Emulsifying properties of soy protein isolate fractions obtained by isoelectric precipitation Bernard E Chove, Alistair S Grandison* and Michael J Lewis School of Food Biosciences, The University of Reading, PO Box 226, Whiteknights, Reading RG6 6AP, UK Abstract: Soy protein isolate SPI) fractions were produced by isoelectric precipitation based on results of isoelectric focusing carried out on the crude soy extract. The fractions were produced from crude protein extract pH 9.0) sequentially and non-sequentially at isoelectric points pIs) of 5.6, 5.1 and 4.5. Emulsions stabilised by soy proteins with pIs between 5.6 and 5.1 had the highest P < 0.01) emulsion stability index ESI), while those stabilised with proteins having pIs between 5.1 and 4.5 resulted in the lowest ESI for sequentially precipitated fractions. Non-sequential fractionation at pI 5.1 produced fractions with higher emulsifying activity index EAI) than sequential fractionation. SDS- PAGE pro®les indicated that the fractions exhibiting high functionality in terms of ESI and EAI were also richer in 7S globulin protein subunits. # 2001 Society of Chemical Industry Keywords: soy protein; isolate; isoelectric focusing; isoelectric precipitation; fractionation; emulsifying activity; emulsion stability INTRODUCTION The rapidly increasing demand for food-grade pro- teins has pushed up the cost of traditional animal protein sources. There is a growing need for less expensiveproteins,andthemainchallengehasbeento modify non-traditional proteins so as to endow them with the requisite functional properties. 1 Vegetable protein is the most abundant source of protein on earth, and numerous vegetables such as alfalfa leaf, cottonseed, winged bean, peanut and soya have been investigated for possible incorporation into formulated foods. 2 Soy proteins are used in foods as functional and nutritional ingredients, but in their native form they lack the required functionality. Critical functional properties necessary in protein ingredients include solubility, water and fat absorption, emulsion stabili- sation, whippability, gelation and good organoleptic properties. 3 Many native proteins possess limited functionality, and physical, chemical and enzymatic modi®cations have been used to expand the range of functional properties available. 4 These modi®cations can be costly in terms of the process itself as well as losing some other properties at the expense of improving the targeted ones. Chemical modi®cation also poses the problem of removing any unreacted reagents from the ®nal product. 5 Fundamental knowledge about emul- sifying properties of soy proteins is scant in compari- son with their other properties such as gelling and foaming. 6 The aim of the present work was to fractionate the proteins in soy protein isolate SPI) on the basis of charge and to assess the functionality of the resulting protein fractions. Isoelectric focusing was also carried out in an attempt to relate the functional properties to individual protein species. Two emulsi- ®cation functional properties were used as tests for functionality: the emulsion stability index ESI) and the emulsifying activity index EAI). EXPERIMENTAL Materials Defatted soy ¯our supplied by Food Ingredient Technology Limited Great Gransdon, UK) was used as the starting material. The protein dispersibility index PDI) was speci®ed as 80%. Analytical reagent- grade chemicals were supplied by Sigma-Aldrich Co Ltd Poole, UK). Crude protein extract The defatted soy ¯our was dispersed in distilled water at 22 °C100gkg 1 ) and the pH was adjusted to 9.0 using 1.0 M NaOH) to solubilise the proteins. 4 The suspension was mixed using a magnetic stirrer for 3h. The insoluble solid was separated out by centrifuga- tion Model RC 5C plus; Sorvall Products, Newtown, CT, USA) at 10000  g for 25min. Fractionation of the extract Starting with the extract at pH 9.0, different fractions were obtained by lowering the pH of the extract using 1.0 M HCl) to 5.6, 5.1 and 4.5 and separating the precipitate by centrifugation 31000  g for 30min). The three pH values correspond to the isoelectric (Received 17 November 2000; accepted 26 February 2001) * Correspondence to: Alistair S Grandison, School of Food Biosciences, The University of Reading, PO Box 226, Whiteknights, Reading RG6 6AP, UK Contract/grant sponsor: Association of Commonwealth Universities # 2001 Society of Chemical Industry. J Sci Food Agric 0022±5142/2001/$30.00 759 Journal of the Science of Food and Agriculture J Sci Food Agric 81:759±763 online: 2001) DOI: 10.1002/jsfa.877