This is helpful, but it means sugar to juice ratio is dependent on acidity.. Still looking for an equation... but a work around may be reducing the amount of pectin used?
The Role Of Acid In Jelly Formation
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.
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