Foams, formation and stability

Wilde, P.J. and Clark, D.C. 1996. Foam formation and stability. In Methods of Testing Protein Functionality (G.M. Flail, ed.) pp. 110-152. Blackie Academic and Professional, New York. 

In contrast to protein stabilized foams the foam formation and stabilization mechanisms in whipping cream are supposed to depend on the bubble stabilization by means of fat rather than proteins. The fat content can therefore not be less than 30% butterfat, and whipping cream is expected to fulfil certain criteria, namely whipping time, foam firmness, foam volume increase (or overrun), and volume of dripping-off (or drainage). 

The physicochemical models consistent with foam formation and stabilization are derived from the work on pasteurized and UHT processed miUc-creams advanced by Besner (1997). Foam formation and stabilization can basically be explained as a multistage process, with some significant differences between non-homogenized (pasteurized cream) and homogenized (UHT cream) systems. These differences... 

The final filtration step is not meant to remove significant amounts of particles or to reduce turbidity. For economic reasons, there should not be many particles left from the first filtration step when entering into the second (final) filtration step. Only if this condition is maintained the costs for the secondary filtration can be kept low. Also, the filtration should only remove microorganisms, and not retain other useful components of beer, i.e., those proteins that have a role in foam formation and stability. On the other hand, bacteria, which should be separated from beer during final filtration, typically have sizes down to 0.5 p,m. This small difference in size between the desirable ingredients and those particles that should be removed, such as bacteria, shows that the selection of the filtration technique and media needs to be done very carefully. 

In addition to their importance for white wine stability, proteins seems to be involved in other aspects of wine quality. For example, it is recognized that proteins can interact with aromatic compounds (Lubbers et al., 1994), influence the perception of wine body in the mouth, and, due to their surface properties, affect foam formation and stability in sparkling wines (Brissonet and Maujean, 1993). 

Frothing is an imwanted effect of surface active water soluble pol5nners. D5mamic surface properties of the solution-air interface due to the presence of the polymer play an important role in foam formation and stability. The surface tension decrease due to adsorbed polymer plays a lesser role (see for example Ref. [84]). 

From the plot of the values of IFT measured at different surfactant concentrations, such as shown in Figure 12, the critical micelle concentration (CMC) can be determined as the concentration at which the change of slope occurs. Figure 12 shows such a plot made at reservoir conditions in a particular field. The CMC is a useful reference concentration for a particular surfactant, although its full significance for foam formation and stability within porous media is not yet known. 

The most important functional properties of proteins are solubility, water absorption and binding, rheology modification, emulsifying activity and emulsion stabilization, gel formation, foam formation and stabilization, and fat absorption [1-6]. They reflect the inherent properties of proteins as well as the manner of interaction with other components of the system under investigation. 

Examples of industrial relevance for the first two phase combinations are the adsorption of pollutants from waste air or water onto activated carbon. Combinations three and four are relevant, for example, related to foam formation and stabilization in the presence of surfactants on water/air interfaces or at the interface of two immiscible liquids (e.g., oil and water). This book deals mainly with the case most typical for preparative chromatographic separations, that is, the exploitation of solid surfaces, liquid mobile phases, and dissolved feed mixtures. The following definitions are made The solid onto which adsorption occurs is defined as the adsorbent. The adsorbed molecule is defined in its free state as the adsorptive and in its adsorbed state as the adsorpt. There are typically different solutes, which are often called components (for example, A and B, Figure 2.1). 

By understanding the basic laws governing foam formation and the physical and chemical characteristics of materials that produce and sustain foams, or prevent and destroy them, the investigator or operator is well equipped to maximize (or minimize) the desired foaming effect. In the following sections, some of the basic physical principles of foam formation and stabilization will be covered along with some practical approaches to problems of foam characterization and control. 

Foams, like emulsions, are inherently unstable systems. Because they are encountered in so many technological areas, they have been the subject of a significant amount of investigation and discussion in the literature. A number of reviews have been published over the years that cover most aspects of foam formation and stabilization (see BibUography). While the theoretical aspects of stabilization are reasonably well worked out, a great deal remains to be understood concerning the practical details of foam formation, persistence, and prevention. 

For this reason, some American wineries have adopted the use of products that increase surface tension. This process reduces foam formation and stability. Two anti-foaming agents are gaining popularity dimethyl polysiloxane and a mixture of oleic acid mono- and diglyceride. They are used at a concentration of less than 10 mg/1 and do not leave a residue in wine, especially after filtration. Due to their efficiency, red wine tanks can be filled to 75-80% capacity and white wine tanks to 85-90%. These products are not toxic. The Office International de la Vigne et du Vin recommends the exclusive use of the mixture of oleic acid mono-and diglyceride. 

The existence of electrical charges at any interface will give rise to electrical effects, which will, in many cases, determine the major characteristics of that interface. Those characteristics will affect many of the properties of a multicomponent system, including emulsion and foam formation and stability, solid dispersions, and aerosols. The theoretical and practical aspects of electrical double layers are the subject of a vast amount of literature and for that reason have not been addressed in any detail so far. Such details can be found in bibliographic references cited for this chapter. 

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