Effects of fast dehydration at low temperature and relative humidity on the phenolic composition of Nebbiolo grapes

Abstract

Grape postharvest dehydration is a widely used technique for the special wines production, where genetic features, ripeness degree and environmental factors strongly influence the metabolic processes [1]. Low temperatures allow a better preservation of secondary metabolites by limiting oxidation reactions, but relative humidity and airflow are also important parameters in regulating the dehydration rate, which induces metabolic responses to water stress [1]. In this study, a cold dryer system (FD), operating at an air temperature and relative humidity of 15 °C and 40%, respectively, was tested for fast grape dehydration. This system was compared to long-term dehydration at 10 °C (LD), allowing better anthocyanin preservation [2], and to an uncontrolled process at 25 °C (MD) to assess the effect on the preservation and extraction of polyphenols in dehydrated grapes. ‘Nebbiolo’ winegrapes were used and samples from all trials were collected when a 35% of berry weight loss was reached. Standard parameters, total anthocyanins (TA), total flavonoids (TF), condensed tannins (PRO), flavanols reactive to vanillin (FRV) and colour traits were determined [3]. The potential phenolic content in skins and seeds [4] and the extracted content during 14 days of simulated maceration with a model wine solution [4] were determined.

The results showed that dehydration was faster for FD, requiring only 15 days to reach 35% berry weight loss compared to 55 days for LD and 27 days for MD. The latter showed higher potential phenolic contents in skins for TA, TF and PRO than FD, LD and fresh grapes (prevailing concentration effect), whereas presented similar extracted concentrations to LD, showing FD samples the lowest values. Extracted TA contents peaked at 24 h of maceration and then decreased, whereas TF remained constant due to the progressive extraction of PRO and FRV. FD samples also showed the lowest potential and extracted FRV contents in skins, with a decrease of around 50% compared to fresh grapes. On the other hand, the potential FRV and PRO contents in seeds increased in dehydrated grapes, particularly for FD, but the extracted content increased similarly across the three treatments for PRO and achieved the highest values for FRV in FD. This study highlights that the dehydration rate, either too fast or too slow, rather than temperature could promote cell-wall degradation releasing phenolic compounds but inducing oxidation reactions that lead to significantly lower contents and red pigments loss.

References

[1] Mencarelli, F., Tonutti, P. (2013). Sweet, Reinforced and Fortified Wines, pp. 105-115. John Wiley & Sons.

[2] Mencarelli, F., Bellincontro, A., Nicoletti, I., Cirilli, M., Muleo, R., Corradini, D. (2010). J. Agric. Food Chem., 58, 7557–7564.

[3] Scalzini, G., Giacosa, S., Paissoni, M. A., Río Segade, S., De Paolis, et al. (2023). J. Sci. Food Agric., 103, 6105-6118.

[4] Giacosa, S., Ferrero, L., Paissoni, M. A., Río Segade, S., Gerbi, V., Rolle, L. (2023). Food Chem., 424, 136463.