The potential of fluorescence spectroscopy for characterizing the deterioration of extra virgin olive oil (EVOO) during heating was investigated. Two commercial EVOO were analysed by HPLC to determine changes in EVOO vitamin E and polyphenols as a result of heating at 170 degrees C for 3 h. This thermal oxidation of EVOO caused an exponential decrease in hydroxytyrosol and vitamin E (R(2)=0.90 and 0.93, respectively) whereas the tyrosol content was relatively stable. At the same time, amounts of preformed hydroperoxides (ROOH), analysed by an indirect colorimetric method, decreased exponentially during the heating process (R(2)=0.94), as a result of their degradation into secondary peroxidation products. Fluorescence excitation spectra with emission at 330 and 450 nm were recorded to monitor polyphenols and vitamin E evolution and ROOH degradation, respectively. Partial least-squares calibration models were built to predict these indicators of EVOO quality from oil fluorescence spectra. A global approach was then proposed to monitor the heat charge from the overall fluorescence fingerprint. Different data pretreatment methods were tested. This study indicates that fluorescence spectroscopy is a promising, rapid, and cost-effective approach for evaluating the quality of heat-treated EVOO, and is an alternative to time-consuming conventional analyses. In future work, calibration models will be developed using a wide range of EVOO samples.
Analyzing the optical properties of fruits represents a powerful approach for non-destructive observations of fruit development. With classical spectroscopy in the visible and near-infrared wavelength ranges, the apparent attenuation of light results from its absorption or scattering. In horticultural applications, frequently, the normalized difference vegetation index (NDVI) is employed to reduce the effects of varying scattering properties on the apparent signal. However, this simple approach appears to be limited. In the laboratory, with time-resolved reflectance spectroscopy, the absorption coefficient, μa , and the reduced scattering coefficient, μs ', can be analyzed separately. In this study, these differentiated optical properties were recorded (540-940 nm), probing fruit tissue from the skin up to 2 cm depth in apple (Malus × domestica 'Elstar') and plum (Prunus domestica 'Tophit plus') harvested four times (65-145 days after full bloom). The μa spectra showed typical peak at 670 nm of the chlorophyll absorption. The μs ' at 670 nm in apple changed by 14.7% (18.2-15.5 cm(-1) ), while in plum differences of 41.5% (8.5-5.0 cm(-1) ) were found. The scattering power, the relative change of μs ', was zero in apple, but enhanced in plum over the fruit development period. This mirrors more isotropic and constant structures in apple compared with plum. For horticultural applications, the larger variability in scattering properties of plum explains the discrepancy between commercially assessed NDVI values or similar indices and the absolute μa values in plum (R < 0.05), while the NDVI approach appeared reasonable in apple (R ≥ 0.80).
The non-invasive detection of chilling injury (CI) symptoms in banana may potentially be approached by means of monitoring changes in the pigment contents and texture of the exocarp. In the present study, laser diodes emitting at 660 and 785 nm were applied to acquire images of backscattered light from intact banana fruits. The idea was to monitor chlorophyll and texture changes by means of relevant wavelengths, respectively. Bananas were stored for 2 days at 13 °C (control), 6 °C (chilling temperature), and subsequently 1 day at ambient temperature to allow the symptom development. Parameters obtained from the backscattering images and their combinations were applied for detecting chilling injury. Significant (P < 0.05) interaction of backscattering properties and treatment factors (temperature, ripening stage, and treatment time) were found. Classification of control and chill-injured samples in ripe fruits measured at 660 nm and 785 nm resulted in misclassification error as low as 6% and 8% for early detection, and 0.67% and 1.33% for detection after storage, respectively. The physiological relevance of the variation measured at the two wavelengths was pointed out by means of destructive pigment and water analyses.
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