Impact of cowpea addition on the Protein Digestibility Corrected Amino Acid Score and other protein quality parameters of traditional African foods made from non-tannin and tannin sorghum

Abstract

Protein malnutrition is a problem in Africa where sorghum is a staple foodstuff. Improvement in the protein quality of traditional African sorghum (Sorghum bicolor L. Moench) foods through the addition of cowpea (Vigna unguiculata L. Walp), an indigenous African legume, was investigated. Two sorghum cultivars, a red, tannin-type (NS 5511) and a white tan-plant, non-tannin type (Orbit) were complemented with cowpea (70:30 ratio). Ugali (thick porridge), uji (fermented thin porridge) and injera (fermented flatbread) were prepared. The protein lysine scores of cowpea-complemented foods were about double the levels of sorghum-only foods. The in vitro protein digestibility of the foods increased by 13–62%. The increase in lysine and protein digestibility improvement resulted in three- and two-fold improvement in the Protein Digestibility Corrected Amino Acid Score (PDCAAS) of NS 5511, and Orbit foods, respectively. Addition of cowpea to tannin, as well as non-tannin sorghum is a viable option for improving the protein quality of a wide range of traditional African foods.

Introduction

Protein malnutrition is a serious problem in Africa where sorghum is a staple food. The nutrient content of sorghum grain is generally similar to other cereals (Food and Agriculture Organization (FAO), 1995). However, the lysine content is particularly low because the kafirin storage proteins are very low in lysine. For example, Taylor and Schüssler (1986) studying the protein composition of different parts of the sorghum grain reported an average of about 2% lysine in kafirin, while the recommended value for a 1–2 year-old child (WHO/FAO/UNU Expert Consultation, 2007) is 5.2%. In addition, the digestibility of sorghum proteins decreases especially on wet cooking (Duodu, Taylor, Belton, & Hamaker, 2003). The work by Maclean, De Romana, Placko, and Graham (1981) has shown that poorly digestible, high lysine sorghum proteins might not provide substantial protein nutritional benefits to children who consume sorghum. Further, the tannins in tannin-containing sorghum cultivars bind proteins (Butler, Riedl, Lebryk, & Blyit, 1984), which can further reduce the protein digestibility. It has been shown that higher sorghum tannin levels result in a greater reduction in in vivo protein digestibility of cooked high tannin sorghum compared to low tannin sorghum (Bach Knudsen, Munck, & Eggum, 1988). Therefore, sorghum-based foods require improvement to enhance their protein nutritional value. Cowpea is an important legume in the tropics, particularly in Africa. With an average of 24 g protein per 100 g, and about 7 g lysine per 100 g protein (USDA, 2009), cowpea is protein-rich. Therefore cowpea could be used to complement traditional sorghum-based foods (Pelembe, Erasmus, & Taylor, 2002).

To assess the protein nutritional adequacy achieved through cowpea addition, it is important to measure the protein quality of the foods in terms of the digestibility and biological value of the protein. This is particularly crucial because, the protein quality of the plant sources is a major concern in meeting the protein nutritional sufficiency of their consumers (Millward, 1999). The biological value of protein in a food can be predicted by the Protein Digestibility Corrected Amino Acid Score (PDCAAS) (WHO/FAO/UNU Expert Consultation, 2007). PDCAAS, which is the amino acid score (in this case the lysine score as this is the first limiting indispensable amino acid) of the food protein × digestibility, was introduced as the means of assessing the protein quality of both individual food sources and food mixtures. The reasoning is that the utilization of a protein will be limited by its digestibility, which determines the overall available amino acid nitrogen from the food (WHO/FAO/UNU Expert Consultation, 2007).

The total lysine values from the amino acid analysis do not always reflect lysine availability to the body for metabolism (Moughan & Rutherfurd, 2008). This is because the ε-amino group of lysine can react with many other food components including reducing sugars, fats and their oxidation products, polyphenols, vitamins, food additives and other amino acids, rendering the lysine nutritionally unavailable (Hurrell & Carpenter, 1981). Therefore, apart from increasing the lysine content of sorghum foods, it is necessary to increase the protein digestibility as well, in order to improve the protein nutritional adequacy derived from a plant food such as sorghum, where it presents a problem (Millward, 1999). As already mentioned, the approach used must also take into account the presence or absence of tannins in sorghum. Accordingly, this study examined the effects of adding cowpea to both tannin- and non-tannin sorghums as the means of enhancing the protein quality of traditional African sorghum-based foods.