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Replacement of acetate with ammonium buffer to determine apparent amylose content of milled rice

Juliano B.O., Tuaño A.P.P., Monteroso D.N., Aoki N., Mestres C., Duldulao J.B.A., Bergonio K.B.. 2012. Cereal Foods World, 57 (1) : p. 14-19.

DOI: 10.1094/CFW-57-1-0014

An improved iodine colorimetric method based on ammonium-buffered pH 9 solution (NH4Cl-KI) is reported. The method produces blue color and measures apparent amylose contents (AC) of nonwaxy rice, using potato amylose standard, and results in values similar to those observed when using differential scanning calorimetry (DSC), based on the exotherm of amylose-lipid complex formation, which has no amylopectin and lipid interference. Using the iodine colorimetric method, waxy rice had AC values of 2-3%, while undefatted, milled rice samples had AC values similar to those observed using the DSC method, probably because interference from amylopectin-iodine and amylose-lipid complexes cancelled each other. Using the iodine-NH4I method, AC values for five nonwaxy rice varieties were identical to DSC values, while AC values for the iodine colorimetric method with acetate buffer were not identical to DSC values when amylose alone was used as the standard. Using an amylose-waxy rice standard, the ammonium buffer method resulted in 4 nonwaxy rice varieties with AC values identical to DSC values. Iodine-NH4I resulted in 14 AC values identical to DSC values, and AACC International Approved Method 61-03.01 resulted in 7 of 16 rice samples with AC values identical to DSC values. AC determined using DSC and iodine colorimetry, both based on amylose helical complexes, were higher than AC (true amylose plus long-chain amylopectin) determined using high-performance size-exclusion chromatography (HPSEC) of debranched starch and amylopectin. The ammonium buffer method produced a blue color, AC values for nonwaxy rice that were similar to those for the DSC method, and allowed use of undefatted rice samples and an amylose standard alone without waxy rice (amylopectin). (Résumé d'auteur)

Mots-clés : riz; amylose; teneur en glucides; colorimétrie; ammonium; acétate; iode; philippines

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