Why is sugar needed to make preserves?

Why is sugar needed to make preserves?

Apart from adding sweetness, sugar helps prevent bacteria from growing in the preserves and it also aids in the gelling. Sugar holds onto the water molecules and lowers the water activity (the amount of water in food) which makes it harder for microorganisms to grow. Sugar is what makes it shelf stable for so long. For example, strawberries are 90% water, so they need equal parts of fruit to sugar. The sugar draws the water out of the fruit and they form a chemical bond preventing anything else from reacting and bonding to the water. The sugar dissolves in the juices released by the fruit when the heat breaks down its cell walls. As it’s boiling, the sugar concentration is increasing and it becomes thicker. If there is too much water the pectin can’t form a strong enough gel and the jam becomes runny. The sugar also helps to cover the sour taste of the citric acid found in the lemon juice which is necessary for the pectin strands to form chains to gel into a network. Although with too much sugar your jelly will crystalize and the fruit will become hard.