Fructooligosaccharides (FOS) are short-chain oligosaccharides consisting of fructose units linked by β(2→1) bonds, typically with a terminal glucose residue. They occur naturally in plants such as chicory root, onions, and bananas. FOS function as prebiotic dietary fibers that are not digested in the small intestine but are fermented by beneficial gut bacteria in the colon, promoting the growth of Bifidobacteria and Lactobacilli. They are used as low-calorie sweeteners and functional food ingredients to improve digestive health, modulate blood glucose, and enhance mineral absorption. Chemically stable under acidic conditions but can undergo hydrolysis at high temperatures. Average degree of polymerization ranges from 2 to 10.