The mechanism of pectin jelly formation described above is also applicable to agar-like substances. However, the nature of these gelling agents, the structure of their molecules, and their physicochemical properties require some features of the gelling process to be noted.
The molecular weight of pectin is 5 times higher than that of agar, and 20 times as large as that of agaroid. Comparing the structure of the molecules of these substances, it is easy to see that in the agar and agaroid molecules there are no carboxyl groups COOH, the methyl groups OCH3so characteristic of a pectin molecule. In the agar molecule (and possibly agaroid) there is a sulfate group HS04. This means that the potential on the surface of agar and agaroid molecules in solution will be significantly lower than on the surface of pectin molecules. Accordingly, the thickness of the diffuse layer will also be small. These characteristics of high-molecular ions, as noted above, determine the magnitude of the electrostatic repulsive forces in the association of molecules.
From the functional groups of the molecules, it can also be concluded that the molecules of agar-like substances are less polar than the molecules of pectin; therefore, their aggregation can also occur with small amounts of sugar in the liquid phase. Even with a small concentration of hydrogen ions or potential-determining ions, barrier-free coagulation conditions are ensured. Apparently, for the molecules of agar-like substances, neutralization coagulation prevails in the solution, when the energy barrier is suppressed by the adsorption of potential ions HE ”, Ca++, Mg++ and etc.
The most important characteristic of agar and agaroid jellies, determining their physicochemical and structural-mechanical properties, is the charge of a high molecular weight anion, which is determined by the number of sulphate groups per unit mass of the gelling agent and the nature of the cation.
By introducing one or another cation into an agar or agaroid molecule, it is possible to change the charge of a high molecular weight anion in a well-known direction, and, consequently, the physicochemical properties of the gelling agent.
S.N. Stavrov and F.P. Bonegrog found that with an increase in electrolytic dissociation of the cation, substituted agaroid samples, their viscosity in dilute solutions and the degree of swelling in water increase, but at the same time decreases the ability to gel formation.
They have the best ability studneobrazuyuschey cation-exchanged samples Ba++ and Ca++- agaroid, i.e. just those on the surface of which molecules in solution will have a minimum potential.
When acidifying agar jelly masses, the concentration of potential-determining OH ions “in the liquid phase decreases, which causes a decrease in gel-forming ability. The previous interpretation of the hydrolyzing effect of acid on a molecule of agar-like substances must be considered erroneous.
The difference in the effect of acids and their sodium salts on the strength of jellies is due to a different change in the solubility of agar and agaroid. When organic acids are added to the jelly mass, forming insoluble or slightly soluble salts with calcium, the dissociation of partially liberated agaric or agaric acid molecules is largely suppressed by the acidic medium, and therefore there are no noticeable changes in the strength of the jellies. With the introduction of sodium salts of these acids, which is often practiced in the production of jelly marmalade, agar (agaroid) is enriched with sodium, the solubility of its molecules increases, which increases the total concentration at a given temperature and contributes to the formation of more durable jelly.