Different types of pectin, different behaviour of gelation
The optimal conditions for gelation of the different types of pectin are shown in the table below.
|Parameters||HM pectin||LM pectin|
|Degree of methoxylation (DM)||> 50%||< 50%|
|Soluble solid content (%)||≥ 65%||10 - 70%|
|pH||2.0 - 3.8||2.6 - 7.0|
|Bivalent ions, Ca++ (mg/g)||-||≥ 15|
HM pectin will only gel if the soluble solids content is higher than 65% and the pH is 2.0 - 3.8.
The gel is formed by cross-linking the polymer at junction zones in which mainly hydrogen bonds and the repulsion of methyl groups create areas where sugar and water can be trapped into the pectin network. The low pH value reduces the negative charges along the pectin chain, originated by the carboxylic group disassociation. Thus an electrostatic repulsion occurs which is responsible for gel formation and the pectin - pectin bond formation is avoided.
- The lower the DM level, the longer the setting time and the lower the setting temperature.
- The higher the soluble solids content, the lower the numbers of hydrogen bonds. These bonds are stronger and thus the strongest gels are formed.
LM pectin forms gels in the presence of polyvalent cations, typically calcium (minimum quantity 15 mg Ca++ / g pectin). LM pectin forms gel over a wide range of pH, from 2.6 to 7.0, and with a soluble solid content between 10 - 70%.
The binding of Ca++ to pectin is not a simple ionic interaction, but involves intermolecular chelate binding of the cation, leading to the formation of macromolecular aggregates (“egg-box” cavity).
- The higher the solids level and the lower the pH, the pectin is more reactive.
- The higher the calcium content and lower the pH, the higher the setting temperature.