Abstract
A rapid polarimetric method applicable to whole wheat, granular, and patent flour is described in which the starch is dissolved in hot calcium chloride solution under uniform conditions, soluble proteins being removed by the addition of stannic chloride or uranyl acetate prior to volume adjustment, filtration, and polarization.Approximately 2.5-gm. samples (40 mesh) are placed in tall beakers and suspended uniformly in 10 cc. of water. Calcium chloride solution (60 cc., d. 1.30, pH 5.5) and 2 cc. of 0.8% acetic acid are added. The mixture is boiled at constant salt concentration for 15 min., foaming being prevented by dropwise addition of n-octanol as required. After cooling, 2.5 to 5.0 cc. of 4% stannic chloride or 10 cc. of 5% uranyl acetate (each dissolved in calcium chloride solution) is added, the mixture is diluted to 100 cc. with calcium chloride solution, mixed, and filtered. After discarding the first portion, the filtrate is polarized in 2 dm. tubes. [α]D = + 203° is used in the calculations; % starch = 2 dm. reading in degrees × 10 when 2.463 gm. flour samples are employed.Stannic chloride is shown to depress the specific rotation value of starches. A large error is introduced when it is used on wheat flour in the amount prescribed by Mannich and Lenz. Using the above uranyl acetate treatment, starch may be rapidly determined in corn, barley, rye, rice, grain sorghum, and buckwheat by the calcium chloride polarimetric method. It is shown that interfering wheat proteins may be removed by preparatory extraction with aqueous ethanol and dilute sodium hydroxide, as well as by precipitation. The latter being more rapid is judged most suitable for routine applications. Levorotatory hemicelluloses of bran and endosperm are dissolved in considerable amount by boiling calcium chloride solution but their small effect is balanced by the error of overestimation arising from insoluble wheat solids. Hemicelluloses are shown to be capable of causing high results in applications of the diastase–hydrochloric-acid method to wheat products. The improved calcium chloride polarimetric method is considered the most reliable of existing macro-methods for the determination of starch in wheat.
Published Version
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