What It Is and Where It Comes From
FOS is built from fructose units joined by a specific type of chemical bond (β-(2,1) linkages), the same bond type found in inulin. The difference is chain length: FOS chains typically have fewer than ten fructose units, while inulin chains run longer. Both are most commonly sourced from chicory root, with sugar beet as another source.
How It's Made
FOS is produced in one of two ways. The first is partial enzymatic hydrolysis of inulin, using an enzyme (inulinase) to break the longer inulin chains down into shorter fragments, typically averaging around four fructose units per chain once the process is complete. The second is enzymatic synthesis directly from sucrose, using a different enzyme (fructosyltransferase) to build short fructose chains onto a sucrose molecule rather than breaking a longer chain down. Both routes produce a similar end result: a short-chain fructan with broadly similar properties to inulin, though the exact chain-length distribution can differ depending on which process and which starting material was used.
This matters for labelling and sourcing: an ingredient made by hydrolysing chicory inulin and one made by building chains onto sugar beet sucrose can both legitimately be called FOS, even though they started from different raw materials and took different manufacturing routes to get to a similar finished product.
How It Behaves in Food and Drink
FOS dissolves in water and has a noticeably mild sweetness of its own, well below table sugar, which is part of why it's used in some products as both a fibre source and a partial sweetener. It doesn't gel or thicken the way psyllium does; it behaves more like inulin, dispersing into liquid with relatively little change to texture.
How It's Classified as Dietary Fibre in Australia
FOS is explicitly named in Schedule 11 of the Food Standards Code, which sets out AOAC method 997.08 for measuring "inulin and fructooligosaccharide" together as a combined category, alongside a separate method (999.03) for inulin specifically. This means FOS has clear, established recognition as dietary fibre under the Code, grouped with inulin rather than requiring its own standalone pathway.
A Note on Naming
FOS appears on labels under a few different names: "fructo-oligosaccharides," "FOS," "oligofructose," and sometimes "short-chain fructooligosaccharides" to distinguish it from inulin specifically. These generally refer to the same category of short-chain fructan, though exact chain length and purity can vary between manufacturers and production batches. Where a product lists a specific brand name for its FOS ingredient, that's a trade name layered on top of the generic ingredient, not a different substance.
FODMAP Status
FOS is a fructan, one of the carbohydrate groups the FODMAP framework measures directly, so it's high FODMAP by definition rather than by individual product testing. See our page on which fibre supplements are low FODMAP for how this compares to other common fibre ingredients.
Sources
- Australia New Zealand Food Standards Code, Schedule 11, Calculation of Values for Nutrition Information Panel
- Australia New Zealand Food Standards Code, Standard 1.2.8, Nutrition Information Requirements