Biospeckle Laser Method IDs Fruits’ Peak Ripeness

There is a relatively small window of “peak edibleness” for fruits such as apples and tomatoes. Imagine being able to fit in that window at precisely the right time for an optimal harvest. That is now possible with a new laser technique.

A team from Saint Joseph University in Lebanon and the University of Western Brittany in France has developed the laser biospeckle technique, allowing them to noninvasively detect the peak of ripeness of climacteric fruits — apples, bananas, pears, melons and tomatoes, among others.

A laser beam, polarizers and quarter-wave plates produce a speckle pattern on an apple. Images courtesy of Rana Nassif.

In a study, the researchers used a laser beam, polarizers and quarter-wave plates to generate different incident polarizations. Also, they used CMOS and CCD cameras to record the speckle pattern created when photons scattered by a medium interfere with the incident light field.

“A group of sparkling and dark grains called ‘speckle grains’ make up this pattern. If the medium is biological … its speckle pattern will show changes with time,” said Rana Nassif, a postdoctoral researcher affiliated with both universities. “And this pattern depends on the medium’s scattering properties, as well as its own nature.”

The team monitored two batches of harvested Golden apples going through the ripening phase in low- and room-temperature environments to “determine speckle parameters and measure the emitted ethylene concentration using gas chromatography as a reference method,” they wrote in the study.

The researchers explored the diffusion properties and inner activity aspects of the apples via speckle grain size, consecutively recording the temporal correlation between a set of images in different light polarizations. They found that speckle-grain-size changes corresponded to the ripeness peak for apples stored at room temperature.

Digital cameras were used to record the speckle pattern that resulted from illuminating the fruit.

The researchers have supplemented their images with fluorescence and biochemical measurements, demonstrating that as glucose content increases, the degree of circular polarization decreases.

“We also noticed a decreasing speckle grain size, which can be attributed either to increasing glucose or the decreasing of absorption,” Nassif said.

The researchers are working to turn their technique into a portable tool for farmers that would noninvasively assess their fruit’s maturity in orchards and fields, allowing them to detect optimal harvest times.

“This is of great interest to fruit farmers, especially since most tests used today are either destructive or based on visual criteria that are often wrong,” Nassif said.

The research was published in Applied Optics (doi: 10.1364/AO.53.008276).

For more information, visit www.usj.edu.lb.