EVALUATING CEREAL STARCH AND PROTEIN DIGESTIBILITY
Grain Quality and Structure Research Unit
2012 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: The rate and extent of granular starch digestion are related to its nutritional properties. Slowly digrestible 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. To determine the rapidly digestible starch, the addition of amylogulcosidase is used to convert hydrolyzates from alpha-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 alpha-amylase addition, less resistant starch was produced than without amylogulcosidase 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 alpha-amylase and burrowing with amyloglucosidase as well as the degree of digestion between samples. To understand roles of amyloglucoisdase and alpha-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.
MEASUREMENT OF SORGHUM PHYTATE CONTENT: Phytate decreases sorghum protein digestibility through the formation of pepsin resistant protein-phytate complexes. Addition of the phytate degrading enzyme phytase to animal diets results in a significant increase in protein utilization especially in poultry. In order to determine the distribution and content of phytate in sorghum a subset of the KSU sorghum diversity panel was used. A representative sample set of 46 sorghum lines composed of 28 non-tannin and 18 tannin containing samples were examined. Phytate contents ranged from 0.55 to 9.28 mg/g with an average of 5.45 mg/g. No significant difference was observed between non-tannin samples which averaged 5.05 mg/g and tannin samples that averaged 5.88 mg/g.