High pressure gelation of soy proteins: Effect of concentration, pH and additives Pedro A. Alvarez, Hosahalli S. Ramaswamy * , Ashraf A. Ismail Department of Food Science and Agricultural Chemistry, Macdonald Campus of McGill University, 21, 111 Lakeshore Road, Sainte-Anne-de-Bellevue, QC, Canada H9X 3V9 Received 13 January 2008; received in revised form 13 February 2008; accepted 19 February 2008 Available online 26 February 2008 Abstract The global demand for soybean protein has increased dramatically over the last few years due to its versatility. High pressure (HP) processing is emerging as an effective alternative to thermal processing of foods. The HP treatment of protein solutions at different pro- cess conditions can cause partial unfolding of proteins that can lead to the irreversible gelation of the product. In this study, the influence of protein concentration (5–20% w/v), pH (3–7), sugar (5% w/v), CaCl 2 (5% w/v), pressure level (up to 650 MPa) and holding time (0.1 and 10 min), and process temperature (20 and 40 °C) on the dynamic rheology of soybean protein concentrate (SPC) solutions was evaluated. Furthermore, the protein structural changes caused by HP were studied, through the use of the extrinsic fluorescence of the probe 8-anilino-1-naphthalene sulfonic acid (ANS) and Fourier transform infrared (FTIR) spectroscopy. Results indicated a strong influence of protein concentration on both elastic (G 0 ) and viscous (G 00 ) moduli, increasing with concentration. Increase in pressure and holding time produced an increase on both G 0 and G 00 for SPC concentrations higher than 10%; at 15% SPC concentration, a relatively low pressure treatment of 250 MPa achieved the cross-over of G 0 over G 00 . The structure of the soybean proteins suffered limited changes after HP treatment; hydrophobicity increased, as well as the relative proportion of random coil, while the b-sheet content decreased. HP treatment can be used to enhance the viscoelastic behavior of SPC after which SPC can be used to enrich both protein content and textural properties of foods. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: High pressure; Dynamic rheology; Molecular structure; Soy protein concentrate; Gelation; Viscosity 1. Introduction Soybean is an industrial crop extensively cultivated for its oil and protein content. The global demand for soy protein has increased dramatically over the last few years. Recent publications refer to the widespread use of soy protein in the manufacture of many goods, mainly for its gelling behavior and emulsification properties (Maltais et al., 2005; Molina et al., 2001; Renkema et al., 2001). Most authors agree that there is a need for more research to exploit the possibilities of the soybean protein as a func- tionality-enhancing ingredient. One venue of interest in soy research is the study of soybean protein fractions. Soybean proteins are composed of four major protein fractions, known as 2S, 7S, 11S and 15S, characterized by differences in sedimentation coefficients. The 11S and 7S fractions constitute about 70% of the total protein in soybeans. The ratio 11S/7S may vary from 0.5 to 3 (Wolf et al., 1961). The changes that soy proteins may undergo during and after the variety of processing conditions found in the food industry is of particular interest to researchers in the field. There has been considerable interest in high pressure (HP) treatment which is emerging as an alternative to the more traditional thermal processing of foods. HP treatment can kill spoilage and pathogenic bacteria, and can also be used for improving functionality of food ingredients and finished foods. During HP treatment, the pressure creation 0260-8774/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2008.02.018 * Corresponding author. Tel.: +1 514 398 7919; fax: +1 514 398 7977. E-mail address: hosahalli.ramaswamy@mcgill.ca (H.S. Ramaswamy). www.elsevier.com/locate/jfoodeng Available online at www.sciencedirect.com Journal of Food Engineering 88 (2008) 331–340