Feasibility of filter‐based NIR spectroscopy for the routine measurement of olive oil fruit ripening indices

Abstract

The aim of this work was to apply and validate a NIR spectrometer based on a discrete filter system for the rapid measurement of the moisture, oil, sugar, and phenolic compounds contents of olive oil fruits. The batches of olive oil fruits were collected during seven crop seasons from several farms located in Tuscany and then they were crushed into olive paste. The water content was measured gravimetrically, oil content was measured using the Soxhlet method, sugar content was measured enzymatically and phenolic compound content was measured by HPLC. NIR spectra were recorded from 1400 to 2400 nm at 19 selected wavelengths. Calibration and validation models were processed using PLS regression. In PLS models built for moisture, oil, and sugar contents, the r2 in calibration ranged between 0.90 and 0.93 with low standard error of calibration (SEC) values (i.e. 2.5, 3.6 and 4.0, respectively). For these parameters the standard error of prediction (SEP) and the standard error of laboratory (SEL) values proved to be comparable (i.e. 2.4 vs. 3.2, 6.0 vs. 4.4, and 6.7 vs. 4.7, respectively). Instead, the calibration and validation results concerning the phenolic compounds were not satisfactory, probably because the necessary wavelengths in the section of absorbance from 1100 to 1400 nm were not covered. Practical applications: NIR spectrometers based on discrete filter systems may be interesting since they are cost‐saving compared to the more sophisticated FT‐NIR and NIR‐AOTF instruments (i.e. the cost is approximately half). Our study also showed that with this instrument it was possible to build some effective models for predicting moisture, oil, and sugar contents in olive paste. The results obtained for moisture and oil contents are comparable with those obtained with other spectrometers, and a predictive model was obtained for sugar content for the first time. Instead the tool did not prove suitable for obtaining predictive models for total phenolic compounds or oleuropein contents. The additional use of a filter‐based NIR spectrometer is therefore to be suggested to rapidly monitor olive fruit ripening on the basis of moisture, oil, and sugar contents. The used filter‐based NIR spectrometer, relevant spectra, and calibration‐validation results.

Non-destructive Evaluation of the Quality Characteristics of Pomegranate Kernel Oil by Fourier Transform Near-Infrared and Mid-Infrared Spectroscopy

“…For total phenolic content, FT-MIRs in the region of 3,996 and 758 cm −1 , have been shown to provide rough predictions (RPD = 1.57), while those developed in the NIR region were shown to be unreliable (RPD = 1.26). Contrary to our results, Trapani et al (2016) reported that the NIR spectral region of 12,500 to 4,000 cm −1 provided relatively good prediction statistics ( R 2 = 0.71, RMSEP = 0.08 mg/kg dm) for total phenolic content in olive oil. Model development for the yellowness index gave relatively poor prediction statistics for both FT-NIRs and FT-MIRs ( Table 2 ).…”

Near Infrared Spectroscopy as a Green Technology for the Quality Prediction of Intact Olives

“…In the considered drupes, moisture content ranged from 39.3 to 87.2%. The obtained results agree with previously published data [ 18 , 35 ], considering that the moisture mean value was 63.3%, while the highest values (>80%) were obtained only in three out of thirteen cultivars, all from Calabria region. Excluding those three cultivars, the maximum value for moisture was 73.7%.…”

“…Similar model performances in calibration and cross-validation were obtained for 1/TPC for all the spectral acquisition systems (R 2 range, 0.76–0.89), whereas in prediction FT-NIR spectra, gave better results (R 2 pred = 0.77–0.76) than Vis/NIR spectra (R 2 pred = 0.69). FT-NIR models are better than those reported in the literature for a filter-based NIR spectrometer [ 35 ]. The authors attributed the unsatisfactory output of their model (R 2 cal , 0.72; SEP, 13.35 g oleuropein/kg dm ) to the exclusion of 8600–6900 cm −1 range from the spectral bands, which was instead here considered.…”

“…Similarly to the second overtone of CH stretching vibrations at 8300 cm −1 , the two bands at 5800 and 5650 cm −1 represent the first overtone of CH-stretching vibrations present in the same CH 3 , CH 2 , and CH=CH functional groups. At the far end of the FT-NIR spectral range, two peaks at 4335 and 4262 cm −1 represent CH and CH 2 s overtones, respectively [ 35 ]. However, the intermediate bands between the overtones (i.e., 8600, 5800–5650, and 4350–4250 cm −1 ) have been attributed to the oil content of the drupes [ 40 ].…”

Application of Infrared Spectroscopy for Functional Compounds Evaluation in Olive Oil: A Current Snapshot

“…e results in terms of R 2 for the calibration, prediction, and cross validation ranged between 0.930 and 0.998, 0.874 and 0.942, and 0.837 and 0.992, respectively. A recent preliminary study of Trapani et al [109] applied cost-saving NIR based on a discrete filter system for the rapid measurement of total phenolic content and oleuroperin of olive fruits, in addition to the moisture, oil, and sugar. Although PLS models built for the latter ones were satisfactory, the instrument did not prove itself suitable for obtaining predictive models for phenolic compound contents.…”