This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Syneresis occurs in some fruit jellies. It is probably more common in cranberry jelly than in other fruit jellies. Tarr found that, with a hydrogen-ion concentration greater than pH 3.1, fluid exuded from the jelly. Myers and Baker have reported that syneresis in jellies may be brought about by the hydrogen ion alone, or by the hydrogen ion and cation of an added salt together, but not by the cation of the salt alone. They have also reported that the anion of a salt by acting as a buffering agent may prevent syneresis.
Whether syneresis occurs in a jelly also depends on the source of the pectin. The writer has never seen syneresis occur with gooseberry jelly, even when the pH of the jelly is 2.6 or lower. Some citrus pectins made into jelly show no syneresis at pH 2.0 or lower.
The rate of dehydration or setting and mechanical disturbance after the jelly starts to set may also influence syneresis.