Abstract
A handheld Raman spectrometer was used to determine the ripeness of peppers. Raman spectra were recorded non-invasively on the fruit surface. The spectroscopic data were transformed into a classification scheme referred to as the maturity index which allowed for attribution of the fruit stadium to four levels from immature to fully mature. Hot pepper and tomato ripening includes pectic polysaccharide depolymerization, chlorophyll degradation and carotenoid formation, among others. The latter were followed non-invasively by Raman spectroscopy. Two portable systems and one benchtop system were compared for their applicability and robustness to establish a suitable maturity index. Spectral acquisition, data treatment and multivariate data analysis were automated using a Matlab script on a laptop computer. The automated workflow provided a graphic visualization of the relevant parameters and results on-site in real time. In terms of reliability and applicability, the chemometric model to determine the maturity of fruits was compared to a univariate procedure based on the average intensity and ratio of three characteristic signals. Portable Raman spectrometers in combination with the maturity index or a chemometric model should be suitable to assess the stage of maturing for carotenoid-containing fruits and thus to determine ripeness on-site or during a sorting process in an automated manner.
Highlights
The narrow, focused laser beam followed a circular motion over a larger sample area to yield a better resolution and intensity since an alternating color was observed over the fruit surface due to the maturing process, which the raster orbital scanning (ROS) technology took into account better than the sampling technique of the other Raman spectrometers
The non-invasive determination of the maturity index enabled the assessment of the maturation state of hot peppers
Using a handheld Raman spectrometer at 785 nm laser wavelength, it was possible to investigate the maturation of the fruits
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. On a cell-size scale, the depolymerization and size of pectic and hemicellulose polysaccharides were associated with fruit maturation, of tomatoes and hot peppers [6,7,8]. The determination of these parameters except fruit size and base color can only be achieved by destruction of the fruit, such as homogenization, extraction, staining or labeling, and at the expense of the producer. Raman carotenoid signals typical for hot peppers are described and the signal evolution during maturing is followed. A multivariate chemometric model and a straightforward univariate model are proposed and compared, allowing a four-stage classification of the ripeness of hot pepper fruits
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