from the attenuation and the temporal distortion of the beam. This time-resolved reflectance spectroscopy technique is nondestructive and is mostly unaffected by variables in an apple's skin. The absorption coefficient provides information on tissue constituents such as chlorophyll and sugars, and the scattering coefficient details texture, firmness and fiber content. Part of the goal of the Diffruit Project team -- which also includes researchers from the Universidad Politécnica de Madrid, Horticulture Research International in the UK and Vela Srl in Italy -- is to minimize the cost of technologies to provide quality control for common products. Other approaches requiring more complex lasers would have been too costly, Cubeddu said. Time-resolved reflectance spectroscopy can be done with low-cost laser sources such as pulsed laser diodes. A pulse duration of anywhere from 70 to 200 ps is sufficient to carry out the measurement, which takes just 100 ms to complete. The Diffruit Project team predicts that a portable instrument based on this approach would cost about $25,000. Although this cost limits potential users to large supermarkets that are testing significant amounts of fruit, the team plans to develop simplified and less-expensive units for smaller grocery stores.