How ordinary starch becomes resistant

Starch is a chain of glucose units held together almost entirely by one type of bond, called an alpha-1,4 glycosidic bond. The human small intestine is very good at breaking that specific bond, which is why starchy food is digested and absorbed quickly.

To make resistant dextrin, manufacturers heat starch to a high temperature with a small amount of acid, typically hydrochloric or citric acid, in a process called dextrinisation or pyroconversion. The heat and acid break some of the original bonds and reform them in different configurations, including alpha-1,6, alpha-1,2, beta-1,6 and beta-1,2 linkages. Human digestive enzymes are built to recognise the original alpha-1,4 bond and largely cannot act on these new ones. The result is a starch-derived molecule that passes through the small intestine mostly intact, the defining behaviour of a dietary fibre.

Where it comes from and how it's made

Resistant dextrin can be produced from a range of starch sources, most commonly corn, potato or tapioca. Manufacturing research categorises the resulting ingredients three ways, according to how they arise: dextrins deliberately manufactured as a fibre ingredient, traditional maltodextrins that happen to carry resistant bonds as a side effect of their structure, and resistant dextrin that appears unintentionally during other starch processing, such as extrusion. What reaches a food label as "resistant dextrin" or "resistant maltodextrin" is almost always the first category, made on purpose for its fibre content.

How FSANZ classifies it as dietary fibre

Under the Australia New Zealand Food Standards Code, dietary fibre is defined as material resistant to digestion and absorption in the small intestine. Meeting that definition on paper is one thing. Measuring it accurately in a finished food is another, and for a long time the standard laboratory method for dietary fibre undercounted resistant dextrin, because it wasn't designed to detect the new bond types created during dextrinisation.

Food Standards Australia New Zealand addressed this directly through Application A491, Resistant Maltodextrin as Dietary Fibre, finalised in August 2004. The application added a specific analytical method, AOAC 2001.03, to the table of approved dietary fibre test methods in Standard 1.2.8, Nutrition Information Requirements. That method measures resistant maltodextrin accurately, which is what allows a food or drink containing it to count that content toward the dietary fibre figure declared on its nutrition information panel.

This followed the same pattern set a few years earlier for inulin and fructo-oligosaccharides, which gained recognition as dietary fibre under Application A277 in 2000, also by adding new approved test methods to the Code rather than changing the definition of fibre itself.

How it behaves in food and drink

Resistant dextrin's main practical advantage in manufacturing is solubility. Research on resistant dextrins produced from potato starch found water solubility as high as 74% at 20 degrees Celsius, well above many other fibre ingredients, which is why it dissolves cleanly into liquids rather than settling or clouding. It is generally low in viscosity, meaning it doesn't thicken or gel the way psyllium or inulin can, which is one reason it turns up in beverages, sauces and baked goods where a manufacturer wants to add fibre without changing texture.

Resistant dextrin isn't one fixed chemical structure. FSANZ's own assessment describes it as poorly defined at the molecular level, identified more by how it behaves, resisting digestion, than by a single consistent formula.

A note on labelling

Resistant dextrin is sold commercially under several different trade names, each a specific manufacturer's version of the ingredient. On an Australian ingredient list it may appear as "resistant dextrin," "resistant maltodextrin," or simply as a contributor to the "dietary fibre" figure in the nutrition information panel, depending on how the manufacturer has chosen to declare it.

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