Effect of foliar treatment with chitosan on phenolic composition of ῾Fetească neagră’ grapes and wines

Abstract

The objective of this study was to determine the influence of chitosan treatments, applied in vineyard for plan protection, on the phenolic quality of grapes. The study was conducted for two consecutive years (2020-2021) in the wine center of Murfatlar, Romania, on ‘Fetească neagră’, an ancient but well-known indigenous Romanian grape variety for red wines. Chitosan is meant to offer an alternative for the classic treatment with Bordeaux mixture (BM), which represented the control variant. A combination treatment with half dose of chitosan and half dose of BM was also applied and evaluated.  Chemical composition at harvest time was evaluated by measuring the content of sugar, total acidity, pH, total anthocyanin potential (ApH1), anthocyanins extractable at wine pH (ApH3.2), total polyphenol index, skin tannins, seed tannins and seeds maturity. In wines the colour parameters were determined by spectrophotometry and individual phenolic compounds by UHPLC-HRMS. In grapes, accumulation of anthocyanins increased with the total dose of chitosan applied in the vineyard, with higher values in 2020 when temperatures were higher and the rainfall values were typical for the region. Accumulation of tannins in grape skins followed a similar trend. In wine, chitosan determined a significant 72% increase in colour intensity in 2020. Among the individual phenols gallic acid was predominant, with higher values in the rainier year (2021) and significant increases determined by chitosan treatment (especially in 2021 when it increased by 97% as compared to BM treatment). Catechin and epicatechin recorded important increases in the less favourable year (2021), with confirmed increases in both years elicited by the chitosan. Quercetin and myricetin were not influenced by the chitosan treatment, but their increase was correlated with higher temperatures and inversely correlated to the amount of rainfall. Trans-resveratrol ranged between 4.3-8.0 mg L-1 in 2020 and 5.0-6.5 mg L-1 in 2021, with an important increase determined by the chitosan treatment in 2020 (89% increase compared to BM treatment).