Extrusion of a hard-to-cook bean (Phaseolus vulgaris L.) and quality protein maize (Zea mays L.) flour blend

Abstract

Bean seeds undergo physical, biological and chemical changes duri ng storage. Physica l factors such as seed moisture content, temperature, seed conditio n and avai labl e oxygen in storage have a decisive effect on degradation because they in­ fluence seed q uality (Rodriguez, 1992). Beans stored under high humidity (> 75%) and high temperature conditions (30-40 °C) experience serious l osses in quality characteris­ tics, particularly increased cooking time due to hardening (Ki­ gel, 1999). Some mechanisms proposed to explain hardening include conversion of l ipids to oxygenated polymers, forma­ tion of insol u ble pectates, lignification, protein denat u ralization and hydrolysi s; all occur mainly i n the cotyledon. Harden i ng leads to what is called th e h ard-to-cook phenomenon. This in­ volves changes in cell adherence that inhibit cell separation during cooking, which affects cooked seed texture, and limits protein availabi l ity due to denaturalization and hydrol ysi s, lowering seed nutritional contribution (Garcia, Filisetti, Udaeta, & Lajolo, 1998). A number of alternative technologies have been proposed for use of hard-to-cook beans, such as dry and wet fractionat­ ing, soaking in saline solutions, alkaline thermal treatment and extrusion. Of particular interest is extrusion, since it i s already w i dely used to incorporate hard-to-cook seeds into cereals which are then used to produce precooked flours, infant food and expanded snacks. These extruded products have advan­ tages in tetms of their sensory characteristics (texture, fla vour, smell and colour) and nutriti onal properties (increased protein content and balanced amino acid profile). A food exuder is a high temperature, sh01t processing time bioreactor that can transf01m a variety of ingredients into inter­ mediate or finished products such as precooked flours, ex­ panded snacks, breakfast cereals, pastas and texturi zed protein (Gonzalez, Torres, & Degreef, 2002). During extru sion