THE INFLUENCE OF ACID ON ASTRINGENCY OF ALUM AND PHENOLIC-COMPOUNDS

Astringency of aqueous solutions of phenolic compounds (grape seed tan nins, tannic acid, catechin and gallic acid) increased upon addition o f citric acid, whereas the astringency of alum was reduced. Astringenc y of alum was decreased equivalently by addition of equi-sour levels o f lactic acid, citric acid or hydrochloric acid. The difference betwee n alum and the phenolic compounds is speculated to result from chemica l modifications affecting binding of the astringents with oral protein s rather than cognitive differences. Chelation of the aluminum ion in alum by acids reduces its availability for interacting with salivary p roteins or epithelial proteins. In contrast, the increased astringency produced upon acidification of phenolic compounds is speculated to re sult from the pH driven increase in the affinity of the phenols for bi nding with proteins. These results suggest that alum cannot be used in terchangeably with phenolic astringents in psychophysical studies.