Preparation, structure, digestibility of starch spherulites with different morphologies and allomorphs

Abstract

Starch is the most important source of food energy, but information about the metabolic qualities of starchy foods is scarce. There is growing interest in understanding the relationships between digestion of starch and its effect on human health, particularly diet-related disorders such as obesity, diabetes, and cardiovascular disease. Our long-term goals are to understand how the structure, morphology and crystalline type affect the digestibility of starches, control the assembly of starch molecules for desired functional and nutritional properties, develop new starch-based ingredients and foods to improve human health, and design foods with controlled digestibility as well as desirable texture, quality, and shelf life. The specific objectives of this dissertation were to: (1) form spherulites with different allomorphs from debranched waxy potato starch; (2) produce starch spherulites with positive and negative birefringent signs from debranched high-amylose maize starch (longer chain length) and investigate the digestion mechanisms; (3) make products with an A-type crystalline structure from the debranched high-amylose maize starch by using high solid concentrations (> 25%) and heat-moisture treatment (HMT). Spherulites with a negative birefringent sign were formed from the debranched waxy potato starch (25% solids content), and the resistant content (RS) content was 74.1%. HMT changed the spherulites from B-type allomorph to A-type allomorph and reduced the RS content to 51.6%. However, due to their greater thermal stability, the RS of cooked spherulites was 33.1% while the RS content of cooked waxy potato starch was very low (1.9%). When the debranched high-amylose maize starch (25% solids content) was used, all the products exhibited a B-type allomorph, and the spherulites with positive birefringence showed high resistance to enzyme digestion. After HMT, the B-type allomorph was changed to A-type without significant effect on the morphology, and thermal stability increased. By increasing the solid content to 30-50%, products with A-type and C-type allomorphs were formed by the debranched high-amylose maize starch at higher crystallization temperatures. The RS content of the spherulites with positive birefringent sign were approximately 90%, and HMT did not significantly change the RS content. Microscopic images revealed that [alpha]-amylase hydrolyzed the out layer of a spherulite, and the birefringence sign could still be observed after digestion. Overall, this study provides significant information on the digestibility of spherulite with different structures and helps to understand how the structure, morphology and crystalline type affect the digestibility of starches.