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RESEARCH TOPIC – Comparison of the Protein Nutritional Value of Food Blends Based on Sorghum, Bambara Groundnut and Sweet Potatoes

Comparison of the Protein Nutritional Value of Food Blends Based on Sorghum, Bambara Groundnut and Sweet Potatoes



The protein quality of four blends based on sprouted sorghum, bambara groundnuts and fermented sweet potatoes had been evaluated by rat feeding experiments; casein served as a reference protein. The test proteins were incorporated to make up 1.6% total nitrogen. There was an inverse relationship between % nitrogen disgestibility and the proportion of sorghum protein in the blend; being highest (89.7%) in the diets based on sorghum: bambara groundnut: sweet potatoes with protein ratios of 52:46:2. This blend proved to be optimum when the biological value (93.6%) and the net protein utilization (84%) were used as protein indices. The findings imply that foods with good protein quality could be formulated from a blend of sorghumbambara
groundnut and sweet potatoes, provided appropriate processing and blending are taken into consideration.


Protein deficiency, particularly in the diets of young children, is one of the major nutritional problems facing the developing countries. Poor food quality has been identified as one of the causative factors (WHO, 1997). In Nigeria, the traditional complementary foods consists mainly of unsupplemented starchy staples like sorghum, maize, millet, yams, cassavas, coco yams and sweet potatoes. They are poor in protein and limited in food value due to the presence of antinutrients which chelate or complex with essential nutrients. Adequate processing is necessary to improve their utilization. Germination and fermentation are two household-level food technologies which have been extensively reviewed as means by which the nutritive value of plant food could be improved L Correspondence to: N. M. Nnam (Obizoba, 1998).

They increase amnio acid and  mineral availability, protein and carbohydrate digestibility, nutrient and energy densities of gruels, levels of the B-complex vitamins and decrease toxic and antinutritional factors like phytates, tannins and alpha-galactosides (Obizoba & Nnam, 1992; FAO, 1995; Obizoba, 19%).
In Nigeria, there exist a lot of cheap, nutritious and readily available indigenous foods which when adequately processed and judiciously combined could serve as an improvement of the existing traditional complementary foods. This will help alleviate the protein gap experienced in the country particularly
among young children. Sorghum is a popular cereal in Nigeria, particularly in the northern part of the country where it is produced.

It constitutes a major source of energy and protein in areas where it is a staple. The ~rotein varies from 7 5 to 9.0% and like other cereal proteins, it is limited in the amnio acids, lysine, threonine, tryptophan and methionine (Ihekoronye & Ngoddy, 1985). Supplementation with other foods whose protein would
complement the amino acid composition is necessary. Bambara groundnut is an important legume produced extensively in northern Nigeria. The legume is sweet and pleasant to eat either as dry or immature seeds. The motein content in the forage range from 12-15% and in grains from 20-25% (dry matter basis) (Arora, 1995). It has more methionine than is found in other grain legumes and would serve as a good supplement to sorghum protein. Sweet potato  is a root crop that provides significant amounts of energy and protein. The tuber is rich in carotene (pa~ticularlyth e yellow variety), minerals and the  B-complex vitamins.

It has a fair quantity of ascorbic acid. The protein is rich in the amino acids lysine and tryptophan (LIFE, 1994) which are low in sorghum. Combining sweet potatoes with sorghum will provide complementary effect on the amino acids. No detailed work had been published on the protein quality of blends of processed sorghum, bambara groundnuts and sweet potatoes. The objective of this study was to evaluate the protein quality of various combinations of
sprouted sorghum, bambara groundnuts and fermented sweet potatoes in rats. Nitrogen (N) digestibility and utilization were the criteria used for the evaluation.


Comparison of the Protein Nutritional Value of Food Blends Based on Sorghum, Bambara Groundnut and Sweet Potatoes


Materials and methods


White sorghum (Sorghum biocolor) (S), cream bambara groundnuts (Vigna subterranea (L) Verde) (BG) and reddish-purple sweet potatoes (Ipomoea batatas) (SP) were used as source of protein for the study. They were all purchased from the Nsukka market in Enugu State of Nigeria.

Preparation of materials

Fivekg of sorghum and 2kg of bambara groundnuts were ,soaked separately in cold deionized water for 8 h in a ratio of 1 :3 (wt/vol) grain to water. The soaked grains were drained at the end of the soaking period. They were separately spread on wet jute bags and covered with moistened muslin cloth to sprout for 48 h at an average room temperature of 28.0 f 2°C. The grains were washed twice daily to avoid the growth of mold. The sprouted grains, (sprouted sorghum, SS and sprouted bambara groundnuts, SBG) were dried separately in a convection air oven (Gallenkamp and Co. Ltd, London, England)
at 50°C for 12 h to 96% dry matter. The vegetative parts were carefully removed in sorghum. Bambara groundnut grains were dehulled mechanically using PRL mini-rollover dehuller (Nutana Machine Co., Saskatoon, Canada).

All the sprouted samples were milled separately in a laboratory hammermill (Thomas Wiley Mill, Model ED-5) to a fine powder (70 mm mesh screen).
One kg of sweet potatoes were hand-peeled, wet-milled using a Gallenkamp mixer Kenwood- MPR2OI and fermented in a bell jar for 48 h by the microflora present in the paste. The fermented sweet potato (FSP) sample was dried and remilled using the same procedure as for sorghum and bambara groundnuts.
Animal feeding experiments The composition of the diets is presented in Table 1. The diets were formulated based on the N content of the foods. The ratios of 60:40:0, 57:42: 1, 55:44: 1, 52:46:2 (protein basis) ofsorghum, bambara groundnuts and sweet potatoes were used in the formulation.

All the diets were formulated to provide 10% protein or 1.6 gNJ100 g diet. Casein served as the control. Oil, surcrose, vitamin and mineral mixes were
added to balance the diets. Animal housing and feeding Thirty male adult rats weighing from 80-150g were obtained from the Department of Veterinary
Pathology, University of Nigeria, Nsukka. The wide range in the weight of the rats was because of a long period (over 6 months) of a scarcity of rat feed in the country, which led to lack of rats. The animals were divided into five groups of 6 rats each on the basis of body weight.

Comparison of the Protein Nutritional Value of Food Blends Based on Sorghum, Bambara Groundnut and Sweet Potatoes

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