Adsorption and Foaming Characteristics of Soy Globulins and Tween 20 Mixed Systems Victor Pizones Ruı ´z-Henestrosa, Cecilio Carrera Sa ´ nchez, and Juan M. Rodrı ´guez Patino* Departamento de Ingenierı ´a Quı ´mica, Facultad de Quı ´mica, UniVersidad de SeVilla, C/ Prof. Garcı ´a Gonza ´ lez, 1, E-41012-SeVilla (Spain) The manufacture of food dispersions (emulsions and foams) with specific quality attributes depends on the selection of the most appropriate raw materials and processing conditions. In this contribution, we have analyzed the addition of Tween 20 (at 5 × 10 -6 and 1 × 10 -4 M, lower and higher than the critical micelle concentration, CMC, respectively) to improve the dynamic interfacial characteristics (adsorption and surface dilatational properties) and foam properties (foam capacity and stability) of soy globulin (7S and 11S at 0.1 wt %) acidic aqueous solutions. We have observed that (i) the adsorption (presence of a lag period, diffusion, and penetration at the air-water interface) of soy globulins depends on three factors: first the particular protein, second, the level of association/dissociation of these proteins by varying the pH (pH 7 was included as a control), and, finally, on the competitive adsorption between protein and Tween 20 in the aqueous phase. (ii) The surface dilatational properties reflect the fact that soy globulin and Tween 20 adsorbed mixed films exhibit viscoelastic behavior. The surface dilatational modulus decreases with the addition Tween 20 into the aqueous phase. (iii) The rate of adsorption and surface dilatational properties (surface dilatational modulus and phase angle) during adsorption at the air-water interface play an important role in the formation of foams generated from aqueous solutions of soy globulins. However, the stability of the foam correlates with the properties of the film at long-term adsorption. Introduction Foams are present in many foods, either in the finished product or incorporated during the production as a preliminary processing stage, which can be subjected to further processing before the product is completed. Among the functional effect of emulsifiers, foaming is of particular interest because it provides the texture for many traditional (ice cream, whipped topping, breads, cakes, meringues, beers, champagne, etc.) and new (cheese, butter, spreads, confectionary, sausages, etc.) aerated food products. 1-5 The positive benefits of aerated food products are primarily to do with texture. However, foaming may also create important problems during processing. 6,7 Thus, knowledge of the mechanism of foam formation and stabiliza- tion is essential if a foam of the required characteristics is to be produced. Foam formation and the stability of the resulting foam depend on the properties of the surface active components in the formulation. 6-16 Foam formation is influenced by the adsorption of the foaming agent (emulsifier) at the air-water interface and its ability to reduce surface tension and develop a mechanical protection to new formed bubbles. 8-11,17 Stability is an important property of food foams because consumer perception of quality is influenced by appearance. Foams are thermodynamically unstable, and their relative stability is affected by factors such as drainage of liquid previously present in the foam, dispro- portionation, and coalescence. Bubbles in a foam are stabilized by a bilayer of emulsifier molecules separated by the continuous aqueous phase. Thus, the characteristics of this thin film determine the formation and stability of the foam. 9,17,15,17-20 An important topic is how the macroscopic characteristics of a foam can be improved by controlling the nanostructure formation at fluid interfaces (molecular or interfacial engineering). Two types of emulsifiers are used in foods: 2,21 low molecular weight emulsifiers (mainly lipids, phospholipids, surfactants, etc.) and macromolecules (proteins and some hydrocolloids). These surface-active substances are used as foaming agents because of their amphiphilic character. In this work, we have analyzed how formulation engineering can improve the interfacial and foaming characteristics of soy globulin acidic aqueous solutions by the addition of a surfactant (Tween 20). Vegetable proteins (i.e., soy globulins) are of equivalent quality to those of meat, milk, and eggs, and their production requires substantially fewer natural resources. The popularity of these protein and peptide derivatives is increasing, mainly because of their health benefits. 1,22,23 They are grouped into two types according to their sedimentation coefficients, -conglycinin (a 7S globulin) and glycinin (an 11S globulin). A notable feature of soy proteins is the strong pH dependence of the molecular conformation and the associated functional properties. 24 Optimum functionality occurs at pH <5, which limits their application as food ingredients. In fact, in many food dispersion formulations emulsifiers (i.e., soy globulins) are used in acidic aqueous solutions. 1,4,5,21,24 Thus, different strategies have been used to improve soy globulin functionality by means of the correlation between specific product property (foam formation and stability) and interfacial properties (property function), including the choice of suitable processing conditions, such as pH, ionic strength, the addition of sucrose and Tween 20, and so forth. 25-27 Some surfactants (i.e., Tween 20) are used as foaming agents in the food industry (i.e., in margarines, low- fat spreads, baked products, creams and toppings, as a fat substitute in low calorie foods, 28 etc.) in combination with proteins to achieve positive synergistic effects on the charac- teristics of the adsorbed film and foams. 29-34 Materials and Methods Materials. Samples for interfacial characteristics of soy protein films were prepared using Milli-Q ultrapure water and * To whom correspondence should be addressed. Tel.: +34 95 4556446. Fax: +34 95 4556446. E-mail: jmrodri@us.es. 2876 Ind. Eng. Chem. Res. 2008, 47, 2876-2885 10.1021/ie071518f CCC: $40.75 © 2008 American Chemical Society Published on Web 04/08/2008