Protein foam formation

In a gas and liquid system, when gas is introduced into a culture medium, bubbles are formed. The bubbles rise rapidly through the medium and dispersion of the bubbles occurs at surface, forming froth. The froth collapses by coalescence, but in most cases the fermentation broth is viscous so this coalescence may be reduced to form stable froth. Any compounds in the broth, such as proteins, that reduce the surface tension may influence foam formation. The stability of preventing bubbles coalescing depends on the film elasticity, which is increased by the presence of peptides, proteins and soaps. On the other hand, the presence of alcohols and fatty acids will make the foam unstable. 

Adsorbents that remove proteins or polyphenols are used to treat a number of beverages to delay the onset of haze formation. Protein adsorbents include bentonite and silica. Bentonite removes protein nonspecifically (see Fig. 2.19) and so is unsuitable for stabilizing beverages where foam is desirable (beer and champagne). Silica, on the other hand, has remarkable specificity for HA proteins while virtually sparing foam-active proteins in beer (Siebert and Lynn, 1997b) (see Fig. 2.20). Silica removes approximately 80% of the HA protein from unstabilized beer, while leaving foam-active protein nearly untouched at commercial treatment levels. 

Cumper and Alexander ( ) and Cumper (T) explained that during foam formation, a monolayer of surface denatured protein surrounded by liquid is rapidly adsorbed at the interface of the colloidal mixture, trapping air and forming bubbles (Table I,... 

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. 

Foam properties related to salt. The addition of sodium chloride to soybean protein suspensions caused them to form high-capacity, low-stability foams (13). It was suggested that foam capacity increased because salt improved protein solubility at the interface of the colloidal suspension during foam formation, but retarded the partial denaturation of the surface polypeptides of proteins that are necessary for protein-protein interaction and stability. 

It is essential to consider the physico-chemical properties of each WPC and casein product in order to effectively evaluate their emulsification properties. Otherwise, results merely indicate the previous processing conditions rather than the inherent functional properties for these various products. Those processing treatments that promote protein denaturatlon, protein-protein Interaction via disulfide interchange, enzymatic modification and other basic alterations in the physico-chemical properties of the proteins will often result in protein products with unsatisfactory emulsification properties, since they would lack the ability to unfold at the emulsion interface and thus would be unable to function. It is recommended that those factors normally considered for production of protein products to be used in foam formation and foam stabilization be considered also, since both phenomena possess similar physico-chemical and functionality requirements (30,31). 

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 Figure 2, the interfacial tension of coffee oil with a high content of volatile flavours against CC>2 is depicted. Mixtures like this are of particular interest for high pressure spray extraction. At increasing density of the fluid CO2 -phase, interfacial tension is decreased by dissolution of CO2 at the interface. In this case, presence of surface active material in the liquid phase, e.g. proteins, rather seem to be of subordinate importance. With respect to foam formation these surfactants neither show their known stabilising effect as long as no polar phase such as water is added. 

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. 

If a barbotage technique is employed in foam formation and the foams produced are of low stability, it is possible to reach a steady-state at which the rate of foam formation becomes equal to the rate of the decrease in foam column height, and during a long period of time the volume of the foam remains constant. It should be emphasised that a certain inaccuracy in the measurement of the foam column height can originate from an non-distinct (diffuse) liquid/foam boundary or roughness of the upper foam boundary (especially in structured foams, e.g. from proteins). 

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 majority of the nitrogen compounds in beer have molecular weights between 5 and 70 kDa. The protein components of this fraction are of particular importance in brewing, as some of them contribute to foam formation (positive effect) while others, in association with polyphenols, lead to haze formation (undesirable effect) [10]. 

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). 

A look at Figure 10.4 shows that 1% ethanol added to water (i.e., about 0.22 molar) causes a significant decrease in surface tension of about 5mN m 1. Half a percent of protein is sufficient for protein-covered foam bubbles to form, but—as discussed in Chapter 11—a small-molecule surfactant is more efficient than protein in the formation of foam and the breaking up of large bubbles into smaller ones. This is because the protein has a low molar concentration and needs to unfold upon adsorption to cause considerable lowering of surface tension, which takes a relatively long time. Hence the ethanol should indeed promote foam formation. 

There have been a limited number of studies on the effects of enzymic modification of protein concentrates on functional properties other than solubility. Studies on functional properties, as modified by enzymic treatments, emphasize foam formation and emulsifying characteristics of the hydrolysates. Treatment of chicken egg albumen alters the functional properties of the egg proteins in terms of foam volume and stability and the behavior of the proteins in angel food cakes (25). Various proteolytic enzymes were used to degrade the egg albumen partially. However, proteolytic enzyme inhibitors indigenous to the egg proteins repressed hydrolysis of the egg proteins compared with casein. 

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. 

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