Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: February 1, 1996
Publication Date: N/A
The chemical composition of rice determines its nutritional and processing characteristics. The rice grain is composed almost entirely of starch which is made up of amylose and amylopectin. Starch is made up of thousands of glucose molecules bonded together in a linear chain by a-1,4 bonds or in a branched chain by a-1,6 bonds. Chromatographic instruments that utilize computer analysis of light scattered from a laser source (MALLS) have become available to determine the molecular weight of large polymers like starch. Other recent innovations such as anion exchange columns (AEC) that can operate under extreme alkaline conditions (1 N NaOH) where carbohydrates become negatively charged can be used to separate glucose chains of from 1-50 units. These two complementary methods were used to determine the branch chain structure of amylopectin. Rice samples ranging from 0 to 31% apparent amylose were analyzed for branch chain length. The most common branch chain length was 12 glucose units. These are A chains o chains without branch points. A continuum of glucose oligomers out to about 50 glucose units could be separated. These include polymers that are not completely debranched by the enzyme either due to steric hindrance or short A chains that are not recognized by the enzyme. The average molecular weight (MW) for rice varieties having up to 25% apparent amylose was about 4000 daltons. Above 25% amylose, the MW tended to rise. This suggests that the amount of true amylose may be limited to 25% or less and higher apparent amylose is the result of amylopectin with branch chains of sufficient length to complex with iodine and give a false apparent amylose reading. There were no correlations between average MW of branch chains with gelatinization temperature or amylographic profile.