Studies on Alkali Decomposition of Rice Kernel : 3. A new alkali test using rice flour

Abstract

Method for eveluating the alkali digestibility of rice flour was investigated. And a rapid alkali test could be done by measuring light transmission rate of alkali-treated flour paste with a modified turvidmeter. The procedure of test is as follows. 500 mg sample of rice flour is put in a flat-bottomed glass container 50 mm in internal diameter. After being wetted wits 1.2ml of distilled water, the sample is added with 25ml of 2.2% potassium hydroxide solution. The flour-alkalii suspension is then stirred magnetically at 120 r. p. m. for 10 minutes on a water bath which is also stirred magnetically and regulated at 40±0.5°C. The paste is cooled with runnihg water to room temperature and set in the measuring apparatus together with glass container. A modified turvidimeter having a light beam of vertical direction was used for measuring light transmission of the paste. Alkali decomposition value is represented by percent light transmission. The procedure can be satisfactorily applied to rice starch sample as well. Compound starch granules in alkali-treated starch pastes of various light transmission rates were observed through a phase-different microscope. And a close relation was revealed between alkali decomposition value and disintegration of starch granules. Rate of light transmission or alkali decomposition value, therefore, may be said to be an adequate indicator for evalualuating the alkali digestibility of pulverized rice whose major component is starch. Rice flour pulverized finer than 120 mesh or coarser than 40 mesh considerably affected the decomposition value. So the test should be done on a fixed mesh of flour within the range of 40 to 120 mesh.