Teferra, Tadesse Fikre (2019-05). Effect of Combining Highly Digestible Protein and Waxy Starch Traits on Sorghum Endosperm Functionality and Protein Digestibility. Doctoral Dissertation.
Sorghum is known for its resilience to climate change and suitability to arid areas, having several potential health benefits such as controlling blood glucose and inflammation. However, functionality of sorghum as a food ingredient is limited due to the hydrophobic proteins. Sorghum mutant with a highly digestible (HD) protein (irregularly shaped) has potential for food application. The goal of this research was to establish the relationship between combined HD - waxy starch in hard endosperm sorghum on its functionality and protein digestibility. First, the suitability of the standard in vitro pepsin assay to identify the HD protein traits in hard endosperm sorghum was checked. The pepsin assay was not reliable in distinguishing the HD and regular low digestible (LD) protein as the digestibility was not consistent across locations and year. For the first time, field emission scanning electron microscope (FE-SEM) was effective to qualitatively identifying the HD protein body. The physicochemical properties of improved HD sorghum lines in waxy versus normal starch background were compared to the LD counterparts. HD-waxy combinations had better functionality in terms of water solubility (%) (7.10 versus 4.68 for normal LD), pasting properties (higher peak (more starch swelling) and lower final (slower retrogradation) viscosities), that indicated better interaction with water. The HD sorghum also had higher lysine content (2.15%) compared to the wild LD control (1.47% of total protein). Finally, the improved sorghum lines with the HD protein traits were tested in model foods in comparison with the LD ones and wheat controls. Pancake, cookie and bread were selected as model foods. The HD-normal gave pancakes with higher diameter (mm, 89.6 versus 87.5 for the LD normal) as well as higher moisture (% db, 45.2 versus 41.2 for the LD-normal) at 100% sorghum, most likely due to higher level of water absorption by the HD-protein. The same treatment also resulted in cookies of lower hardness (g, 1715 versus 4705 for the LD-normal). The HD sorghum resulted in denser bread crumb with faster firming, likely due to retained moisture migrating during storage (staling). The implication is that the improved HD-normal sorghum gave pancakes and cookies with desirable attributes making it an appealing ingredient in batter-based products as a partial substitute for wheat, or as gluten-free healthy grain with improved processing and nutritional qualities.