Sugar in high concentrations acts as a preservative due to osmosis. Sugar attracts all available water and water is transferred from the microorganisms into the concentrated sugar syrup. The microflora is dehydrated and cannot multiply further.
The concentration of sugar in sugar preserved products must be 68 per cent or more, which does not allow microorganisms to grow. Lower concentrations may be effective but for short duration unless the foods contain acid or they are refrigerated. The critical concentration of sugar required to prevent microbial growth varies with the type of microorganisms and the presence of other food constituents.
Some of the most popular preserves with sugar are jelly, jam and marmalade. These are the stable gels. Pectin, a natural component of fruits, forms a gel only in the presence of sugar and acid. Sugar prevents spoilage of jams, jellies, and preserves even after the container is opened.
With the use of sugar, the water activity cannot be reduced below 0.845. This level of water activity is sufficient to inhibit mesophillic bacteria and yeast but does not check mould attack. Due to this, various other methods are also adopted to prevent mould development in sugar preserved products like finished product pasteurization (jams, jellies, etc.) and use of chemical preservatives in order to obtain the antiseptisation of the product surface.
Most of these products are made of acid fruits. When foods low in acid are used, they are usually combined with some acid fruit. Besides contributing flavour to the product, the acid aids in the preservation. The amount of sugar used in manufacture of these products varies widely.