3.1 Efficiency in foam formation
Since the foaming protocol determines that the nitrogen flow stops once the foam height reaches 120 ml, the foaming time, the gas required during the bubbling, and the remaining liquid will depend on the ease of the solution to form the foam, which would relate to the overall foaming capacity in each case. In Table 1 it can be seen the Final Time of Foaming (FTF), the Total Gas Volume (TGV) incorporated into the foam, and the Final liquid volume (FLV), corresponding to the remaining liquid after foam formation; for proteins at 2 and 6%p/p and mixed systems. Thus, we can assume a better efficiency in foam formation when the Final Time of Foaming is low, the Total Gas Volume is incorporated, and the liquid is incorporated, which means, a low remaining FLV [8].
Table 1
Final Time of Foaming (FTF), Total Gas Volume (TGV), and Final Liquid volume (FLV) for WPI, at 2 and 6%w/w and the polysaccharides mixed systems (E4M and E50lv at 0.25%w/w).
| System | FTF(± 10 s.) | TGV(± 5 cm3) | FLV(± 3 cm3) |
| Wpi2% | 205.5 | 150 | 14.75 |
| Wpi6% | 147.5 | 107 | 7.8 |
| E4M0.25% | 153 | 112 | 9.8 |
| E50lv0.25% | 131 | 94.5 | 8.8 |
| W2%/E4M0.25% | 154.5 | 112.5 | 10 |
| W6%/E4M0.25% | 156 | 113 | 9.9 |
| W2%/E50lv0.25% | 414 | 80 | 14.3 |
| W6%/E50lv0.25% | 442 | 86 | 13.1 |
Table 1 shows that the three parameters were acceptably low when PSs were alone and WPI at a higher concentration (6%). However, when mixed systems were studied, a remarkable improvement was observed for WPI + E4M systems.
This implies a better performance with the E4M addition, which means, a good foam formation, with lower gas quantity needed and more solution incorporation. By comparing with the PSs behavior alone, it could be seen that mixed systems with E4M followed the E4M alone behavior, even with the bulk viscosity increase after its addition.
Perez et al. (2007) [9] determined the mixed performance of whey protein concentrate and HPMC systems at air-liquid interfaces. They studied the competitive behavior of mixed films adsorbed using E4M, E50lv, and F4M with different interfacial properties, studied by measurement of the dynamics of adsorption. The interfacial behavior difference may be attributed to differences in the molecular weight between PSs.
In the same way, the mixture approached E4M behavior, due to polysaccharide higher surface-activity at liquid interfaces.
On the other hand, E50lv showed no help to foam formation, taking more time and liquid incorporation to reach 120 ml, even at high WPI concentrations. However, the gas quantity was the lowest (TGV), performing as E50lv alone as well. All these results showed that the forming process of foams is a complicated physical phenomenon where not all macromolecules behave in a homogenous mode. That is to say, the molecules that act as better foam volume forming do not necessarily make them more efficient.
3.2 Effects of mixed systems on parameters for foam formation and stability
In Fig. 1 (a-d) it can be seen the mixing effect for WPI and PSs, Ε4Μ and E50lv for, Cf (%), FE; MD, and FC. It can be seen, in general terms, that there were no substantial changes in FE and MD (Figs. 1b-c), however, there were slight changes for FC, (Fig. 1d) where the PSs addition provoked an increase to every WPI concentration.
As FC includes the final foam volume for every system (120 ml) and the gas incorporated (TGV), the changes in this parameter would correspond to differences in the gas used. More precisely, as was described above (Table 1), a significant decrease in the TGV used to get 120 ml of foam for the WPI + E50lv foamed systems was found. Therefore, these systems showed the higher FC parameters in Fig. 1d.
On the other hand, it can be seen a good concordance between Cf (%) (Fig. 1a) and the described previous results. The FLV (Table 1) showed a good E4M performance in the mixed systems, whereas the same results were obtained in the Cf(%) parameter analysis. The highest values were found for these same samples (WPI2%+E4M and WPI6%+E4M). The conclusion for this fact would be the existence of a good correlation between the liquid in the foam and conductivity. Although bubble size or shape was not studied here, a positive relation was found that makes the conductivity a reliable physical parameter to analyze the foams performance.
Table 2 shows the Liquid Stability for the systems studied. More time of stability indicates more time of liquid permanence into the foam, representing a more stable system against the drainage of liquid. In the Table can be seen that the Liquid Stability was higher when WPI was at 6%, which is logical, from a point of view of viscosity effects between others. When PSs were added, different results were obtained depend the PS used. The mixed systems with E50lv showed extraordinary stability against liquid. It needs to be in mind that, these foams incorporated less quantity of liquid compared with the WPI + E4M systems (FLV, Table 1), nevertheless, they resulted in more stable foam against drainage of liquid. The stability of foam could be explained by the tension-activity of molecules that led to determined interfacial film viscoelasticity formation with time and the phase continuous viscosity [10].
Table 2
Liquid Stability for systems WPI and E4M, E50lv mixed systems.
| System | Liquid Stability (s)(± 40) |
| WPI2% | 158 |
| WPI2%/E4M | 237 |
| WPI2%/E50lv | 623 |
| WPI6% | 341 |
| WPI6%/E4M | 193 |
| WPI6%/E50lv | 530 |
Previously, contrary results we found. It studied the effects of ultrasound of high intensity on the foamability of soy protein isolate and whey protein isolate at similar concentrations at pH 7. It was seen for soy protein isolate that an increment of foaming time and density increase at the same time, whereas, a decrease of foaming time and lower foam density was found for whey protein isolate [11]. Thus, more solution incorporated into foam would lead to more amount of surfactant molecules present (protein and polysaccharides in this case) at the air-liquid interface, promoting a better viscoelastic interfacial film formation, increasing the liquid retention and consequently, time to drainage delay [10, 12–13]. Taking into account that electrostatic protein charge makes some different molecular interaction, the PSs presence would alter completely the relation: time of foaming-liquid incorporation, denoting the complexity of phenomena.
As a result, more studies at the interfacial level should be done with other types of PSs such as anionic ones to determine which factor is more relevant at so low pH.