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What’s In A Food? Defining And Testing For Fiber

by Jyllian N. Kemsley
December 6, 2010 | A version of this story appeared in Volume 88, Issue 49

Inclusion Needed
Credit: Jonathan DeVries/Medallion Laboratories
Several types of fibers were outside the realm of analysis for old methods that quantitated them in foods.
Credit: Jonathan DeVries/Medallion Laboratories
Several types of fibers were outside the realm of analysis for old methods that quantitated them in foods.

Starting in the early 1990s, the Codex Alimentarius Commission—created in 1963 by the United NationsFood & Agriculture Organization and the World Health Organization to develop international food standards and guidelines—took up the task of defining “dietary fiber.” The organization finally adopted a definition last year. A concurrent effort by AOAC International led to a new analytical method to test for fiber in foods in a way that would match the test to the definition.

The new Codex definition states that dietary fiber is carbohydrate polymers that are not hydrolyzed by endogenous enzymes in the small intestine of humans. Although the definition states that the polymers must have 10 or more monomeric units, a footnote adds that national authorities can decide whether to include carbohydrates with 3–9 monomeric units. The polymers can come naturally from foods, be isolated from raw food material, or be synthesized. (The U.S. Institute of Medicine, in contrast, defines dietary fiber as something intrinsic to plants, functional fiber as isolated carbohydrates used as ingredients, and total fiber as the sum of the two.)

With the new Codex definition also came a need for a test that would quantitate fiber as defined by Codex, says Jonathan W. DeVries, a senior technical manager at Medallion Laboratories and a member of the AOAC team that developed the new method.

Previous AOAC methods for dietary fiber in foods included steps that did not reflect digestive processes, DeVries explains. The tests also didn’t capture fibers that were soluble in alcohol.

The new method, the AOAC Official Method 2009.01 for total dietary fiber in foods, starts with a 16-hour incubation of a food sample with the pancreatic enzymes α-amylase and amyloglucosidase. The incubation is designed to match digestion that would normally occur in the small intestine. The carbohydrates that are not digested by the enzymes are considered to be fiber. Fat is also released from the sample during this step.

After incubation, the α-amylase and amyloglucosidase are destroyed by the addition of base and heat, and a protease is added to digest any protein.

After the protein is broken down, alcohol is added to the sample solution. Fiber that’s insoluble in the alcohol precipitates and is filtered, washed, dried, and weighed. Alcohol-soluble fiber in the filtrate is desalted, concentrated, and quantitated by high-performance liquid chromatography. The sum of the insoluble and soluble fibers is the total amount of fiber in the food.



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