EVALUATING CEREAL STARCH AND PROTEIN DIGESTIBILITY
Grain Quality and Structure Research Unit
2010 Annual Report
1a.Objectives (from AD-416)
The objectives of this research are: (1) Evaluate starch and protein digestibility of select sorghum lines and (2) Develop rapid reference methods to assess amylose-amylopectin ratios as well as amylopectin branch ratios in wheat and sorghum starch.
1b.Approach (from AD-416)
Sorghum samples currently used for development of gluten-free foods will be analyzed for starch and protein digestibility using conventional pepsin digestion (protein) and the Englyst method (starch). The results will be used to evaluate rapid methods using specific labeling, gel permeation chromatography with pulsed amperometric detection (PAD) for determination of amylose amylopectin ratios and amylopectin branching. Digestibility will be performed using rapid fluoremetric methods.
MECHANISM AND IN VITRO TEST METHOD OF DIGESTION OF GRANULAR STARCHES WITH DIFFERENT AMYLOSE CONTENTS: Investigate the exact role of amyloglucosidase in determining the digestibility of starch and understand the mechanism of enzymatic actions on starch granules. Starch digestion rate and extent are related to its nutritional property. Slowly digestible starch (SDS) and enzyme resistant starch (RS) are desirable. Type 2 resistant starch is a granular starch that, in portion or whole, escapes the small intestine undigested. Both rate and extent of starch digestibility are important. To determine the rapidly digestible starch, the addition of amyloglucosidase is used to convert hydrolyzates from a-amylase digestion to glucose. Four maize starches differing in amylose content: waxy maize (0% amylose), normal maize (˜26% amylose) and two high amylose starches (˜50% and ˜ 70% amylose) were examined. Without amyloglucosidase addition, the outcome of the expected slowly digestible starch fraction was 20% lower, while the resistant starch increased with increasing amylose content. In the method without a-amylase addition, less resistant starch was produced than without amyloglucosidase added, except in maize at 70% amylose content. The molecular weight distributions of the digestive residues were compared with gel permeation chromatography (GPC). Scanning electron microscopy (SEM) revealed the digestive patterns of pinholes with a-amylase and burrowing with amyloglucosidase as well as the degree of digestion between samples. To understand roles of amyloglucoisdase and a-amylase in the in vitro test, multiple analytical techniques were used, showing that amyloglucosidase has a significant impact on SDS and RS content of granular maize starches.
ENDOSPERM PROTEIN CHARACTERISTICS IN NORMAL AND HARD WAXY WHEAT: Investigate the composition and solubility of hard waxy wheat flour proteins and their changes from flour to dough. Six waxy wheat flours, one normal wheat flour and one partial waxy wheat flour were examined. Flours were also analyzed for their mixing and biaxial extensional properties. Waxy wheat flour had higher water absorption and lower mixing time than did normal wheat flour. Biaxial extension properties classify a dough from waxy wheat flour as in-elastic. Free thiol content, protein composition of flour and dough were analyzed. Flour proteins (i.e. gliadins and glutenins) extracted using various solvents and sonication, were analyzed using size exclusion high performance chromatography. Waxy wheat flours had higher free thiol content, less of 50% propanol soluble proteins and lower soluble polymeric proteins than did normal wheat flour. Additionally, waxy wheat flours had higher residual protein after detergent and sonication extraction. The measurable gliadin content of waxy wheat samples significantly decreased from flour to dough, while remaining constant in normal wheat. Waxy starch affects protein hydration but not protein extractability after optimum dough mixing. The presence of high non protein free thiol content and some gliadins acting as chain terminators could be the underlying reason for waxy wheat flours producing slack dough.