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A Critical Assessment of the Quantification of Wheat Grain Arabinoxylans Using a Phloroglucinol Colorimetric Assay

¹Department of Crop and Soil Sciences, Washington State University, Pullman, WA 99164-6376; affiliated with the USDA-ARS Western Wheat Quality Laboratory. ²Laboratory of Food Chemistry and Biochemistry, University of Leuven, Kasteelpark Arenberg 20, B-3001 Leuven, Belgium. ³U.S. Department of Agriculture (USDA), Agricultural Research Service, Western Wheat Quality Laboratory, E-202 Food Quality Bldg., Washington State University, P.O. Box 646394, Pullman, WA 99164-6394. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. ⁴Corresponding author. Phone: (509) 335-4062. Fax: (509) 335-8573. E-mail: morrisc@wsu.edu

Arabinoxylans (AX) of wheat (Triticum aestivum L.) play a critical role in processing, end-use quality, and human health and nutrition. Consequently, an efficient, accurate method of AX quantification is desirable. The objective of this work was to evaluate a standard phloroglucinol colorimetric method for quantification of wheat AX. The method is based on the formation and spectrophotometric quantification of a phloroglucide product that results from the reaction of furfural produced during the condensation of pentose sugars with phloroglucinol. Method parameters, including reaction reagents and reaction times, were varied to identify areas for improved accuracy and consistency. Phloroglucide formation at three xylose concentrations was examined over time. The optimal reaction reagents and reaction times were determined based upon improved consistency in xylose quantification. The optimized method was used on xylose and arabinose standards and on whole meal wheat samples for total and water-extractable AX content. Glucose was shown to be unnecessary in the reaction and was eliminated. A second-order polynomial equation provided a slightly better fit to the nearly linear standard xylose curve. A reduced concentration of phloroglucinol of 10% was found to give equivalent results to the standard 20%. Optimum reaction time was 25 min, and it required the inclusion of all reagents. The phloroglucide product decreased in absorbance over time such that, within the range of xylose concentration examined, about 40–50% of the colored product was lost over 100 min; however, the rate of loss was linear over time. Four operators performed the optimized method on whole wheat meal samples for total and water-extractable AX. Inter- and intraoperator variation was identified as an area requiring further study and improvement. However, all operators tended to rank the samples in a consistent manner. Compared with a gas chromatography–flame ionization detection method, the phloroglucinol method underestimated total AX by about 2.3% and water-extractable AX by about 0.08%.