Nondestructive evaluation of internal maturity of tomatoes using spatially offset Raman spectroscopy

Authors: QIN, JIANWEI - University Of Maryland; Chao, Kuanglin - Kevin Chao; Kim, Moon

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2012
Publication Date: 6/15/2012
Citation: Qin, J., Chao, K., Kim, M.S. 2012. Nondestructive evaluation of internal maturity of tomatoes using spatially offset Raman spectroscopy. Postharvest Biology and Technology. 71:21-31.

Interpretive Summary: Maturity evaluation is important for tomato production because inappropriate or nonuniform maturity within a batch of fruit can reduce fruit quality, and increase food safety risks and postharvest losses during storage and processing. The tomato ripening process is associated with the accumulation of carotenoids in the fruit, including lutein, ß-carotene, and lycopene. Carotenoids are first generated within the interior of tomato fruit; exterior color changes generally lag behind the internal composition changes. Therefore, the exterior surface appearance is an unreliable indicator for the actual degree of fruit maturity. This study investigated the potential of using a spatially offset Raman spectroscopy (SORS) technique for nondestructive evaluation of the internal maturity of tomatoes. SORS spectra were measured for slices of the outer part of the tomato cut from green and red tomatoes. Analysis of SORS spectra of the tomatoes detected the internal development of carotenoids in tissue based on the two carotenoid peaks associated with decreasing lutein and increasing lycopene content. Analysis of these spectra by a spectral information divergence method effectively differentiated tomatoes in the immature green, mature green, and breaker stages. This nondestructive evaluation method will enable processors to more accurately determine fruit maturity that can help reduce postharvest safety and quality problems related to spoilage and inconsistent ripening of fruit and thus reduce costs for both processors and consumers.

Technical Abstract: This research explored the use of spatially offset Raman spectroscopy (SORS) for nondestructive evaluation of internal maturity of tomatoes. A Raman spectroscopy system using a 785 nm laser was developed to collect spatially-offset spectra in the wavenumber range of 200 – 2500. The SORS measurements were conducted using an offset source-detector distance ranging from 0 to 5 mm with a step size of 0.2 mm. One hundred and sixty tomatoes were tested, including fruits at seven ripeness stages (i.e., immature green, mature green, breaker, turning, pink, light red, and red). The feasibility of the SORS technique for subsurface detection was examined by using a Teflon slab placed under 5-mm and 10-mm thick slices of outer pericarp that were cut from green and red tomatoes. Self-modeling mixture analysis of the offset spectra following fluorescence correction effectively separated the Raman signals from the outer pericarp layer and the Teflon layer. Three Raman peaks due to carotenoids inside the tomatoes first appeared in fruits at the mature green stage. Two peaks appeared consistently at the 1001 and 1151 wavenumbers, and the third peak gradually shifted from the 1525 wavenumber (lutein at mature green stage) to the 1513 wavenumber (lycopene at red stage) owing to the loss of lutein and ß-carotene and the accumulation of lycopene during tomato ripening. The Raman peak changes were evaluated by spectral information divergence (SID) with the spectrum of pure lycopene as the reference. The SID values decreased as the tomatoes ripened, and thus these values can