The hydrates of chlorogenic acid in its aqueous solution and in stored food products

Chlorogenic acids (CQAs), a large family of naturally occurring esters of quinic acid with one, two or even three cinnamic acids moieties and/or moieties of cinnamic acids derivatives, supplied to human organisms mainly with coffee, tea, fruit and vegetables, have been among the most frequently studied polyphenols. Of these, caffeic acid esters predominate, and 5-O-caffeoylquinic acid (5-CQA) is their main and most widespread representative. Recently performed studies have shown that after removing water from the aqueous solution of 5-CQA, its dry residue contains two hydrates of this compound, each consisting of two 5-CQA molecules and two molecules of H₂O (2 × 5-CQA*2xH₂O). The results presented in the paper not only define the characteristic features of 5-CQA but, more importantly, offer solid evidence that 5-CQA hydrates begin to form already in the aqueous solution of 5-CQA immediately after the dissolution of this compound and that the analogous process can occur in food products containing 5-CQA during their storage. Moreover, the performed experiments indicate the validity of the hypothesis that 5-CQA hydrates are formed by active collisions of 5-CQA molecules with monomeric/dimeric water molecules existing in trace amount in strongly associated aqueous systems. The presence of 5-CQA hydrates in liquid food products may have a significant impact on the assessment of some of their physicochemical properties as well as their biochemical activity.