Protein solutions, foam formation

Bovine serum albumin (BSA), a globular protein, is often applied as a model protein for foam formation. The surface tension of BSA solutions as a function of time indicates that, depending on the BSA concentration, it can take 15 to 20 h to attain an equihbrium surface tension which is independent of the BSA concentration. The coagulation rates are shght and the loss of native protein in the svuface film due to adsorption and denaturation is compensated by the quick and continuous diffusive transport of native protein to the surface. Therefore, the critical micelle concentration (CMC) and Ocmc can be evaluated from these measmements. 

Kinsella (13, 14) summarized present thinking on foam formation of protein solutions. When an aqueous suspension of protein ingredient (for example, flour, concentrate, or isolate) is agitated by whipping or aeration processes, it will encapsulate air into droplets or bubbles that are surrounded by a liquid film. The film consists of denatured protein that lowers the interfacial tension between air and water, facilitating deformation of the liquid and expansion against its surface tension. 

Solubility is a critical functional characteristic because many functional properties depend on the capacity of proteins to go into solution initially, e.g. gelation, emulsification, foam formation. Data on solubility of a protein under a variety of environmental conditions (pH, ionic strength, temperature) are useful diagnostically in providing information on prior treatment of a protein (i.e. if denaturation has occurred) and as indices of the potential applications of the protein, e.g. a protein with poor solubility is of little use in foams). Determination of solubility is the first test in evaluation of the potential functional properties of proteins and retention of solubility is a useful criterion when selecting methods for isolating and refining protein preparations (1). Several researchers have reported on the solubility of extracted microbial proteins (69,82,83,84). In many instances yeast proteins demonstrate very inferior solubility properties below pH 7.5 because of denaturation. 

Take a series of numbered reaction tubes and into each pour 2 c.c. of protein solution. To tube No. i add 6 c.c. of water, mixing the contents. Then add 2 c c. of the saturated solution of ammonium sulphate, shaking the tube carefully to avoid the formation of foam, and allow to stand. In tube No. 2 and the following add salt solution, increasing the amount each time by c.c., while diminishing the water added, so as to have in all the tubes a volume of 10 c c. The first tube in which appears a lasting opacity will indicate the lower limit of precipitation. This limit will be expressed by the number of cubic centimeters of the sulphate solution used. 

The main components of foam formation in cultivation media are proteins. Therefore, several authors have apphed solutions of particular proteins and used them as model media to investigate the behavior of biological foams. In this section foams formed by various protein solutions and foams produced by cultivation media are characterized. In addition, the properties of foams suppressed by antifoam agents (AFAs) are described. 

Very recently, pioneer work has been performed on the potentials of gas bubbles in protein solutions to obtain insight on formation and collapse of protein foams. As these foams play an important role in a number of processes, including foam formation in bioreactors, foam fractionation for protein recovery, and production of protein-based food and drinks, this work extends the study of the electric properties of the gas-liquid interfaces to biotechnology and food technology. 

Cultivations are often accompanied by foam formations due to the high foaming capacity of protein solutions. This capacity results from the stabilization of the gas liquid interface caused by the denaturation and strong adsorption of the surface proteins (21). 

The main factors, which determine the foam formation ability and physical stability of the foams in mixed Lys-MR solutions, are surface activity and complex dilatation modulus. With increase of MR concentration in certain range a surface activity and complex dilatation modulus of interfacial layers increased and phase angle decreased. It means that viscoelasticity of interfacial layers became higher. In these conditions the foam volume and mutiplicity as well as stability of foams were growing. This effect may be used for creation of mixed protein-MR system foam type for pharmaceutical applications with improved physical stability and wide range of antibacterial actions. 

Figure 10.6 shows sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) blots. On the left side the protein solution is shown after extraction, on the right side the fractionated proteins, pure cruciferin, and pure napin are shown. The cruciferin fraction has good emulsifying, film and gel formation properties, whereas the napin fraction is characterized by a high solnbility and foam stabilization properties. 

Foam fractionation (or separation) is an adsorptive bubble separation technique in which soluble, surface-active substances can be removed from solution by preferential adsorption at the gas-Uquid interface (Wang and Liu, 2003). Proteins contain both hydrophilic and hydrophobic amino acid residues that are surface active. During foam formation, bubbles... 

Systematic studies of the influence of border pressure on the kinetics of foam column destruction and foam lifetime have been performed in [18,24,41,64-71], Foams were produced from solution of various surfactants, including proteins, to which electrolytes were added (NaCI and KC1). The latter provide the formation of foams with different types of foam films (thin, common black and Newton black). The apparatus and measuring cells used are given in Fig. 1.4. The rates of foam column destruction as a function of pressure drop are plotted in Fig. 6.11 [68]. Increased pressure drop accelerates the rate of foam destruction and considerably shortens its lifetime. Furthermore, the increase in Ap boosts the tendency to avalanche-like destruction of the foam column as a whole and the process itself begins at higher values of foam dispersity. This means that at high pressure drops the foam lifetime is determined mainly by its induction period of existence, i.e. the time interval before the onset of its avalanche-like destruction. This time proves to be an appropriate and precise characteristic of foam column destruction. 

The results on formation and stability of black foam films, on the first place those on bilayer foam films (NBF) (see Sections 3.4.1.2 and 3.4.4) have promoted the development of methods which enable lung maturity evaluation. The research on stability of amphiphile bilayers and probability for their observation in the grey foam films laid the grounds of the method for assessment of foetal lung maturity created by Exerowa et al. [20,24]. Cordova et al. [25] named it Exerowa Black Film Method. It involves formation of films from amniotic fluid to which 47% ethanol and 7-10 2 mol dm 3 NaCl are added [20,24]. In the presence of alcohol the surface tension of the solution is 29 mN m 1 and the adsorption of proteins from the amniotic fluid at the solution/air interface is suppressed, while that of phospholipids predominates. On introducing alcohol, the CMC increases [26], so that the phospholipids are present also as monomers in the solution. The electrolyte reduces the electrostatic disjoining pressure thus providing formation of black foam lipid films (see Sections 3.4.1.2 and 3.4.4). 

While stirring the solution gently with a magnetic stirrer, slowly add 54 ml of cold, saturated ammonium sulfate solution (solution E in step 10-2) for each 100 ml supernatant II solution. The final concentration of ammonium sulfate will be 35%. Ammonium sulfate addition may be conveniently accomplished using a 10 ml pipette and should require 5 to 10 minutes for completion. If the solution is stirred too rapidly some of the proteins will denature. This becomes apparent by the formation of an off-white foam at the surface of the solution. Such denaturation is to be avoided. 

Soy proteins are applied in a wide range of food products. In this context most attention has been paid to the ability of soy proteins to form a gel upon heating. Heat denaturation and subsequent gel formation of soy proteins in bulk solutions have been extensively studied.1-5 Besides this, studies have been performed on the interfacial tension and adsorbed amount of soy proteins and on their suitability for formation and stabilisation of emulsions and foams.6-10 A question that, to our knowledge, has not been discussed is how far these different functional properties are mutually related. 

Proteins in solution reduce the surface tension of the water and consequently aid in formation of food foams. Measure foam persistence, foaming power, and foam stiffness—whipped toppings, frozen desserts, chiffon mixes, angel food cakes, meringues... 

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