Rapid methods for the evaluation of total phenol content and extractability in intact grape seeds of Cabernet-Sauvignon: instrumental mechanical properties and FT-NIR spectrum

Abstract

<p style="text-align: justify;"><strong>Aims</strong>: Fourier Transform-Near Infrared (FT-NIR) spectrum and instrumental texture parameters were assessed as total phenol content and extractability predictors in intact grape seeds.</p><p style="text-align: justify;"><strong>Methods and results</strong>:The study was carried out on Cabernet-Sauvignon seeds from grapes harvested at six different advanced physiological stages throughout ripening and calibrated by flotation to reduce the in-field heterogeneity inside each sample. Among the instrumental mechanical properties tested (i. e., break force, break energy, Young’s modulus of elasticity and deformation index), the seed Young’s modulus of elasticity showed an increase during the first four weeks of ripening. This parameter also showed significant correlations with phenol content and extractability, although with low R coefficients. These correlations highlighted that the springier seed tissues greatly increase phenol extractability. Nevertheless, the best prediction of seed phenol content, performed directly on intact seeds, was found using FT-NIR spectroscopy in transmittance mode. The standard error of prediction for total phenol content was less than 8 %, while that for phenol extractability was worse.</p><p style="text-align: justify;"><strong>Conclusion</strong>: On the basis of these results, the two analytical methods could be applied in oenology for the rapid monitoring of seed phenolic maturity.</p><p style="text-align: justify;"><strong>Significance and impact of the study</strong>: The phenolic composition of grapes at the harvest time is a key factor determining their quality, and thus the quality of the finished wine. The chemical methods used for the determination of seed phenol content and extractability are generally slow because they require a preliminary extraction. Therefore, a rapid evaluation of these parameters could be highly interesting for the oenological sector.</p>

Berry Heterogeneity as a Possible Factor Affecting the Potential of Seed Mechanical Properties to Classify Wine Grape Varieties and Estimate Flavanol Release in Wine-like Solution

“…This work confirmed the relationship reported in a preliminary study performed on Cabernet Sauvignon seeds, where the strongest and most significant correlation for the A 280 values per seed weight measured in the extracts obtained after seed treatment with a wine-like solution was found with E s (Rolle et al, 2012a). In the present study, wine grape varieties that belong to cluster 2, including Cabernet Sauvignon, showed a significant correlation between these two parameters, but the correlation coefficient improved (R = 0.406 instead of 0.190, p < 0.01).…”

“…In contrast, significant differences were found in the compression parameters of Cabernet Sauvignon seeds (W s , E s and DI s ) when the ripening effect was studied for several weeks (Rolle et al, 2012d). Nevertheless, the changes occurred only in seed stiffness (E s ) for sorted Cabernet Sauvignon grapes sampled at different harvest dates, even if one density class per sampling date was selected to emphasise the physiological differences among grape ripening stages (Rolle et al, 2012a). Some researchers also noted that most of the texture parameters of the seeds became steady three weeks after the end of véraison (Letaief et al, 2013).…”

“…The compression parameters are closely related to the extractable content of phenolic compounds during ripening, particularly in the seeds. The resistance of the seed to axial deformation is negatively correlated with the phenol extractability (Rolle et al, 2012a), and the seed break force is also intrinsically linked to the extractable content of oligomeric flavanols (Torchio et al, 2012). Nevertheless, the robustness of these correlations was low, probably due to the fact that the studies were conducted either for total phenols or on heterogeneous berries in terms of ripeness at each harvest date.…”

Sensory and instrumental texture measurements for assessing grape seed parameters during fruit development

“…The results obtained in this work showed that all seed textural parameters increased logarithmically during ripening, including hardness, toughness, fracturability, elasticity, stiffness and crispness. This did not agree with another study previously performed on Cabernet Sauvignon grape seeds, which reported that, among the instrumental mechanical properties tested (break force, break energy, Young's modulus of elasticity and deformation index), only the seed Young's modulus of elasticity increased during ripening . In the literature, this parameter was wrongly expressed by the slope of the initial part of the curve, which should correspond to the seed stiffness (measure of the resistance offered by the seed to deformation) and not to the Young's modulus of elasticity expressed as the ratio of stress (pressure) to strain (dimensionless) as reported in the present study.…”

“…This did not agree with another study previously performed on Cabernet Sauvignon grape seeds, which reported that, among the instrumental mechanical properties tested (break force, break energy, Young's modulus of elasticity and deformation index), only the seed Young's modulus of elasticity increased during ripening . In the literature, this parameter was wrongly expressed by the slope of the initial part of the curve, which should correspond to the seed stiffness (measure of the resistance offered by the seed to deformation) and not to the Young's modulus of elasticity expressed as the ratio of stress (pressure) to strain (dimensionless) as reported in the present study. Taking this into consideration, it results that stiffness is the seed texture property that most affects the phenol extractability, and not the seed elasticity as reported in the literature .…”