The preservative properties of sugar are exploited in products such as jams, juices and pickling solutions. Foods are preserved to avoid or inhibit the growth of microorganisms that destroy the product and to avoid pathogenic microorganisms that cause diseases.
Microorganisms need water in order to grow. They absorb water via the outer layer of the cell. If the concentration of sugar in food is raised to a certain level, all water is bound by the sugar. This inhibits the growth of microorganisms, because the availability of water is reduced and water activity drops.
Adding sugar to the solution increases the osmotic pressure, thereby reducing the opportunities for microorganisms to grow. By creating the most unfavourable combination of e.g. pH, water activity and temperature for microorganisms, it is possible to reduce the proportion of preservatives. Sugar can play an important role in this.
Since sugar binds water, adding sugar reduces water activity. The less water activity there is, the longer the product will last. Microorganisms can only live in “free” water. If the water activity level is low enough, microorganisms cannot destroy the food.
A sugar level of 500 g per kg of berries or pulp is equivalent to a water activity of 0.97, while a sugar level of 1 kg per kg of berries or pulp reduces the water activity to 0.92. When the mixture is boiled, this proportion of sugar inhibits moulds.