Inulin, FOS and GOS: chain length is what you are actually specifying

Two inulins at the same declared purity can behave nothing alike in a formulation. The parameter that separates them is chain length, and it is often missing from the quote entirely.

Degree of polymerisation

Degree of polymerisation (DP) is the number of sugar units in the chain. For chicory-derived fibres it typically spans DP 2 to 60, and where a given material sits in that range drives almost everything a formulator cares about:

• Short chain (DP 2–9, generally sold as FOS or oligofructose) — more soluble, noticeably sweet, around 30–35% the sweetness of sucrose. Useful where you want fibre and some sugar replacement together.
• Standard inulin (DP 2–60 as extracted) — the general-purpose material, mildly sweet.
• Long chain (DP ≥ 10, sold as HP or high-performance inulin) — barely sweet, less soluble, and forms a gel that is used to mimic fat in dairy and bakery applications.

Specifying "inulin, 90% purity" without a DP range leaves the most important property of the ingredient to chance.

Process tolerance

Fructans hydrolyse under heat in acid conditions. A fibre that survives a neutral-pH bake may be substantially degraded in an acidic beverage held hot during filling — and degraded fructan is free fructose, which changes both the sugar declaration and the fibre claim. If your process has a low-pH hot step, ask for hydrolysis data at your actual pH, temperature and hold time rather than a general stability statement.

Measuring fibre content

Fructans are not fully captured by older total dietary fibre methods. AOAC 997.08 and 999.03 are the fructan-specific methods, and a fibre figure is only meaningful with the method named. This matters directly for nutrition labelling, where the declared fibre content has to be defensible.

What to put on the specification

• DP range and average DP, not just the fibre percentage.
• Fibre content with the AOAC method number.
• Free sugars — glucose, fructose and sucrose, individually. These carry into your sugar declaration.
• Source — chicory root, agave or enzymatically synthesised from sucrose, which affects both labelling and non-GMO status.
• Solubility and pH in solution, at the concentration you will actually use.

Send a sample through your own process

Prebiotic fibres are the ingredient class where paper specifications diverge most often from real behaviour. A pilot run through your actual thermal profile tells you more than any datasheet, and it is worth doing before committing to a container.