Foams Tween 20 system

Solutions containing mixtures of a-lactalbumin (a-la) and Tween 20 are a classic example of a two component system that displays Type II behavior [21]. A summary of foam stability, film thickness and FITC-a-la surface diffusion is given in Figure 17. Alone, a-la produced less stable foams than 8-lg, and it was necessary to increase the stock protein concentration to 0.5 mg/ml (35.4 / M). 

An improvement in foam stability was observed as R was increased to >0.15 (Figure 17). This was accompanied by the onset of surface diffusion of a-la in the adsorbed protein layer. This is significantly different compared to our observations with /8-lg, where the onset and increase in surface diffusion was accompanied with a decrease in foam stability. Fluorescence and surface tension measurements confirmed that a-la was still present in the adsorbed layer of the film up to R = 2.5. Thus, the enhancement of foam stability to levels in excess of that observed with a-la alone supports the presence of a synergistic effect between the protein and surfactant in this mixed system (i.e., the combined effect of the two components exceeds the sum of their individual effects). It is important to note that Tween 20 alone does not form a stable foam at concentrations <40 jtM [22], It is possible that a-la, which is a small protein (Mr = 14,800), is capable of stabilizing thin films by a Marangoni type mechanism [2] once a-la/a-la interactions have been broken down by competitive adsorption of Tween 20. 

Regarding the systems used in this study, we use the same proteins as in the previous section (whole casein and P -casein) and they are mixed with Tween 20, respectively. This is a low molecular weight surfactant used in the food industry, which is water soluble and nonionic. The different behavior of these two mixed systems is again discussed on the basis of fundamental magnitudes such as surface tension and foam film thickness. 

Let us first evaluate the behavior of the foam formed by the surfactant and the different effect caused by adding each protein into the bulk solution. The stability of the foams formed by each of the systems presented in this work is characterized by their half-lifetime. Figure 10.3 shows the halflifetime of the foam formed by the pure Tween 20 along with that of the mixed systems. These were obtained with a fixed concentration of protein of 0.1 g/L for both cases and increasing the concentration of Tween 20 between 10" and 10 M. 

Concerning the stability of foams formed by mixed systems, it can be seen in Figure 10.3 that the surfactant affects very differently the stability of the foams formed by whole casein and P-casein, respectively. On the one hand, the stability of foam of P-casein/Tween 20 system decreases monotonously with increasing concentration of surfactant. This continues until practically reaching the... 

Let us evaluate first the static surface properties of the adsorbed layer of these same systems used in the formation of foams. Hence, Figure 10.4 shows the measured surface pressure isotherms for Tween 20 and for the mixtures with whole casein and P-casein. For each of the concentrations... 

Contrary to the case of P-casein/Tween 20 systans, although the surface pressure isotherms shed some light on the surface structure of whole casein/Tween 20 systems, the information extracted does not provide a full explanation to the behavior of foams stabilized by this system. Taking into account the key information provided by the confinement in thin liquid films as regards the foam stability of whole casein and P-casein solutions, which was examined in the previous section, let us evaluate the properties of the foam films stabilized with whole casein/Tween 20 mixtures. Table 10.2 shows the thickness of foam films stabilized by pure whole casein, pure Tween 20 and two mixed systems under similar conditions to the foam stability, and the surface pressure experiments. The film thickness is measured by using Scheludko s microinterferometric method (Maldonado-Valderrama and Langevin, 2008). 

Whereas Tween 20 displaces completely p-casein from the surface, whole casein appears to be more resistant to the displacement. This feature is reflected in the foam stability and in the drainage of thin liquid films of whole casein/Tween 20 mixtures. The reason for this might be related to the more compact structure of K-casein and its resistance against displacement (Maldonado-Valderrama and Langevin, 2008), also suggesting that this fraction governs the foam stability of whole casein. This section illustrates the important effect of the nature of the components on foam stability of mixed protein/surfactants systems and the important relationship with surface behavior. 

---