System parameters and product properties response of soybean protein extruded at wide moisture range Feng Liang Chen, Yi Min Wei * , Bo Zhang, Anthony Okhonlaye Ojokoh Institute of Agro-Food Science and Technology, Chinese Academy of Agricultural Sciences, P.O. Box 5109, 100193 Beijing, China article info Article history: Received 14 May 2009 Received in revised form 13 July 2009 Accepted 21 July 2009 Available online 25 July 2009 Keywords: Soybean protein Extrusion texturization System parameters Product properties abstract In order to explore the effect of water during extrusion process, soybean protein isolate (SPI) was extruded using a pilot-scale twin-screw extruder at 28%, 36%, 44%, 52% and 60% moisture content and 140, 150 and 160 °C cooking temperature. The extrusion system parameters like in-line viscosity at die, mean residence time and specific mechanical energy (SME), product textural properties including tensile strength, hardness, chewiness and degree of texturization, and the molecular weight distribution characterized by SDS–PAGE were investigated. And the interrelationship between system parameters and product properties were analyzed. The results showed that moisture content was a more important factor on system parameters and product properties than cooking temperature. Higher moisture content resulted in lower viscosity of dough in the extruder, shorter residence time and lower conversion ratio of extruder mechanical energy into heat energy, finally reducing significantly the tensile strength, hard- ness, chewiness and the degree of aggregation. The data from extrusion system parameters and product properties correlate well and could be used to explain and control the characteristics of extrudate. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Extrusion as a continuous efficient cooking, mixing and forming process, has been used increasingly to produce breakfast cereals, baby foods, flat breads, snacks, meat and cheese analogues, modi- fied starches etc. (Harper, 1979; Ding et al., 2006). Despite in- creased use of extrusion technology, extrusion process is still a complicated multi-input–output system that is yet to be mastered. A simplified system analysis model has been proposed (Meuser and Van Lengerich, 1984), which sorts extrusion parameters into three groups, namely, process parameters (including screw speed, moisture content, barrel temperature, screw configuration, die dimension, raw material characteristics etc.), system parameters (including energy input, residence time etc.), and products proper- ties (including color, nutrition, texture, taste etc.). Among these three kinds of parameters, process parameters have effects on the properties of final products by means of affecting extrusion system parameters. As a result of the ‘‘black box” characteristic of extruder and limitation of in-line detection, many researchers focus on the influence of process parameters on product properties, disregarding system parameters (Pham and Del Rosario, 1984; Wang et al., 2001; Ding et al., 2006), and so there are a few litera- tures related to extrusion system parameters alone, without con- sidering products properties (Akdogan, 1996; Wang, 2005; Kang et al., 2007a,b). In order to better understand the effect of extrusion process on the characteristics of products and to obtain various extrudate with ideal structure and texture, it is imperative to re- search the correlation between process parameters, system param- eters and product parameters comprehensively. Feed moisture content, as one of the vital process variables, was considered a significant factor affecting the properties of final products (Lin et al., 2000; Wang et al., 2001), and is also an impor- tant basis to divide low moisture extrusion and high moisture extrusion (Akdogan, 1999). Due to the limitation of extruder and raw materials, previous studies about the effect of process param- eters on system parameters and product properties were imple- mented at a relative narrow range and numerously at low moisture content level, resulting in incomprehensive even contra- dictory conclusions. The purposes of this research were to investi- gate extrusion of soybean protein meat analogue using twin-screw extruder at relative wide moisture range spanning low moisture and high moisture, to study the effect of moisture content and cooking temperature on system parameters and product proper- ties, and to analyze their interrelationship. 2. Materials and methods 2.1. Raw material Soy protein isolate (SPI) was obtained from Yuwang Group Ltd. (Shandong Province, China). The approximate composition of raw 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.07.014 * Corresponding author. Tel.: +86 10 62815956; fax: +86 10 62815846. E-mail address: weiyimin36@126.com (Y.M. Wei). Journal of Food Engineering 96 (2010) 208–213 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng