Effect of Temperature on the Separation of Soybean 11 S and 7 S Protein Fractions during Bipolar Membrane Electroacidification Laurent Bazinet, †,‡ Denis Ippersiel, † Raynald Labrecque, ‡ and Franc ¸ ois Lamarche* ,† Agriculture and Agri-Food Canada, Food Research and Development Centre, 3600 Casavant Blvd. West, St. Hyacinthe (Que ´bec), Canada J2S 8E3, and Hydro-Que ´bec, Laboratoire des Technologies E Ä lectrochimiques et des E Ä lectrotechnologies, 600, Av. de la Montagne, Shawinigan (Que ´bec), Canada G9N 7N5 The purpose of this study was to evaluate the effect of temperature (10 and 27 °C) on the efficiency of bipolar membrane electroacidification (BMEA) to fractionate soybean proteins. BMEA is a technology derived from electrodialysis, based on the isoelectric precipitation of proteins. It appears that temperature has a significant effect on the selective precipitation of the soybean protein fractions, mainly 11 S and 7 S, during BMEA. At 27 °C, the precipitation profile of the four protein fractions is situated in a pH range from 6.6 to 4.4, with no possibility of separating any of theses fractions. However, at 10 °C, the 11 S globulin precipitates at a higher pH than at 27 °C, pH 6.7 vs 5.9, allowing the fractionation of 11 S from the other fractions. Using electroacidi- fication it is possible to obtain a precipitate solution enriched in the 11 S fraction (71.8% of 11 S and 10.8% of 7 S) and a supernatant solution enriched in the 7 S fraction (46.6% of 7 S and 4.6% of 11S). Introduction The nutritional and technological importance of soy protein (63% of plant protein consumed worldwide) (Soyatech Inc., 1993) in agricultural and food products has, for several years, been a driving force behind the study of this class of food products and the development of industrial processes intended to separate the various protein fractions and improve their properties and quali- ties. The two major reserve soybean proteins, the globulins 7 S or -conglycinin (37-39% of total protein) and 11 S or glycinin (31-44% of total protein) have different intrinsic properties leading to different functional proper- ties. Saio and Watanabe (1978) found soy 11 S and 7 S globulins to be distinctly different in their functional properties: the 11 S globulin was reported to make much harder tofu gels than 7 S globulin, to precipitate faster, and to form larger aggregates relative to 7 S gels. Ning and Villota (1994) found that, after extrusion, the 11 S globulin fraction appeared to favor expansion and water- holding capacity of the finished product. Numerous processes have been patented to separate these proteins on the basis of their different isoelectric points and the capability of the 11 S fraction to precipitate at low temperatures (Kolar et al., 1985). Hence, Davidson et al. (1979) have set up a process favoring the precipitation of the 11 S fraction, by cooling the extraction solution while recovering the other proteins by acid precipitation at pH 4.5 or ultrafiltration. Shermer (1980) patented a process allowing the production of an isolate rich in 7 S globulins by extraction at pH between 5.1 and 5.9; in these conditions, the 11 S protein fraction has a very low solubility. Howard et al. (1983) and Lehnhardt et al. (1983) have achieved, by addition of salts and sulfuric ions, respectively, a selective precipitation and a selective extraction of the 11 S and 7 S fractions. In this process, the separation of intermediary fractions, containing 30% of 7 S and 70% of 11 S, is necessary to obtain pure fractions of 11 S and 7 S (Shermer, 1980). Similar separations are necessary during the extraction of soy- bean protein by isoelectric precipitation (Howard et al., 1983). The disadvantages of these methods include denatur- ation of protein on exposure to alkali and acid treatment, high ash content, and alteration of protein solubility after rehydration (Nash and Wolf, 1967). Local extremes in pH can cause irreversible denaturation of the proteins, which will alter the precipitation behavior (Fisher et al., 1986; Kilara and Sharkasi, 1986). Moreover, these procedures generate high volumes of effluents. In this context, the aim of this work is to demonstrate the efficiency of bipolar membrane electroacidification (BMEA) on the fractionation of soybean proteins without adding acid. BMEA is a technology derived from elec- trodialysis, based on the isoelectric precipitation of proteins (Bazinet et al., 1996; 1997a-c; 1998a,b; 1999). Protons are generated by dissociation of water molecules at the interface of a bipolar membrane, using a suf- ficiently large cathode/anode voltage difference (Mani, 1991). Under the influence of the electric field, the protons generated are able to migrate toward the cathode and acidify a protein solution (Bazinet et al., 1996). The specific objective of our study is to evaluate the effect of temperature (10 and 27 °C) on the efficiency of BMEA to fractionate soybean proteins. Efficiency was evaluated in terms of energy and percent of soluble protein. The percent soluble protein determined in the supernatant and the precipitate permitted us to follow the evolution of protein fractionation as a function of temperature during the decrease of pH by the BMEA process. † Agriculture and Agri-Food Canada. ‡ Hydro-Que ´bec 292 Biotechnol. Prog. 2000, 16, 292-295 10.1021/bp000001v CCC: $19.00 © 2000 American Chemical Society and American Institute of Chemical Engineers Published on Web 02/29/2000