Variability in the Nutritive Value of Three New Lima Bean (phaseolus Lunatus L. Walp.) Breeding Lines

Chemical composition and protein quality of raw and heat-processed (boiled and autoclaved) samples of selected advanced breeding lines of lima beans (Tpl 1, Tpl 7 and Tpl 175), part of a larger collection developed by the International Institute for Tropical Agriculture (IITA), and evaluated in agronomic field trials in Nigeria, were studied. Protein quality was evaluated using weanling albino rats fed diets which we formulated to supply 10% protein using lima bean samples with casein as a control. Raw seeds contained 24.0-27.8% crude protein, 6.8-7.7 mg iron per 100g, 55.7-79.0 mg polyphenol per 100g and 118.8-225.9 mg phytic acid per 100g seed flour. Tpl 1 had higher crude protein and mineral (calcium, iron and sodium) contents, and lower amounts of polyphenol and phytic acid compared with Tpl 7 and Tpl 175. Boiling (moist heat) was more effective than autoclaving (dry heat) for reducing the levels of antinutrients and improving the protein quality of the beans, as shown by the higher values for weight gain, protein efficiency ratio (PER), net protein ratio (NPR) and true digestibility (TD) of boiled samples. There were no significant (p > 0.05) differences between the values obtained for PER, NPR, and TD of diets formulated with boiled samples of Tpl 1 and casein, indicating the superiority of the protein quality of this lima bean line compared with Tpl 7 and Tpl 175. INTRODUCTION Lima beans (Phaseolus lunatus L. Walp.) are an important grain legume and source of protein and other nutrients in many developing countries. In Nigeria, lima beans are reported to provide from 25 to 30% of the protein in the diet and are second only to maize and cowpea as a staple food (Luse, 1979). Efforts aimed at breeding and improvement of lima bean cultivars for specific characteristics, such as high yield, early maturity, improved nutritional properties, desired seed quality and resistance to insects and diseases have been carried out by the International Institute for Tropical Agriculture (IITA), Ibadan, Nigeria. Advanced lines of lima beans, with good agronomic performance (high yield, early maturity and resistance to diseases) have been developed and released for cultivation by the IITA and they include Tpl 1 Tpl 7 and Tpl 175. The presence of antinutrients, such as polyphenol and phytic acid has been identified as one major factor limiting the use of lima beans as food (Ologhobo, 1986). Genetic manipulation in breeding new lima bean cultivars may produce wide variation in the contents of these antinutrients, their chemical composition and protein quality. The chemical composition and protein quality of lima beans and other legumes have been shown to vary according to cultivar (Apata and Ologhobo, 1986; Singh et al., 1991; Giami et al., 2001a; Giami 2002). Removal or reduction of antinutrients and evaluation of the protein quality of newly developed lima bean cultivars is necessary for their effective utilization in human nutrition. Heat treatment has been shown to improve the nutritive value of legumes, oil seeds and other edible seeds by decreasing the levels of antinutrients and increasing protein digestibility (Kadam et al., 1987; Giami and Wachukwu 1997; Giami et al., 2001b; Giami, 2002). Characterization of these improved lines of lima beans (Tpl 1, Tpl 7 and Tpl 175) for physico-chemical properties and cooking quality has been reported (Giami, Legume Res., 29 (1) : 25 31, 2006