Foams whole casein

Table I shows that the foaming properties of whole casein improved by slight phosphorylation. The lowest phosphorylated form of casein (4 mol P/mol protein) showed higher foam hydration and stability than the native whole casein. However, the highly phosphorylated whole casein (11 mol P/mol protein) showed poor foaming properties. The foam hydration of as-casein deteriorated while that of K-casein improved by phosphorylation. This discrepancy seemed to be caused by a different initial hydrophobic/ hydrophilic balance of the proteins in their native states. However, foam stabilities of all casein fractions were reduced by phosphorylation, with K-casein being only slightly affected.

Let us first evaluate the stability of foams formed by whole casein and P-casein under similar experimental conditions (Figure 10.2). It can be clearly seen in the figure that the foam of P-casein is more stable at this bulk concentration. The stability of the foam formed is often estimated by the half-lifetime of the foam (ty, the time taken by the foam to decay to half the original height after the air flow is stopped. Hence, the foam of P-casein (0.1 g/L) has a half-lifetime of 35min whereas the foam of whole casein (0.1 g/L) has a half-lifetime of 15 min. 

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. 

Accordingly, the behavior foam formed by pure whole casein and P-casein has been described in detail in the previous section. The stability of the foam formed by pure Tween 20 under these experimental conditions is described in detail by Maldonado-Valderrama et al. (2007). In short. Tween 20 produces poorly stable foam only above a certain concentration in bulk. Regarding whole casein and P-casein, their behavior at this concentration is discussed in detail in the previous section, being the foam of the latter more stable. 

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... 

FIGURE 10.3 Half-lifetime of the foams formed by Tween 20 (straight line), p-casein + Tween 20 (dashed line), and whole casein -I- Tween 20 (dash-dotted line). The concentration of protein in the mixtnres is of 0.1 g/L, and the solutions are prepared in a buffer solution at pH 7.4,1= 16.4mM, T= 20°C. 

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). 

Regarding the drainage of a foam film stabilized by sole whole casein, this has been discussed in the previous section. The thickness of the whole casein film diminishes in a continuous manner until reaching a finite final thickness that corresponds to a bilayer of caseins with somehow screened electrostatic repulsion between layers and a higher intermolecular interaction between the different monomers composing the whole casein (Maldonado-Valderrama and Langevin, 2008). 

Thickness of the Foam Films Formed by the Pure and Mixed Solutions of Whole Casein and Tween 20... 

Firstly, the stability of the foam formed by P-casein and whole casein appears very different, the former being more stable. In order to further investigate this issne, we evalnate several surface properties of these two proteins. The surface tension and surface rheology do not seem to be accurate enough to account for this large difference in foam stability, since they show very similar values. However, the thickness of the foam films stabilized by the two proteins respectively seems to determine the ultimate behavior of the foam. Hence, the thicker foam film measured for P-casein probably prevents coalesce of air bubbles resulting in more stable foam formed by this protein as compared to whole casein. 

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. 

Maldonado-Valderrama, J. and D. Langevin. 2008. On the difference between foams stabUized by surfactants and whole casein or P-casein. Comparison of foams, foam films, and liquid surfaces studies. J. Phys. 

Colloidal dispersions have a very large surface area for their volume. Therefore the surface properties of the phases have a large influence on the properties as a whole. Ice cream is simultaneously an emulsion (fat droplets), a sol (ice crystals) and a foam (air bubbles), and also contains other colloids in the form of casein micelles, other proteins and polysaccharides in the matrix. 

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