The team's setup to measure speckle patterns involves coherent light, a laser beam, polarizers and quarter-wave plates to generate different incident polarizations, and a CMOS or CCD camera to record the fruits' speckle pattern. Credit: R.Nassif The ability to detect when to harvest "climacteric" fruits—such as apples, bananas, pears and tomatoes—at the precise moment to ensure "peak edibleness" in terms of both taste and texture may soon be within reach for farmers, thanks to the work of a team of researchers from Saint Joseph University in Lebanon and the Université de Bretagne Occidentale de Brest in France. 
 
 As the team reports in a paper published in The Optical Society's (OSA) journal Applied Optics, they recently demonstrated a laser biospeckle technique capable of detecting fruits' climacteric peak. 
 
 What's the significance of this climacteric peak? "Fruits are divided into two categories: climacteric or nonclimacteric fruits," explained Rana Nassif, a postdoctoral researcher affiliated with both Saint Joseph University and the Université de Bretagne Occidentale de Brest. "Climacteric fruits continue their maturation off the tree or vine, so these fruits emit ethylene and are characterized by a climacteric peak—indicating a maximum ethylene release. 
 
 "This peak signals that the fruit has reached its maturity." 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. 
 
 Speckle pattern recorded when illuminating an apple. Credit: R.Nassif 
 
 To do this, the team uses a rather simple setup that involves coherent light, a laser beam, polarizers and quarter-wave plates to generate different incident polarizations, and a digital camera to record the speckle pattern. "Simplicity and low cost are the key advantages of our technique," noted Nassif. 
 
 How does it work? Laser light interacts with any medium through different processes such as scattering, absorption and reflection. Photons scattered by the medium interfere with the incident light field to create a speckle pattern. "A group of sparkling and dark grains called 'speckle grains' make up this pattern. If the medium is biological—meaning that it presents some sort of cell activity—its speckle pattern will show changes with time," she said. "And this pattern depends on the medium's scattering properties, as well as its own nature." 
 
 Once obtained, speckle 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 noninvasive alternative method to respiration rate and ethylene production, which are used today for climacteric peak detection and as a ripening index," said Nassif. 
 
 The team explored the diffusion properties and inner activity aspects of the apples via speckle grain size, recording the temporal correlation between a set of images, consecutively, in different light polarizations. "On one hand, the changes the speckle grain size underwent marked an inflection point corresponding to the climacteric peak for apples stored at room temperature. While on the other hand, the time correlation coefficient behavior demonstrated a rise in the apples' activity until the day of climacteric peak was reached, followed by a decrease that marked the beginning of the deterioration stage." 
 
 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 polarization decreases," Nassif said. In technical terms, she added, "This signature is expected for a medium that constituted a significant portion of small 'scatterers,' in which the Rayleigh diffusion regime outweighs the Mie regime. We also noticed a decreasing speckle grain size, which can be attributed either to increasing glucose or the decreasing of absorption." 
 
 What's next for the team? "In the near term, we're working on speckle theoretical studies based on Monte Carlo simulations—taking into account light polarization and media characteristics," said Nassif. "These simulations can be used to differentiate and quantify the diffusion coefficient variations and its effect from those of absorption on the fruits' speckle image." 
 
 Then, they'll focus on comparing these simulations with experiments done on latex spheres—a mix of spheres of varying diameters—to vary the diffusion and absorption coefficient. 
 
 And, of course, one of the team's ultimate goals is to develop a portable tool to enable farmers to noninvasively 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. 
 
 Explore further: Time is ripe for wine grapes 
 
 More information: "Detection of Golden apples climacteric peak by laser biospeckle measurements," by R. Nassif et al., Applied Optics, Vol. 53, Issue 35, pp. 8276-8282(2014). www.opticsinfobase.org/ao/viewmedia.cfm?uri=ao-53-35-8276&html;=true 
 
 

 References You do not have subscription access to this journal. Citation lists with outbound citation links are available to subscribers only. You may subscribe either as an OSA member, or as an authorized user of your institution. Contact your librarian or system administrator 
 
 or 
 
 Login to access OSA Member Subscription 
 
 Cited By You do not have subscription access to this journal. Cited by links are available to subscribers only. You may subscribe either as an OSA member, or as an authorized user of your institution. Contact your librarian or system administrator 
 
 or 
 
 Login to access OSA Member Subscription 
 
 Figures (9) You do not have subscription access to this journal. Figure files are available to subscribers only. You may subscribe either as an OSA member, or as an authorized user of your institution. Contact your librarian or system administrator 
 
 or 
 
 Login to access OSA Member Subscription 
 
 Tables (1) You do not have subscription access to this journal. Article tables are available to subscribers only. You may subscribe either as an OSA member, or as an authorized user of your institution. Contact your librarian or system administrator 
 
 or 
 
 Login to access OSA Member Subscription 
 
 Equations (2) You do not have subscription access to this journal. Equations are available to subscribers only. You may subscribe either as an OSA member, or as an authorized user of your institution. Contact your librarian or system administrator 
 
 or 
 
 Login to access OSA Member Subscription 

 In the 1989 movie Honey, I Shrunk the Kids, a hobby scientist played by Rick Moranis accidentally blows up an apple with the laser in his shrink ray machine. Now, scientists have found a better use for lasers trained on apples: measuring their ripeness. In a technique known as biospeckle, researchers shined a helium-neon laser on the surface of golden apples and then analyzed how the light was reflected (seen above). The laser wasn’t very powerful—just a couple of milliwatts stronger than the average laser pointer—but it picked up tiny imperfections on the apple’s smooth skin, which caused the laser light to bounce irregularly and result in a grainy appearance at small scales. As the fruits ripened, the “grains” in the pattern became smaller and smaller, the team reports this month in Applied Optics. They attribute the shrinking to a rise in an apple’s cellular activity during the ripening process—the study notes that cellular respiration, for instance, is known to increase as fruits ripen. To confirm that the smallest grains occurred during peak ripeness, the team also monitored the apples’ production of ethylene gas—a known byproduct of the ripening process that peaks just as the fruit reaches optimal edibleness. Once the apples were past their prime, ethylene gas production slowed, apple activity decreased, and the size of the reflected grains began to increase. Because traditional methods of assessing ripeness either destroy the fruit or rely on subjective visual cues, the researchers suggest that the technique could eventually help farmers accurately harvest their crops at optimal times and predict how long a fruit can be stored.
– It can be tough to tell, without biting into an apple, whether it's unripe, just ripe enough, or its tastiest days are behind it. Fortunately, scientists in Lebanon and France are working on a new way to determine ripeness, and instead of using their teeth, they've opted for what they call a "laser biospeckle technique." Researchers shined a laser at the outside of an apple, and it revealed a speckled texture, as shown at Phys.org. The riper the apple got, the smaller the individual speckle grains became, probably because of activity in the apple's cells, the researchers say. After the apple had passed its ideal point of ripeness, those grains started getting bigger again. Researchers checked their work by watching the apple's production of ethylene gas at the same time; the gas is increasingly produced until a fruit is ready to eat. As the grains got bigger again, the gas production got slower, Science reports. Researchers are working on a similar process for pears. The hope is that farmers could eventually use the process to ensure their harvest comes at just the right time, Photonics Online reports. (While you're pondering tasty fruit, you might want to consider the Mediterranean diet, which could reportedly extend life by years.)