The effect of genotype and traditional food processing methods on in-vitro protein digestibility and micronutrient profile of sorghum cooked products

Abstract

Sorghum (Sorghum bicolor (L.) Moench) is one of the principal staple for millions of people in sub-Saharan Africa serving as the main sources of protein. However, protein digestibility is low in sorghum and this may be affected by processing methods. In this study 15 sorghum cultivars and one variety each of maize (Zea maize) and tef (Eragrostis tef) all of Ethiopian origin were investigated for in-vitro protein digestibility (IVPD), activity and concentration of anti-nutritional factors and micro nutrient profile in raw flour and various cooked food samples. Kafirin composition content and composition was also determined from raw flour samples of the sorghum cultivars. IVPD was significantly different between genotypes with both maize and tef superior to sorghum both in cooked and uncooked state except for the high lysine genotype Wetet Be-gunchie. Cooking significantly reduced IVPD in all crops but had only minor effect in maize. Results revealed a highly significant interaction between genotype and food processing methods where, occasionally, genotypes with highest IVPD under one processing method ended up to be the lowest under another. Trypsin inhibitor levels had a significant and negative correlation with IVPD (r2 = 0.1), while changes in phytic acid concentration and intrinsic phytase levels during processing followed opposite trends to each other. Processing increased mineral levels by 20-44% for iron and 4-29% for zinc perhaps due to degradation of phytic acid. Results demonstrated that protein digestibility and the concentration of anti- nutritional factors varied widely depending on the food type. Identification of specific genotypes for a specific food product may help improve the nutritional quality of sorghum based foods.

Fig 1. In-vitro protein digestibility (IVPD) across food products of maize, tef and high-lysine and average of 14 normal sorghum genotypes.

Bars in each panel followed by the same letter are not significantly different.

Fig 2

Contour plots showing 10 sorghum genotypes with the highest (red dots) and lowest (yellow dots) in-vitro protein digestibility in (a) raw flour, (b) fermented flatbread, (c) porridge, (d) unleavened flatbread and, (e) dry-cooked sorghum. Axis values in each plot depict protein digestibility range observed for each cooked product and raw flour.

Fig 3

Venn diagrams showing the distribution of sorghum genotypes with (a) highest and, (b) lowest IVPD score for cooked products. The underlined cultivars are those highest or lowest IVPD score in raw flour. Tannin containing genotypes are marked with a star symbol.

Fig 4. Correlation matrix for total protein content, total kafirin content, gamma-kafirin content and IVPD broken down based on food products and raw flour.

Statistical significance at P ≤ 0.05, 0.01 and 0.001, are shown using *, **, ***, respectively.

Fig 5

Levels of anti-nutritional factors and intrinsic phytase levels in three sorghum cultivars shown to have among the highest IVPD in raw flour as well as in three or more cooked products: (a) phytic acid concentration, (b) phytase concentration and, (c) trypsin inhibitor levels.

Fig 6. Correlation between IVPD, anti-nutritional factors (phytic acid and trypsin inhibitor), phytase concentration, and micronutrient content in raw flour samples of fifteen sorghum cultivars.

*, **, ***, = statistically significant at P ≤ 0.05, 0.01 and 0.001, levels of probability, respectively.