Date of this Version
Arcila-Castillo J.A. (2014), Improving the Health Impacts of Whole Grains through Processing: Resistant Starch, Dietary Fiber Solubility, and Mineral Bioaccessibility, MS thesis, University of Nebraska-Lincoln.
Whole grains are good sources of starch, dietary fiber, and mineral elements. Starch is an important source of energy but also can be transformed to resistant starch to impart lower caloric value and increase total dietary fiber. Dietary fibers impart physiological benefits to human body once they are fermented by the gut microbiota. Mineral elements are important co-factors of a wide range of enzymes involved for instance in glucose and lipid metabolism. In general human body is capable of absorbing only 21-28% of most mineral elements, and gut bacteria are only able to ferment 34% of the dietary fiber from grain-based foods. The objective of this study was to improve the health impact of whole grains using strategies that either increase or modify the accessibility of dietary fiber and mineral elements. First, whole wheat flour was processed with cooking-freezing cycles to convert part of the native starch into resistant starch. Resistant starch is known to be a highly fermentable substrate for beneficial gut bacteria. After processing, resistant starch increased up to 8-fold in whole wheat flour and total beneficial bacterial metabolites increased up to 31% after in vitro fermentation. The second strategy was to extrude wheat bran to increase dietary fiber accessibility to gut microbiota. Combinations of moisture (15% and 30% wb) and screw speed (120 and 250 rpm) were chosen to generate severe and mild extrusion conditions. Severe extrusion conditions of 250 rpm, 15% moisture and 120°C in the barrel using a single screw extruder led to 3-fold increase in dietary fiber solubility and a 1.4-fold increase in beneficial metabolites after in vitro fermentation with human fecal microbiota. Finally, the same experimental design of extrusion of wheat bran was used to conduct a preliminary study on the effect of processing conditions on mineral solubility and phytate content. Wheat bran extrusion reduced phytate content up to 11.3% using high speed high moisture processing conditions; however, element solubility results were inconclusive. In conclusion, thermal processing of whole wheat flour and wheat bran enhanced in vitro fermentation properties associated with benefits to human health.
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