Modified method for apparent amylose content (AC) of milled rice
1988
Perez, C.M. | Juliano, B.O. (International Rice Research Inst., Los Banos, Laguna (Philippines). Chemistry Dept.)
Colorimetric amylose assay based on amylose standard alone overestimates AC [amylose content] of milled rice because of amylopectin-iodine complex interference at acid pH. Addition of amylopectin (waxy rice starch 0.6% amylose) to amylose values, particularly for high amylose rice. These values are probably close to true AC since the potato amylose standard is only 80% pure. Amylose values, however, are high-iodine binding amylopectin; thus apparent AC of high-amylose rice starches is 25-26%. High-amylose rice amylopectin has iodine complexing properties different from waxy rice amylopectin, while waxy and low-amylose rice amylopectins are similar in branching properties. Because classification of milled rice AC into low (10-20 %), intermediate (20-25%), and high (25%) is well accepted and documented, maintaining such classification was proposed but calling it apparent AC. These values can be obtained for defatted rice flour in the amylose, amylopectin standard method by adjusting starch level (amylose + amylopectin) from 90% to 70% of the milled rice weight. Adding a constant correction factor, 1.4% to all amylose values based on the 90% starch standard maintains the above classification scheme. With these method calibration is at pH 4.5 - 4.8 in acetate buffer, reducing errors inherent in the Williams procedure at alkaline pH. Excess iodine characterizes the acid pH procedure as reflected in the greenish amylose-iodine solution. Defatting with refluxing 95% ethanol gives more accurate apparent amylose values than using undefatted samples, because fat content of samples are not necessarily identical at similar milling conditions.
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