Pectin is soluble in pure water. Monovalent cation (alkali metal) salts of pectinic and pectic acids are usually soluble in water; di- and trivalent cations salts are weakly soluble or insoluble. Dry powdered pectin, when added to water, has a tendency to hydrate very rapidly, forming clumps. These clumps consist of semi dry packets of pectin contained in an envelope of highly hydrated outer coating4. Further solubilization of such clumps is very slow. Clump formation can be prevented by dry mixing pectin powder with watersoluble carrier material or by the use of pectin having improved dispensability through special treatment during manufacturing27. Dilute pectin solutions are Newtonian but at a moderate concentration, they exhibit the non-Newtonian, pseudo plastic behaviour characteristics. As with solubility, the viscosity of a pectin solution is related to the molecular weight, degree of esterification, concentration of the preparation, and the pH and presence of counter ions in the solution. Viscosity, solubility, and gelation are generally related. For example, factors that increases gel strength will increase the tendency to gel, decrease solubility, and increase viscosity, and vice versa18. These properties of pectins are a function of their structure. As such, monovalent cation salts of pectins are highly ionized in solution, and the distribution of ionic charges along the molecule tends to keep it in extended form by reason of coulombic repulsion28. Furthermore, this same coulombic repulsion between the carboxylate anions prevents aggregation of the polymer chains. The number of negative charges is, determined by the DE. In addition, each polysaccharide chain, and especially each carboxylate group, will be highly hydrated29. Solutions of monovalent salts of pectins exhibit stable viscosity because each polymer chain is hydrated, extended, and independent. As the pH is lowered, ionization of the carboxylate groups is suppressed, and this results in a reduction in hydration of the carboxylic acid groups. As a result of reduced ionization, the polysaccharide molecules no longer repel each other over their entire length and can associate and form a gel. Apparent pK-values (pH at 50% dissociation) vary with the DE of the pectin29; a 65% DE pectin has an apparent pK of 3.55, while a 0% DE pectic acid has an apparent pK of 4.10. However, pectins with increasingly greater degrees of methylation will gel at somewhat higher pH, because they have fewer carboxylate anions at any given pH. Dissolved pectins are decomposed spontaneously by de-esterification as well as by depolymerisation; the rate of this decomposition depends on pH, water activity, and temperature. In general, maximum stability is found at pH 4. The presence of sugar in the pectin solution has a certain protective effect while elevated temperatures increase the rate of degradation4. At low pH-values and elevated temperatures degradation due to hydrolysis of glycosidic linkages is observed. Deesterification is also favored by low pH. By deesterification a HM-pectin becomes slower setting or gradually adapts LM-pectin characteristics. At nearto-neutral pH (5-6), HM-pectin is stable at room temperature only. As the temperature (or pH) increases, so-called elimination starts which results in chain cleavage and very rapid loss of viscosity and gelling properties. LM-pectins show a somewhat better stability at these conditions. At alkaline pHvalues pectin is rapidly de-esterified and degraded even at room temperature30, 31.
Powdered HM-pectins slowly lose their ability to form gels if stored under humid or warm conditions while LM-pectins are more stable and loss should not be significant after one year storage at room temperature30.
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