Application of commercially available, low-cost, miniaturised NIR spectrometers to the assessment of the sugar content of intact fruit

Recent decreases in costs, and improvements in performance, of silicon arra y detectors open a range of potential applications of relevance to plant ph ysiologists, associated with spectral analysis in the visible and shortwave near infra-red (far-red) spectrum. The performance characteristics of thre e commercially available 'miniature' spectrometers based on silicon array d etectors operating in the 650-1050-nm spectral region (MMS1 from Zeiss, S20 00 from Ocean Optics, and FICS from Oriel, operated with a Larry detector) were compared with respect to the application of non-invasive prediction of sugar content of fruit using near infra-red spectroscopy (NIRS). The FICS- Larry gave the best wavelength resolution; however, the narrow slit and sma ll pixel size of the charge-coupled device detector resulted in a very low sensitivity, and this instrumentation was not considered further. Wavelengt h resolution was poor with the MMS1 relative to the S2000 (e. g. full width at half maximum of the 912 nm Hg peak, 13 and 2 nm for the MMS1 and S2000, respectively), but the large pixel height of the array used in the MMS1 ga ve it sensitivity comparable to the S2000. The signal-to-signal standard er ror ratio of spectra was greater by an order of magnitude with the MMS1, re lative to the S2000, at both near saturation and low light levels. Calibrat ions were developed using reflectance spectra of filter paper soaked in ran ge of concentrations (0-20% w/v) of sucrose, using a modified partial least squares procedure. Calibrations developed with the MMS1 were superior to t hose developed using the S2000 (e. g. coefficient of correlation of 0.90 an d 0.62, and standard error of cross-validation of 1.9 and 5.4%, respectivel y), indicating the importance of high signal to noise ratio over wavelength resolution to calibration accuracy. The design of a bench top assembly usi ng the MMS1 for the non-invasive assessment of mesocarp sugar content of (i ntact) melon fruit is reported in terms of light source and angle between d etector and light source, and optimisation of math treatment (derivative co ndition and smoothing function).