Natural compounds in some foods, particularly acids, corrode metal and make a dark deposit on the underside of jar lids. This deposit is harmless providing the jar has a good seal and the contents have been properly processed. This sudden change of temperature is too great and will crack jars. This can be oftentimes be accomplished by soaking jars for several hours in a solution of 1 cup vinegar and 1 gallon of water. Salt is used for flavor only and is not necessary to prevent spoilage.
Sugar is added to improve flavor, help stabilize color, and retain the shape of the fruit.
It is not added as a preservative. Aspirin should not be used in canning. It cannot be relied on to prevent spoilage or to give satisfactory products. Adequate heat treatment is the only safe procedure. Recommended processing methods must be used to assure safety.