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

Abstract

Abbreviations used: AOTF, acousto-optical tuneable filter; CCD, charge-coupled device; D, distance between the centre of the illuminated area and the detected area of the fruit; FWHM, full width at half maximum; InGaAs, indium gallium arsenide; IR, infra-red; MPLS, modified partial least squares; NIRS, near infra-red spectroscopy; PDA, photodiode array; R, decrease in detector response; R c 2 , regression coefficient of predicted on actual Brix for the calibration set; SEC, standard error of calibration; SECV, standard error of cross validation; SNV, standard normal variance; VR, variance ratio. Abstract. Recent decreases in costs, and improvements in performance, of silicon array detectors open a range of potential applications of relevance to plant physiologists, associated with spectral analysis in the visible and shortwave near infra-red (far-red) spectrum. The performance characteristics of three commercially available 'miniature' spectrometers based on silicon array detectors operating in the 650-1050-nm spectral region (MMS1 from Zeiss, S2000 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 small pixel size of the charge-coupled device detector resulted in a very low sensitivity, and this instrumentation was not considered further. Wavelength 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 gave it sensitivity comparable to the S2000. The signal-to-signal standard error ratio of spectra was greater by an order of magnitude with the MMS1, relative to the S2000, at both near saturation and low light levels. Calibrations were developed using reflectance spectra of filter paper soaked in range of concentrations (0-20% w/v) of sucrose, using a modified partial least squares procedure. Calibrations developed with the MMS1 were superior to those developed using the S2000 (e.g. coefficient of correlation of 0.90 and 0.62, and standard error of cross-validation of 1.9 and 5.4%, respectively), indicating the importance of high signal to noise ratio over wavelength resolution to calibration accuracy. The design of a bench top assembly using the MMS1 for the non-invasive assessment of mesocarp sugar content of (intact) melon fruit is reported in terms of light source and angle between detector and light source, and optimisation of math treatment (derivative condition and smoothing function).

NIR Spectroscopy Applications for Internal and External Quality Analysis of Citrus Fruit—A Review

“…Compared with NIR, IR radiation is strongly absorbed by organic molecules, with the wavelength of absorption characteristic of the molecule. The absorbance in the NIR region of the spectrum is typically 10-100 times weaker than that of the fundamental absorption bands in the IR region of the electromagnetic spectrum (Walsh et al 2000). Weak absorptions in the NIR have been cited as a shortcoming.…”

Near-infrared spectroscopy and imaging in food quality and safety

“…The other line of NIRS instrumentation that holds great promise is the miniaturized NIR spectrometers. Walsh et al compared three commercially available, low-cost, miniaturized NIR spectrometers for the assessment of sugar content in fruit [61]. The three miniaturized NIRS spectrometer models were MMS1 (Carl Zeiss Inc., Thornwood, NY, USA), S2000 (Ocean Optics, Dunedin, FL, USA), and FICS (Oriel Instruments, Stratford, CT, USA) The three models had similar instrument configurations in that they employed diffraction grating as the spectral dispersive element and silicon diode array or charge coupled device (CCD) as detector.…”

“…The FWHM values can be derived by taking the spectra of a known monochromatic light source and calculate the wavelength difference. Walsh et al took a spectrum of a mercury argon lamp and used the discrete emission line at 912.3 nm to calculate the FWHM line width [61]. Wang and Paliwal used a low-cost single-mode diode laser to estimate spectral resolution of their EES and proposed a nonlinear function approximation method to estimate instrument spectral profile [65].…”

“…It can be measured by varying concentrations of test samples to include values at the upper and lower limits [38]. Spectrometric stability can be measured by observing the deviation of spectral response of the instrument within a specific time period [61]. Spectral range and its calibration can be carried out using a mercury argon lamp [46].…”

Assessment of quality defects in macadamia kernels using NIR spectroscopy

“…Typically, the feasibility of using NIRS for a given application is assessed using laboratory grade instrumentation that is unsuitable for industry use with regard to cost and complexity. In this project we have drawn on our prior expertise with modular, low cost instrumentation (Greensill and Walsh 2000b;Walsh et al 2000), comparing their performance with laboratory grade instrumentation.…”