Pectin is mainly composed of polyagalacturonic acid (>70%) and yet is water soluble, whereas the pure acid is completely insoluble in water, even upon heating. The degree of estherification cannot be a reason because methyl groups, found in pectin, make the polymer chain less soluble than the carboxyl groups of the pure acid. What is the form of polygalacturonic acid in pectin that makes it soluble?
Your argument about the degree of esterification is not so obvious.
For instance, cellulose is not soluble, whereas methyl-cellulose is soluble.
The reason usually invoked to explain why hydrophobic groups favor dissolution in water, is the gain of entropy due to the disorder in the water network introduced by methyl.
This may be a track.
Dear Didier Lairez , Thank you very much for sharing your knowledge and for this clear explanation.
Although polygalacturonic acid contains very low amount of neutral sugars and methyl ester groups, It is water-insoluble due its high molecular weight (25-50 Kda). Then you need to dissolve it in diluted alkali agent as sodium carbonate.
Thank you Mohamad Issaoui , my doubt comes from the fact that pectin also has a high molecular weight but it is still water soluble
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