Although acid is not essential for jelly formation its presence in fruit jellies is very important. Singh has reported that "between certain limits the greater the acidity of the juice the lower the amount of sugar required." He adds that it has long been known that juices of high acidity yield firmer jellies than juices deficient in acid but with as high a pectin content.

Spencer has published data showing the increase of rigidity of jellies with increase of acid when the pectin and sugar concentrations are constant. Spencer explains the action of acid in this way. The strength of a jell network depends upon the continuity and the rigidity of the structure. "Continuity of structure, by hypothesis, depends upon the number and proximity of pectin particles at the time of precipitation," which in turn is determined by the degree of dispersion and concentration of the pectin. "Differences in rigidity are due to the amount of water retained by the pectin at the equilibrium established during precipitation." Hydrogen (or hydroxyl) ions lessen the stability of the pectin sol by decreasing the hydration capacity of the pectin. Hence in an acid medium less sugar is required to bring about precipitation. Nearly neutral fruit juices will not form jelly with sugar because the sugar is not soluble enough to allow precipitation of the more stabile pectin. Hence with a definite concentration of pectin the rigidity of the jelly is determined by the sugar and acid concentrations.

Hydrogen-ion concentration and jelly. Tarr has determined the minimum amounts of several acids required to produce a jelly and also the amount of the acids to produce an optimum jelly by the hot evaporation method with pectin, sugar, acid, and water. Optimum jelly is defined as the jelly which in his judgment has the best texture. For jelly formation, when other conditions were standardized, the minimum amounts of the acids were as follows: 8.5 cc. of 0.1 N sulfuric acid; 27.5 cc. of 0.1 N phosphoric acid; 22.7 cc. of 0.1 N tartaric acid; 52.9 cc. of 0.1 N citric acid; and 583.3 cc. of 0.1 N acetic acid. The total acidity of the minimum amounts required for forming a jelly varied, but the acids were all at the same pH, 3.40. For optimum jelly the acids were all at pH 3.1.

But although hydrogen-ion concentration controls the formation and character of the jelly to a certain extent, the salt content of pectin or of fruit juices, the temperature to which the pectin is heated, and the rate of pouring may extend this pH range, as will be seen later. In Tarr's jelly a pH of 3.46 gave a very tender jelly, but increasing the hydrogen-ion concentration below pH 3.1 gave syneresis or weeping. The best jellies were obtained with pH. 3.3 to 3.1. Olsen and others have suggested that with pH lower than 3.1, jelly failure and increased syneresis may result because of the increased rate of setting. Olsen has shown that an increase of hydrogen-ion concentration increases the rate at which the jelly sets.

The hydrogen-ion concentration of ordinary fruit juices depends upon the particular acid present, upon the quantity of acid present, and upon the "buffer" action exerted by the particular juice.

Relation of the time factor to optimum acidity. Olsen assumed that the lowering of jelly strength might be caused by incomplete dehydration of the pectin and lessened precipitation or by a more flexible network of pectin. To test the second postulate he prepared jellies by heating the pectin to 55°C. and to boiling. The jelly at 55° was made as follows: The pectin, 30 grams of sugar, and water to make a total weight of 140 grams were brought to 36°C. The minimum amount of acid, the remaining sugar, and water were boiled together, cooled to 65°C, and adjusted to correct weight with distilled water. The additional acid and the pectin solution were added and stirred, the time was varied from the moment the pectin solution was poured into the warm sirup until the mixture was poured in jelly glasses, as shown in Table 22. The strength was tested with a Tarr and Baker jelly strength tester. Olsen states that a high-grade pectin made by the standardized hot method gives a reading close to 45.

The optimum pH. varies with the time factor. The principal variation is the rate of gelation of the pectin as influenced by the acid concentration. Olsen explains this as follows: "The largest or 'optimum' amount of the acid to be used will be that point at which an additional increase in acid will increase the rate of setting to a point where loss in jelly strength due to a disturbance of the jelly in the stirring or pouring exactly balances the strengthening effect of that same increment of acid." Or in other words, stirring or pouring hinders jelly formation, which is offset by increasing the hydrogen-ion concentration.

Acids in fruits. The acids occurring naturally in fruit juices that are used for jelly are tartaric, malic, and citric. Sometimes acetic acid is added to apple juice to make spiced apple jelly. Tarr's results show that acetic acid is of little value to add to a juice to increase its jellying power, because it volatilizes during boiling and is only slightly ionized in solution.