Title: Evaluating internal maturity of tomatoes using spatially offset Raman spectroscopy Authors
Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: July 6, 2012
Publication Date: July 29, 2012
Citation: Qin, J., Chao, K., Kim, M.S. 2012. Evaluating internal maturity of tomatoes using spatially offset Raman spectroscopy. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE). Paper NO. 12-1337431. Technical Abstract: Spatially offset Raman spectroscopy technique was investigated for evaluating internal maturity of intact tomatoes. A Raman spectroscopy system was assembled to acquire spatially offset spectra in the wavenumber range of 200–2500 cm–1. A 785-nm laser was used as the excitation source and the measurements were performed in a source-detector distance ranging from 0 to 5 mm with a step size of 0.2 mm. The capability of subsurface detection was evaluated by using a Teflon slab placed under outer pericarp slices of 5-mm and 10-mm thicknesses cut from green and red tomatoes. Self-modeling mixture analysis was used to analyze the offset Raman spectra. The signals from the outer pericarp layer and the Teflon layer were effectively separated and identified. One hundred and sixty tomatoes at seven ripeness stages (i.e., immature green, mature green, breaker, turning, pink, light red, and red) were tested. Three Raman peaks due to carotenoids inside the tomatoes started showing at the mature green stage, in which two peaks appeared consistently at 1001 and 1151 cm–1. Due to the loss of lutein and ß-carotene and the accumulation of lycopene during tomato ripening, the third Raman peak was gradually shifted from 1525 cm–1 (lutein at mature green stage) to 1513 cm–1 (lycopene at red stage). Spectral information divergence was used to quantify the Raman peak changes using pure lycopene as the reference. The divergence values to the lycopene reference decreased as the tomatoes ripened, and they can be used to determine the internal maturity of tomatoes.