Submitted to: Proceedings of SPIE
Publication Type: Proceedings
Publication Acceptance Date: 4/29/2011
Publication Date: 8/16/2011
Citation: Qin, J., Chao, K., Kim, M.S. 2011. Evaluating carotenoid changes in tomatoes during postharvest ripening using Raman chemical imaging. Proceedings of SPIE. Interpretive Summary: Commercial tomato operations typically harvest green mature tomatoes that are subsequently stored and undergo postharvest ripening prior to sale. Lycopene content is a good maturity indicator as the fruit progresses through the green, breaker, turning, pink, light red, and red, stages, but currently is difficult to evaluate without cutting the fruit open. Raman scattering signals are specific and sensitive to chemicals, and Raman chemical imaging technique can visualize the quantity and spatial distribution of interesting constituents within complex food matrices. In this study, a laboratory-based Raman chemical imaging system was utilized to acquire hyperspectral macro-sale Raman images from cross-sections of tomatoes. With the use of a reference spectrum acquired for a sample of pure lycopene, a spectral information divergence based image classification method was shown to be useful for evaluating lycopene content in the tomato samples at different maturity stages, providing maps of the lycopene patterns that develop within the fruit during the postharvest ripening process. This preliminary work with cut tomato samples, provides a framework for developing non-destructive Raman evaluation of intact tomatoes that measure lycopene content. Development of a non-destr4uctive method to evaluate lycopene content and thereby determine fruit maturity, will allow processors to avoid the safety and quality problems associated with the mixing of fruits at different maturity stages during postharbest ripening procedures, since spoilage and inconsistent ripening within a batch of fruit can cause by safety concerns and economic loss.
Technical Abstract: During the postharvest ripening of tomato fruits, the increasing presence of lycopene in the tomatoe samples spanning a range of fruit maturity. In this study, Raman chemical images were acquired of tomato samples spanning a range of fruit maturity stages, and were evaluated for the presence and distribution of lycopene in the tomato samples. The tomato maturity stages included green, breaker, turning, pink, light red, and red, and the fruits were cut equatorially to expose a full cross-section for imaging. The system consists of a 785 nm laser, a fiber optic probe, a dispersive imaging spectrometer, a spectroscopic CCD camera, and two-axis positioning table. Hyperspectral Raman images were acquired in the range of 200 to 2500 cm-1 with a spacial resolution of 1 mm. A polynomial curve-fitting method corrected the images for the fluorescence background present in the Raman spectra of the tomatoes. The Raman spectrum of a sample of pure lycopene was acquired for use as reference in developing a hyperspectral image classification method based on spectral information divergence to identify the lycopene present in the tomatoes. The quantity and spatial distribution of lycopene in the tomatoe samples was visualized from the Raman chemical images. The spatial lycopene patterns mapped accross the fruit images revealed the mechanism of lycopene generqation during postharvest ripening. The methods and findings of this stydy form a basis for the future development of a Raman-based nondestructive approach for monitoring internal maturity of the tomatoes.