Laser biospeckle technique to identify the ripeness of fruit
Researchers have developed a non-invasive way to identify the ripeness of fruit. As described in the journal Applied Optics, the team from Saint Joseph University in Lebanon and the University of Western Brittany in France used a laser biospeckle technique to detect the ideal time to harvest apples.
In the future, the research could lead to a handheld device which would help farmers to maintain high quality and minimise disease among climacteric fruits.
Climacteric fruits mature off the tree or vine, and release ethylene as they continue their maturation. The ‘climacteric peak’ is reached when the fruit has given off the maximum amount of ethylene. ‘This peak signals that the fruit has reached its maturity,’ explained Rana Nassif, a postdoctoral researcher affiliated with both of the universities involved in the project. After this point, the fruit is more susceptible to fungal invasion or begins to degrade from cell death.
By tapping biospeckle activity, generated by illuminating a biological medium with coherent light, the researchers studied the evolution of two batches of Golden apples' speckle patterns as they underwent the ripening process in both low- and room-temperature environments.
To do this, the team used a setup that involved coherent light, a laser beam, polarisers and quarter-wave plates to generate different incident polarisations, and a digital camera to record the speckle pattern.
During the process, the laser light interacts with any medium through processes such as scattering, absorption and reflection. Photons scattered by the medium interfere with the incident light field to create a speckle pattern. These patterns can be correlated with a reference standard, which is based on emitted ethylene concentrations obtained by a principal component analysis.
‘This approach allows us to validate biospeckle as a non-invasive alternative method to respiration rate and ethylene production, which are used today for climacteric peak detection and as a ripening index,’ said Nassif.
Beyond apples, Nassif and colleagues are also monitoring the ripening of Conference pears, performing backscattered speckle images on the fruits during the ripening phase. They supplemented these images with fluorescence and biochemical measurements. ‘By doing this, we were able to show that as the glucose content increases, the circular degree of polarisation decreases,’ Nassif said.
In the future, the team hopes that they will be able to develop a portable tool to enable farmers to non-invasively assess their fruits' maturity in orchards or fields to detect the optimal time to harvest their crops. ‘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,’ noted Nassif.