Several old jam or jelly recipes called for Epsom Salt to help the product gel, but this is NOT a recommended practice.
Epsom Salt is a bitter, colorless or white crystalline salt which is a hydrated magnesium sulfate. Magnesium has the ability to form weak links with pectin in the presence of sugar and acid. Epsom Salt was thus used in an old method for testing for natural pectin content in fruit juice before making jelly, as it does cause pectin to gel when magnesium ions are released in solution.
Epsom salt is a laxative, regulated by FDA as a medication or drug, not as a food ingredient. Possible side effects or hazards include nausea, vomiting, abdominal cramps and diarrhea. Whereas there are some food grade forms of liquid magnesium sulfate used in approved food manufacturing situations, the dry (anhydrous) Epsom Salt found in drugstores is usually labeled: may be harmful if swallowed and not intended for ingestion.
Source: http://nchfp.uga.edu/questions/FAQ_jellied.html#6
