Developing analytical methods to determine physicochemical properties, protein and amino acid composition of African staple foods

Abstract

Plant-derived staple foods are important sources of energy, nutrition and income in Sub-Saharan Africa as they are more accessible compared to animal sources of food. This study therefore sought to develop analytical methods to determine physicochemical properties (moisture and particle size distribution), protein and amino acid composition of five major African staple foods; namely, maize, sorghum, cassava, corn-soya and teff. Moisture was determined by Karl Fischer titration, and it was found that corn-soya, sorghum, maize, and teff had moisture levels below the recommended maximum limits. However, cassava revealed a high moisture content which would increase the risk of fungal contamination. Particle size distribution was investigated by laser diffraction which showed corn-soya and teff had coarse particles distributed within a similar size fraction. Maize, cassava and sorghum showed relatively fine particles distributed between multiple size fractions. The protein and amino acid composition was also determined to evaluate the nutritional content of staple foods, as well as to assign reference values for fit-for-purpose reference materials. To determine the amino acid composition, a 72-h hydrolysis method was developed and validated to quantify acid-stable amino acids. A separate hydrolysis protocol was optimised to quantify methionine and cysteine. The method included performic acid oxidation, followed by acid hydrolysis at 145 ⁰C for 4 h. Hydrolysates were analysed using UHPLC-UV and good recoveries between 87% and 117% were obtained. Despite several method optimisation attempts, a reproducible method could not be established for tryptophan analysis, and thus the residue was not quantified in this study. The protein content was quantified using the Dumas combustion method, and the results showed a content of 12.6 g/100 g in corn-soya, 11.3 g/100 g in sorghum, 9.53 g/100 g in teff, 8.88 g/100 g in maize, and 1.13 g/100 g in cassava. Amino acid analysis showed a sum of 12.6 g/100 g in corn-soya, 11.5 g/100 g in sorghum, 9.29 g/100 g in teff, 8.16 g/100 g in maize, and 0.660 g/100 g in cassava, which matched protein results obtained from the Dumas method. Corn-soya revealed the highest composition of essential amino acids, followed by sorghum and teff, respectively. The results for corn-soya are not surprising as the preparation is distributed by food aid initiatives to combat malnutrition. Sorghum and maize showed low concentrations of the limiting amino acids lysine and methionine. Cassava revealed poor concentrations of all amino acids, providing evidence the food source is unlikely to meet nutritional demands in regions where it serves as a primary source of nutrition.