Application of time-of-flight near-infrared spectroscopy to detect sugar and acid content in Satsuma mandarin

Abstract

Time-of-flight near infrared spectroscopy (TOF-NIRS) was applied to the detection of sugar and acid content in Satuma mandarin (Citrus unshiu Marc). The combined effects on the time resolved profiles of soluble solids, free acids, laser beam wavelength, and detection position of transmitted light were investigated in detail. Attenuance of peak maxima (At) and time delay of peak maxima (Δt) increased as sugar content increased. However, these optical parameters decreased with increases in acid content. In the case of a model sample of a polyurethane block immersed in specified solutions, At and Δt decreased by adding sucrose or citric acid. These results suggested that time resolved profile was governed not only by the concentration of soluble solid or free acid but also by other inherent factors of a fruit; for example, an optical characteristic of tissue or difference in refractive index between the tissue substance and the liquid. Results indicated the optimum optical parameters for detection of sugar and acid content were At and Δt, respectively.