Understanding the basis of the slow starch digestion characteristic of sorghum porridges and how to manipulate starch digestion rate
Abstract
The overall goal of this project was to determine the factors causing low starch digestibility or slow starch digestion characteristic of cooked sorghum porridges as opposed to other cereal porridges and their mode of action. First, the roles of major non-starch flour components (protein, fat, dietary fiber) in reduced starch digestibility were investigated by removal of individual components followed by estimation of increase in starch digestibility. All the components contributed somewhat to low starch digestibility, but protein was found to be the single largest contributor. The second study was designed to determine the mode of action by which protein affects starch digestibility. α-Amylase inhibitor assays revealed presence of inhibition activity that diminished with cooking suggesting that the inhibitors are heat-labile and inconsequential to the low digestibility characteristic. Microstructural analysis of flour samples using confocal laser scanning microscopy revealed profound changes in protein microstructure due to cooking. Sorghum proteins formed extended web-like and rigid sheet-like structures that entrapped the starch while maize and rice proteins formed large aggregate structures that seemed to collapse to release gelatinized starch. These results suggest that encapsulation of starch within the sheet-like and web-like structures results in reduced accessibility of starch to degrading enzymes, hence the lower starch digestibility and slow starch digestion characteristic in sorghum porridges. Addition of reducing agent to sorghum during cooking resulted in disruption of the protein structures and increased starch digestibility. These results suggest that the sorghum protein structures formed during cooking are partly due to formation of disulfide bonds, hence the possibility of an oxidizing environment in sorghum flour. To investigate this hypothesis, an oxidizing agent was added to maize and rice flour during cooking. The result was formation of protein structures similar to those observed in cooked sorghum and lower starch digestibility values than those of samples cooked in water. Overall, these studies show that sorghum proteins form resilient web-like and sheet-like structures that interfere with accessibility of gelatinized starch for digestion and that these structures may be as a result of formation of intermolecular disulfide bonds forming large polymeric structures from a potentially oxidizing environment in sorghum.
Degree
Ph.D.
Advisors
Hamaker, Purdue University.
Subject Area
Food science
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