This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
The earliest work on jelly from a school laboratory is that of Gold-thwaite. Since that time many valuable contributions have been made dealing with jelly making. Probably the most extended recent investigations are those of Spencer, of Olsen, and of Tarr and his associates, Baker and Myers. They believe that the formation of jelly "depends almost entirely upon the application of the laws of chemistry, more especially the laws of physical chemistry."
Jelly formation. Spencer offers a new theory for gelation of pectin jells. Stated briefly it is as follows. A pectin sol is stabilized by hydration and a negative charge of the pectin particles. The greatest stability comes in the neutral range as increasing either alkalinity or acidity decreases its stability. In jelly the role of the sugar is that of a precipitating agent: the more acid the solution, the less sugar required for the precipitation of the pectin. Salts may aid in precipitation or tend to prevent it, depending on whether they increase or decrease the stability of the pectin.
For theoretical study of pectin jelly Spencer recommends mixing the constituents of the jelly while cold instead of the usual boiling of the sugar and juice, which she calls the hot evaporation method. The reasons for this are four in number. Decomposition of both sugar and pectin occurs during cooking. The proportions of the various constituents change by evaporation, hence it is difficult always to obtain the concentration desired. The precipitating agent sugar has a limited solubility. There is danger of premature precipitation of the pectin if all the cold sugar is added at one time to the hot pectin mixture. For this experimental work dried pectin or a concentrated pectin solution is necessary.
Olsen summarizes the theory of jelly formation as follows: Granted that pectin is a negatively charged hydrophilic colloid, the following may be assumed: (1) sugar is the dehydrating agent; (2) the hydrogen-ion concentration reduces the negative charge on the pectin, thereby permitting the pectin to precipitate and coalesce in the form of a network of insoluble fibers, provided the concentration of sugar is sufficient; (3) the dehydration of the pectin micelles by sugar is not instantaneous but requires time to come to an equilibrium; (4) the rate of hydration and precipitation increases in direct ratio as the hydrogen-ion concentration; (5) the maximum jelly strength is reached when the system reaches equilibrium, and depends upon the position of that equilibrium; (6) any component added to a pectin jelly system, including salts, which causes a change in the ultimate jelly strength of that system may function either (a) by changing the rate of gelation, or (b) by affecting the ultimate jelly structure, or (c) a combination of these two.