As the pH i s lowered, i.e. , as the hydrogen io n concentratio n
of the solutio n i s increased , ionizatio n of the carboxylat e groups
i s repressed , i.e. , the highl y hydrated carboxylat e groups are
converted int o onl y slightl y hydrated carboxyli c aci d groups. As
a resul t of losin g some of thei r charge, the polysaccharid e
molecules no longe r repe l each other over thei r entir e length ; and
as a resul t of losin g some of the water of hydration , they can
associat e over a portio n of thei r lengt h t o form a gel . Apparent
pK values (pH at 50% dissociation ) vary wit h the DE of the pecti n
(22); a 65% DE pecti n has an apparent pK of 3.55, whil e a 0ί DE
pecti c aci d has an apparent pK of 4.10. However, pectin s wit h
increasingl y greate r degrees of methylatio n wil l ge l at somewhat
higher pH, undoubtedly because they have fewer carboxylat e anions
at any given pH (See late r paragraph). pH affect s ge l textur e
more than ge l strength .