Phenolic acids and polydatin of grape and products of its processing

Abstract

The paper established that in the fruits of red grapes and in the products of its processing, the main phenolic acids are not chlorogenic acids (the acylation product of quinic acid by caffeic one), as indicated in a number of publications, but caftaric acid (the acylation product of tartaric acid by caffeic one) and coutaric acid (the acylation product of tartaric acid by p-coumaric one). For chromatographic analysis of the studied samples, the stationary phase Symmetry C18 and the mobile phases of the “acetonitrile – 10 vol.% of HCOOH-water” system were chosen, being convenient for determining grape anthocyanins. It was shown that caftaric acid in all studied mobile phase compositions is completely separated from coutaric acid and from three isomeric monocaffeoylquinic acids (3-caffeoylquinic, 4-caffeoylquinic and 5-caffeoylquinic), i.e. there is no confusion between the retention times of these acids. But for chromatography, mobile phase compositions should be selected in which there is no co-elution of any of these acids with delphinidin-3-glucoside, which is usually present in extracts, juices and wines. Complete separation of these components is possible both with a relatively low acetonitrile content (less than 6 vol. %) in the mobile phase, and with a relatively high (at least 8.5 vol. %) as well. This made it possible to propose a gradient elution option for separating all compounds of interest. A gradient mode was needed to elute all anthocyanins and associated extractives from the column. It turned out that the proposed separation conditions were favorable for the detection of polydatin (one of the isomeric glucosides of resveratrol). This made it possible to establish another common misconception, according to which it is claimed that resveratrol (and not its glucoside) is one of the most important biologically active substances in grapes. Under the proposed conditions, much more strongly retained resveratrol coelutes with one of the acylated anthocyanins, but analysis of the areas of the corresponding peaks indicates that polydatin is the dominant form of resveratrol in grapes