This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
The role salts play in jelly formation is not emphasized as much as that of pectin, acid, and sugar, but it is an important one. All fruit juices contain salts of the acids found in the fruit. In addition to these, other salts may be found.
Halliday and Bailey have reported that the addition of calcium chloride favors jelly formation, as somewhat lower concentrations of pectin, acid, and sugar are required when it is added. On the acid side, sodium chloride tends to prevent jelly formation, i.e., larger quantities of acid and sugar are required when sodium chloride is added. Spencer has reported that on the alkaline side sodium chloride tends to precipitate the pectin. Spencer has also reported that, with the same anion, chlorine, the precipitating effect of the cation increased with increasing valence, or Al > Ca > Na. With the same cation but different anions Spencer gives the order of absorption of the negative ion by pectin as acetate > citrate > tartrate. This means that the tartrate is not so strongly adsorbed as the citrate or acetate ions, hence has less stabilizing effect and is more easily precipitated by sugar. The chloride anion is more strongly absorbed than the nitrate or sulfate ions.
Salts buffer a fruit juice so that more acid is required to give a definite pH when salts are present. But the hydrogen ion is not the only positive ion in the fruit juice. Myers and Baker have found that the cation of the salt may supplement the hydrogen ion, but only at definite hydrogen-ion concentrations. No jelly was formed with added salts above pH 3.6. Thus jellies from fruit juices may have a slightly wider range of hydrogen-ion concentration for jelly formation than those from pure water, acid, pectin, and sugar. Myers and Baker used sulfuric, tartaric, and citric acids and the sodium salts of these acids in their investigations.
The negatively charged ion of a salt tends to stabilize pectin sols, but those anions absorbed most strongly exert the most stabilizing effect. These negatively charged anions may be neutralized by the positively charged ion of a salt, but the adsorption of cations also varies, so that some have a greater destabilizing effect than others. Since all pectins contain salts, the quantitative recipes worked out for one pectin may not give exactly the same results with other pectins because of a different salt content.