In-vitro Protein Digestibility, Phytic Acid and Phytase Concentrations in Sorghum as Affected by Genotype and Food Processing Method.

Numerous factors both internal and external to the protein bodies are known to affect digestibility of sorghum proteins. Phytic acid and protease inhibitors are among important external factors that impact protein digestibility. The objectives of this study were to determine the effect of genotype and food processing methods on the activity of these anti-nutritional factors, and their effect on in-vitro protein digestibility (IVPD) and mineral nutrient content. Ground samples of four sorghum food products, porridge, popped sorghum, dry roast, fermented flatbread, and unleavened flatbread from 15 sorghum and one each of maize and teff genotypes of Ethiopian origin were analyzed for IVPD, phytic acid concentration (PA), phytase activity (PTA), trypsin inhibitor activity (TIA) and, iron and zinc content. Raw flour samples of each genotype were also included as checks. Results showed significant genotype by food product interaction for all parameters except for iron and zinc. IVPD was significantly and negatively correlated (r²=0.1) with TIA, but was not correlated with either PA or PTA. The across genotype IVPD was highest in the raw flour followed by popped grain and fermented bread while porridge and the unleavened bread were the least. Phytic acid was also highest in the raw flour and least in fermented bread that also showed highest phytase activity across genotypes. Among genotypes, the high-lysine cultivar Wetet-begunchie showed the highest IVPD and PA across food products but also had the highest PTA. However, as compared to normal sorghum genotypes with high IVPD, the high-lysine cultivar showed significant disparity in digestibility across food types. Generally, processes that involve fermentation appear to improve digestibility in normal sorghum. Results suggest protein digestibility in cooked foods can be improved through selection of genotypes for specific food products and optimizing food processing methods. Yet, towards breeding new cultivars, efforts are needed to understand the genetic basis of many factors affecting protein digestibility.