If you have ever bought the "perfect" piece of fruit from the grocery store only to have it rot the next day, you know how tough it can be to find fruit that is ripe enough to eat -- but not overripe. Research being conducted at the University of Coimbra in Portugal may someday help food producers determine when fruit is ready for the table. Alexander A. Kharlamov has discovered that the gases produced by ripening fruit luminesce when exposed to different wavelengths of light.Researchers are characterizing the ripeness of fruit, such as this Granny Smith apple, by monitoring their luminescence spectra. Here, a HeNe laser serves as the excitation source. Courtesy of Alexander Kharlamov.Since discovering the luminescence in 1997, Kharlamov and colleague Hugh D. Burrows have been working to characterize the luminescence spectra with the intention of using the data to show how ripe a piece of fruit is. They have distinguished ripe, unripe and overripe apples, and have begun to study the spectra of other living systems.The researchers measured the luminescence spectra with either a Renishaw plc RM2000 Raman imaging microscope along with a Spectra-Physics Inc. HeNe laser serving as the excitation source or with a Spectra-Physics argon-ion laser for illumination and a Spex monochromator for spectral measurements. They also made measurements under various temperatures and atmospheric conditions using Spex and Jobin Yvon/Spex spectrofluorimeters. They found that the fruit generated spectral peaks at 330 to 460 nm and at 680 to 685 nm.Now the researchers will turn their attention to developing ways to use their discovery, such as for sorting fruit in the food industry. A potential means of determining fruit maturity is proton-transfer-reaction mass spectrometry, but Kharlamov said this technique is complicated and expensive.Biomedical and commercialThere may be other applications for the technique. "Initially, this is a new method to control the quality of fruit with an applelike skin -- apples, pears, et cetera," Kharlamov said. But aroma luminescence also may find biomedical applications, such as evaluating health by monitoring the gas emissions of the human body.He added that, for fruit inspection, it might be possible to develop a simple pocket-size device for quality control testing or for shoppers as they make their way through the produce section. He said he is interested in working with industrial partners because money for developing new instruments is difficult to come by in the Portuguese university system.Now if they could only figure out how to tell if a cantaloupe is ripe.