Jam were developed in ancient times as a means of preserving the fruit that would otherwise rapidly spoil. When preserved, fruit resist spoilage because they lack the water that microorganisms are necessary for growth. Surface shape can be prevented by sealing homemade preserves sealed with paraffin layer.
Preserved fruit sugar will gel via the reacting of acids, fruit pectin and soluble fiber which is composed of cell walls by boiling the fruit. Apples, grapes, and most fruits contain enough natural pectin but in other fruits like apricots and peaches, you have to add it. Low calorie jams with reduced sugar content with special cast pectin determines the lowest acidity and less sugar.
Concentrated fruit juice is added to these products for sweetness and thickness is increased with starch. In Nutritional Value, there is no comparison between the jams and fresh fruit as most of the vitamin C and other nutrients in fruits are destroyed by intensive cooking.