Fructooligosaccharides (FOS) also sometimes called oligofructose or oligofructan, are oligosaccharide fructans, used as an alternative sweetener. FOS exhibits sweetness levels between 30 and 50 percent of sugar in commercially prepared syrups.[unreliable source?] It occurs naturally, and its commercial use emerged in the 1980s in response to consumer demand for healthier and calorie-reduced foods.
FOS can be produced by degradation of inulin, or polyfructose, a polymer of D-fructose residues linked by β(2→1) bonds with a terminal α(1→2) linked D-glucose. The degree of polymerization of inulin ranges from 10 to 60. Inulin can be degraded enzymatically or chemically to a mixture of oligosaccharides with the general structure Glu-Frun (abbrev. GFn) and Frum (Fm), with n and m ranging from 1 to 7. This process also occurs to some extent in nature, and these oligosaccharides can be found in a large number of plants, especially in Jerusalem artichoke, chicory and the Blue Agave plant. The main components of commercial products are kestose (GF2), nystose (GF3), fructosylnystose (GF4), bifurcose (GF3), inulobiose (F2), inulotriose (F3), and inulotetraose (F4).
The second class of FOS is prepared by the transfructosylation action of a β-fructosidase of Aspergillus niger or Aspergillus  on sucrose. The resulting mixture has the general formula of GFn, with n ranging from 1 to 5. Contrary to the inulin-derived FOS, not only is there β(1→2) binding but other linkages do occur, however, in limited numbers.
Because of the configuration of their glycosidic bonds, fructooligosaccharides resist hydrolysis by salivary and intestinal digestive enzymes. In the colon they are fermented by anaerobic bacteria. In other words, they have a lower caloric value, while contributing to the dietary fiber fraction of the diet. Fructooligosaccharides are more soluble than inulins and are, therefore, sometimes used as an additive to yogurt and other (dairy) products. Fructooligosaccharides are used specially in combination with high-intensity artificial sweeteners, whose sweetness profile and aftertaste it improves.
FOS is extracted from the blue Agave plant as well as fruits and vegetables such as bananas, onions, chicory root, garlic, asparagus, jícama, and leeks. Some grains and cereals, such as wheat and barley, also contain FOS. The Jerusalem artichoke and its relative yacón together with the Blue Agave plant have been found to have the highest concentrations of FOS of cultured plants.
FOS has been a popular sweetener in Japan for many years, even before 1990, when the Japanese government installed a "Functionalized Food Study Committee" of 22 experts to start to regulate "special nutrition foods or functional foods" that contain the categories of fortified foods (e.g., vitamin-fortified wheat flour),[unreliable source?] and is now becoming increasingly popular in Western cultures for its prebiotic effects. FOS serves as a substrate for microflora in the large intestine, increasing the overall gastrointestinal tract (GI Tract) health. It has also been proposed as a supplement for treating yeast infections.
Several studies have found that FOS and inulin promote calcium absorption in both the animal and the human gut. The intestinal microflora in the lower gut can ferment FOS, which results in a reduced pH. Calcium is more soluble in acid, and, therefore, more of it comes out of food and is available to move from the gut into the bloodstream.
FOS can be considered a small dietary fibre with (like all types of fibre) low caloric value. The fermentation of FOS results in the production of gases and acids. The latter provide some energy to the body.
All inulin-type prebiotics, including FOS, are generally thought to stimulate the growth of Bifidobacteria species. Bifidobacteria are considered beneficial bacteria. This effect has not been uniformly found in all studies, either for Bifidobacteria or for other gut organisms.[unreliable source?] FOS are also fermented by numerous bacterial species in the intestine, including Klebsiella, E. coli and many Clostridium species, which can be pathogenic in the gut. These species are responsible mainly for the gas formation (hydrogen and carbon dioxide), which results after ingestion of FOS. Studies have shown that up to 20 grams/day is well tolerated.
The Food Safety Authority warned parents of babies that a major European Baby-Formula brand made in New Zealand does not comply with local regulations, (because it contains fructo-oligosaccharides (FOS)) and urged them to stop using it.
FOS use has been approved in the European Union; allowing addition of FOS in restricted amounts to baby formula (for babies up to six months) and follow-on formula (for babies between six and 12 months). Infant and follow-on formula products containing FOS have been sold in the EU since 1999.
FOS is currently not approved for use in baby formula.