INTRODUCTION
Gallic acid (3.4.5-trihydroxybenzoic acid) is one of the most common plant acids. As a part of the plant usually in a bound form (esters, polymers) it is a precursor to a number of polyphenolic substances [1].
This acid is contained in the sewage of pharmaceutical factories, it is present as a component in the beer wort. Gallic acid and its derivatives have a pronounced astringent taste. When being boiled together with hops, they contribute to the formation of trub, reducing the colloidal stability and the quality of beer.
It has all the properties of hydroxycarbon acids. The most reactive is OH group in the position 4 (Figure 1). Of the various reactions of gallic acid (oxidative coupling and esterification) there arise tannins. The tannins are divided into hydrolyzable tannins - carbohydrate and gallic acid esters (or its oligomers) and non-hydrolyzable condensed tannins.
Gallic acid forms oligomers of two types: with a carbon-carbon bond between the phenyl rings and with a complex ester linkage between the fragments. In this regard, gallic acids themselves are divided into non-hydrolyzable (diphenyl derivatives) and hydrolyzable (esters). Both these and those acids form hydrolyzable tannins with carbohydrates, because in an aqueous medium under conditions of enzymatic, alkaline or acid catalysis phenolic acids and carbohydrates are formed. These tannins are esters of monosaccharides with gallic or trigallic acids. Glucose esters with condensed gallic acid can be considered as tannins of dual nature, because they contain hydrolyzable fragments. These completely non-hydrolyzable tannins are derived from flavonols.
Under the influence of atmospheric oxygen tannins form stable black dyes. [1]
Dimerization and polymerization of coniferyl alcohol compounds help to form such derivatives of phenolic acid or, to be more exact, phenolic alcohol groups as lignans and lignins. Lignans are accumulated in all plant organs, they are present in dissolved form in resins and essential oils. Lignins in plants - important components of the cell wall of conducting and supporting tissues that act as tissue mechanical strengthening and cells protecting from biological, chemical and physical effects [1].
