Near-IR Spectroscopy Gauges Tastiness of Plums

Lynn M. Savage

Consumers judge stone fruits such as plums by appearance and texture, seeking sometimes subtle clues about the flavor that awaits beneath the fruit’s flesh. Before you see or feel the fruit at the supermarket, however, growers and shippers must first inspect, classify and sort it for transport.

Currently, plums undergo destructive testing — in small batches — between the orchard and the distribution center. Individual fruits are removed from the production line and subjected to quality tests that determine firmness and soluble solid content. The process takes 15 to 30 minutes and destroys the fruit. Near-IR spectroscopy has been explored as a nondestructive and much speedier analytical method, but widespread adoption of the technique is hampered by the fruit’s high water content, variations in size and shape that affect optical geometry, and wide differences among individual plums, even of the same variety.

Near-IR spectroscopy helped develop calibration data on several varieties of plums. Courtesy of Marìa-Teresa Sánchez, Universidad de Córdoba.

Scientists at Universidad de Córdoba in Spain believe that limitations in the infrared region measured by previous-generation technology are culpable. They sought to determine whether a near-IR diode array instrument with a broad spectral range could provide useful predictions of soluble solid content or firmness in plums that are left completely intact.

They used a spectrophotometer made by Perten Instruments North America Inc. of Springfield, Ill., to scan 720 individual plums of nine varieties at two spots around each fruit’s equator. The instrument captured reflectance spectra across the wavelength range of 400 to 1700 nm at 5-nm steps. The investigators then subjected the samples to traditional destructive tests for comparison, using a refractometer from Atago Co. of Tokyo to acquire data from the juice of each plum and a penetrometer to gauge the firmness of each.

Among the spectral data they found peaks at 570 and 660 nm that were characteristic of the color of the yellow and red plums, respectively; peaks at 950 and 1400 resulting from water content; and bands at about 910 and 1170 that were influenced by sugar content. They noted that the spectra acquired from the plums differed significantly from that recorded by researchers of peaches and nectarines because the flesh of plums is more translucent than that of the other stone fruits.

The scientists also noted that the best results for sugar content prediction were in calibration data sets incorporating the wavelength ranges 515 to 1400 nm and 515 to 1650, but primarily the first range.

For firmness prediction, the best results were in calibration data sets incorporating the wavelength ranges 515 to 1650 nm and 1100 to 1650 nm, but especially the latter.

They believe that the calibration data acquisition and analysis techniques that they employed would be useful for quantifying the sugar content and firmness of intact plums. Ultimately, this should help in the classifying of plums by their sugar content and firmness, allowing increased sampling of each production batch and ensuring a given quality with greater precision and accuracy, further helping distributors evaluate their produce.