Extrusion cooking was used to formulate rice and pigeon pea blend with the aim of providing high energy, protein enriched and affordable foods for malnourished children. Three (3) factors (screw speed, feed moisture content and feed blend composition) affecting the extrusion cooking process were subjected to Central composite design (CCD) and functional properties were used to optimize production variables. Furthermore, the expansion index of 9.56 ± 0.15, bulk density of 0.05 ± 0.01 g/ml and water absorption index of 5.55 ± 0.07 at screw speed of 200 rpm, feed moisture content of 30 % and feed blend composition of 20% (run 12) were obtained. Regression model and response surface plots were developed and tested via coefficient of determination (R2 and adjusted R2), analysis of variance, lack of fit and residual plots. The developed quadratic model showed significant (p < 0.05) effect of the process variables on the functional properties of instant porridges from rice-pigeon pea flour blend. Similarly, the coefficient of determination (R2) was found to be 0.96, 0.93, and 0.88, for expansion index, bulk density and water absorption index of the extrudates. Furthermore, lack of fit for the quadratic models were not significant (p > 0.05), which suggested that the model equations were adequate in describing the functional properties of the rice-pigeon pea extrudates. In order to optimize the quadratic models, four formulations were prepared and subjected to nutritional and sensory properties where formulation 3 showed higher protein (22.10 ± 0.01 g/100g), limiting amino acids, (lysine and methionine) of 3.44 ± 0.04 g/100g and 1.44 ± 0.02 g/100g, calcium (3.41 ± 0.07 mg/100g), iron (12.64 ± 0.03 mg/100g), zinc (9.33 ± 0.02 g/100g) contents and general acceptability (6.68 ± 0.06). In conclusion, convenient complementary food was developed from rice-pigeon pea flour blend through extrusion which improved the nutritional and sensory properties of the products.

1.1 Background of the study
Food processing technology via locally available raw materials and innovative processing technologies that maintain high nutrient compositions and consumer acceptability have been extensively studied (Singh et al., 2000; Anuonye, 2012; Filli et al., 2013). Cereals and legumes are the principal sources of energy and protein for significant population in developing countries including Nigeria (Muller and Krawinkel, 2005). The production of cereal-legume based products to supply additional protein and minerals has increased significantly over the years (Muller and Krawinkel, 2005). Extrusion cooking technology has played a central role in enhancing food security for development of different types of products such as breakfast cereals, flakes, quick cooking paste, instantised legume powders and breakfast gruels (Chaiyakul and Winger, 2009). This technology requires a high temperature short time (HTST) cooking process which provide an alternative for improving traditional food processing (Guy, 2001a). It also has the ability to produce a shelf-stable food of reduced microbial load, minimal anti-nutrient content and low moisture content. (Filli et al., 2011). Products with these characteristics are suitable for countries like Nigeria where infrastructure is limited and inadequate (Filli et al., 2011).

Extrusion method is one of the contemporary food processing technologies applied to foods in order to mitigate the problems associated with processing of traditional cereal based-products in terms of functional properties, physical state and shelf-life (Guy, 2001a). It offers many advantages over spray-drying and roller-drying technologies in terms of preparation of ready-to-eat foods of desired shape, size, texture and sensory characteristics at very low processing cost (Guy, 2001a; Sumathi et al., 2007).

Rice (Oryza sativa) is an important food crop in the world and the number of consumers depend on it as their staple food (Juliano, 2016), which could be linked to its attractive color, bland taste, hypoallergenicity and ease of digestion (Guy, 2001). It has been reported that an average Nigerian, consumes 21 kg of rice per year (WARDA, 2004). In most developing countries, rice accounts for daily supply of 27% of energy, 20% of protein and 3% of fat (Kennedy and Burlingame, 2003). Similarly, extruded products from rice are currently less available compared to those from maize and wheat (Danbaba et al., 2015). Guha and Ali (1998) reported that the free glutinous rice is suitable material for production of the extruded products such as ready-to-eat snacks and breakfast cereal with low bulk density, high expansion and low shear stress.

Pigeon pea (Cajanus cajan) is an indigenous crop and one of the major leguminous crops cultivated in many countries of the tropics and subtropics (Troedson et al., 1990). It is known as Waken Kurawa in Hausa, Fio fio in igbo and Otilli in yoruba (Anuonye et al., 2012). Similarly, it is considered as number four after groundnut, cowpea and Bambara nut and highly cherished for its protein content (17 - 30%) which makes it indispensable along with cereals in human diet (Troedson et al.,1990). The plant has also been listed as one of the under-utilized legumes with broad potentials and its seed could serve as an ideal supplement to traditional african cereal and tuber-based diets which are generally protein deficient (Badifu, 1992; Onu and Okongwu, 2006; Eneche, 2009). Therefore, blending of rice and pigeon pea will produce adequate meals of balanced nutrient compositions for both infant and adult usage.

1.2 Statement of Problem
In Africa, poverty, poor agricultural practices, low productivity and inadequate processing are the major causes of food shortage (Danbaba et al., 2015). Based on this, significant populations survive predominantly on staple food crops such as rice, maize, sorghum, millet, cassava, with little or no animal products to meet the protein need for normal growth and development (Danbaba et al., 2015). These problems are further aggravated by the menace of epidemic diseases that increased the number of vulnerable populations, which resulted in wide spread of maternal and infant malnutrition in most of the countries (Danbaba et al., 2015). Despite Africa’s vast natural resources and a considerable progress in economic growth; hunger and malnutrition have remained endemic amongst the African population, and it is estimated that 200 million Africans are chronically malnourished (WFP, 2011; Mohammed, 2017).

Similarly, several strategies have been adopted to reduce maternal and infant malnutrition including diversification, fortification and complementation of foods with indispensable amino acid sources, supplementation with good quality protein and the utilization of locally grown crops for the production of high protein, shelf stable and affordable recipes for addressing the deepening global nutrition challenges in developing countries (Iwe, 2001).

1.3 Justification of the Study
The production of cereal-legume based products provides an ideal source of dietary protein for human populations; such products include nutritionally enhanced biscuits, breads, cakes, porridges and extruded snacks (Filli et al., 2011). Nutritionally, rice and rice-based products are deficient in lysine, an essential amino acid, that can easily be improved by blending rice with food materials rich in lysine.

In addition, researches on utilization, development and commercialization of pigeon pea and its associated products are limited in Nigeria when compared with other legumes such as cowpea (Akande, 2007). However, several researches have been reported on the use of extrusion cooking for cereals and legumes singly (Bredie et al., 1998; Iwe, 2001; Pelembe et al., 2002; Ding et al., 2006), and no available report in the literature to the best of our knowledge on the development of rice-pigeon pea blend using this technology.

Hence, extrusion technology could contribute in this direction through the production of foods containing cereals and legumes, which are highly acceptable, and meet consumer’s specifications. Furthermore, extrusion processes could help in producing quality products with improvement in nutritive value, colour, flavor and bioactive substances as well as reduction in production cost (Tabo et al., 1995).

1.4 Aim and Objectives
The aim of this research is to optimize and characterize instant porridge developed from rice-pigeon pea flour blend using extrusion cooking method.

1.4.1 Specific Objectives
i. To determine the optimum process conditions for production of instant porridge from rice- pigeon pea flour blend using statistical optimization technique.

ii. To evaluate the characterization on proximate, amino acid and mineral compositions of the developed instant porridge from rice-pigeon pea flour blend.

iii. To carry out sensory evaluation of the developed instant porridge from rice- pigeon pea flour blend.

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