Fructooligosaccharides (FOS) are short-chain oligosaccharides composed primarily of fructose units linked by β-(2→1) glycosidic bonds, typically with a terminal glucose residue. They occur naturally in plants such as chicory, onions, and bananas. FOS function as prebiotic fibers, selectively stimulating beneficial gut bacteria like Bifidobacteria and Lactobacilli. They exhibit mild sweetness (30-50% of sucrose) and are used in functional foods for their low caloric value (1.5 kcal/g) and ability to improve texture and moisture retention. Chemically stable under acidic conditions but susceptible to hydrolysis at high temperatures. FOS are non-digestible in the human small intestine, reaching the colon intact where they are fermented to produce short-chain fatty acids.