Stability in solution

Most reactions which pectin undergoes in use tend to degrade the pectin. As a rule, maximum stability is found at pH 4. The presence of sugar in the solution has a certain protective effect while elevated temperatures increase the rate of degradation.

Breaking strength of pectin gels as a function of previous heat
treatment of the pectin (90EC, 15 minutes) at various pH-values.

At low pH-values and elevated temperatures degradation due to hydrolysis of glycosidic links is observed. De-esterification is also favored by low pH. By de-esterification a high ester pectin becomes slower setting or gradually adapts low ester pectin characteristics.

At near-to-neutral pH (5-6), HM-pectin is stable at room temperature only. As the temperature (or pH) increases, a so-called -elimination starts. The -elimination results in chain cleavage and very rapid loss of viscosity and gelling properties.

LM-pectin shows a somewhat better stability at these conditions as illustrated in the graph above. At alkaline pH-values pectin is rapidly de-esterified and degraded even at room temperature.

Reactions with other electrically charged hydrocolloids

Pectin is a polygalacturonic acid and the chain molecule is negatively charged at neutral pH. The pK-value of pectin is approx. 3.5. Pectin reacts with positively charged macro-molecules, e.g. proteins at pH-values below their isoelectric pH. Pectin precipitates gelatin at low pH-values, but this reaction can be prevented by addition of salt. When pectin is added to milk at the pH of milk (6.6) a two-phase system is formed. At lower pH, pectin may combine with casein particles present to produce a stable acidified milk, which may even be heat treated to extend the shelf life of the product. Without pectin the milk protein would agglomerate to cause "sandy" mouthfeel and separation.

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