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Updating AACC Method 32-45.01 for the measurement of total dietary fiber
B. V. MCCLEARY (1), A. Draga (1), N. Sloane (1). (1) Megazyme International Ireland Ltd., Bray, County Wicklow, Ireland

The definition of dietary fiber (DF) introduced by Codex Alimentarius in 2008 includes resistant starch (RS) and optionally non-digestible oligosaccharides (NDO) of degree of polymerization of 3-9. The integrated TDF method introduced by McCleary in 2007 (AACC Method 32-45.01) appeared to satisfy this definition. However, various analytical challenges have since been identified relating to; incubation time with pancreatic alpha-amylase/amyloglucosidase; underestimation of phosphate cross-linked starch; incomplete hydrolysis of maltodextrins; underestimation of fructo-oligosaccharides (FOS); and use of sodium azide in the buffer. Literature indicates that the residence time of food in the small intestine is 4+1 h. Consequently, in the research reported here, an incubation time of 4 h was chosen, and levels of PAA (6 KCU/assay) and AMG (1.7 KU/assay) were adjusted so that the RS values obtained were in-line with those obtained in ileostomy studies for a defined set of starch samples. With these levels of PAA plus AMG, the resistant maltodextrins that are formed on hydrolysis of starches under the conditions employed in AACC Method 32-45.01 are no longer present. Also, with the new format much higher DF values are obtained for phosphate cross-linked starch (e.g. Fibersym). Problems observed in the separation of fructotriose (in FOS preparations) were resolved by employing TSK-GELR G2500PWXL for HPLC of non-digestible oligosaccharides (NDO). With the shorter incubation time of 4 h, there was no requirement for inclusion of sodium azide in the incubation buffer. Finally, sample preparation for HPLC has been simplified. In conclusion, the integrated total dietary fiber method (AACC Method 32-45.01) has been modified to remove all currently identified challenges, simply by changing the concentrations of PAA and AMG that are employed in the assay.

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