Sugar acts as a preservative because it causes bacteria to lose water, hampering their ability to live and propagate in the preserved food. Food In Jars reveals that sugar also improves the set, or firmness, of jams, jellies and fruit butters. Many recipes for preserved fruits, vegetables and meats combine sugar with salt, which also inhibits bacterial growth. Both sugar and salt also inhibit mold growth and fungal development.
Honey, agave nectar, Splenda and other sugar substitutes are all effective sweeteners, but Food In Jars explains that preserves made with real sugar last much longer than those sweetened with anything else. Sugar substitutes are ideal for short-term preserving, particularly for diabetic-friendly foods with low glycemic indexes.
Although sugar effectively inhibits the growth of many bacterial species, it has no effect on Clostridium botulinum, the anaerobic bacterium responsible for the deadly disease called botulism. According to the Centers for Disease Control and Prevention, acid is lethal to this bacterium, but sugar has no effect upon it. To minimize the risk of Clostridium botulinum growth in preserved foods, always follow the packing, sealing and processing instructions in each recipe.
According to Scientific American, Clostridium botulinum is not the only microorganism capable of thriving in sugar-rich preserves. Several strains in the Zygosaccharomyces yeast family also flourish in sugary preserves. These yeasts have a particular affinity for fruit juice concentrates and syrups.