Proteins foam stabilisers

The low molecular protein, the antibiotic nisin, was successfully concentrated in industrial scale from a culture liquid in a high expansion ratio foam, stabilised with the foaming agent Tween-80. In the absence of a special foaming agent no stable foam was produced from the culture liquid. Nisin could be extracted by adding Tween-80, though the... 

Similar differences in the surfactant nature of soluble and insoluble species can be found in studies of foam stability. Broadly speaking, insoluble or solid-type surfactants do not efficiently stabilise foams (Garrett, 1967b). On the other hand, water-soluble surfactants such as the alkyl sulphonates, phosphates and tri-alkyl ammonium salts, all common detergents, form prolific foams. Wilson (1959) noted the accumulation of albuminoid-N compoimds in coastal sea foams, while Southward (1953), who found a variety of planktonic and benthic oi anisms in such foams, concluded that proteins were probably responsible for the foam stability. Wilson and Collier (1972) have observed the production of such foam-stabilising events in appreciable quantities by various marine organisms such as diatoms. 

Some proteins (especially lipid transfer protein and protein Z, see Section 2.4.5.4.1) and hydrophobic polypeptides resulting from barley proteins in malt, through endogenous proteases and yeast autolysis, are important foam stabilisers in beer. 

A stable foam is likely to have ingredients that are in a low energy state at the air-liquid interface. Substances that fit this description include proteins, emulsifiers some fats and fat components such as diglycerides monoglycerides and fatty acids. Food law uses the term emulsifier and stabiliser to cover the situation where the ingredient is stabilising an emulsion rather than helping to form it. 

Once the product is allowed to emerge from the extruder the water flashes into steam, expanding the product to a foam. Any protein and emulsifiers present help stabilise the bubbles, which set as the product cools. As the water flashed to steam the latent heat of the steam is lost to the product thus cooling it rapidly. 

Abstract. Surface pressure/area isotherms of monolayers of micro- and nanoparticles at fluid/liquid interfaces can be used to obtain information about particle properties (dimensions, interfacial contact angles), the structure of interfacial particle layers, interparticle interactions as well as relaxation processes within layers. Such information is important for understanding the stabilisation/destabilisation effects of particles for emulsions and foams. For a correct description of II-A isotherms of nanoparticle monolayers, the significant differences in particle size and solvent molecule size should be taken into account. The corresponding equations are derived by using the thermodynamic model of a two-dimensional solution. The equations not only provide satisfactory agreement with experimental data for the surface pressure of monolayers in a wide range of particle sizes from 75 pm to 7.5 nm, but also predict the areas per particle and per solvent molecule close to the experimental values. Similar equations can also be applied to protein molecule monolayers at liquid interfaces. 

Tobias, J., Tracy, P.H. 1958. Observation on low fat dairy spreads. J. Dairy Sci. 41, 1117-1120. Tornberg, E., Ediriweera, N. 1987. Coalescence stability of protein-stabilised emulsions. In Food Emulsions and Foams (E. Dickinson, ed.), pp. 52-63, Royal Society of Chemistry, London. 

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. 

The FRAP method has been applied to the measurements of molecular lateral diffusion of molecules adsorbed at the interface of equilibrium common thin foam films and of black foam films [39-43], Initially Clark et al. reported FRAP measurement of surface diffusion of the fluorescence probe 5-N(octadecanoyl)aminofluorescein incorporated into foam films stabilised with NaDoS [39]. Then followed the measurements of protein-stabilised foam films where the protein was covalently labelled with fluorescein [40,41], Studies of FRAP measurements of surface lateral diffusion in equilibrium phospholipid common thin foam films and black foam films were also reported [42,43]. 

Thin liquid films (especially foam films) stabilised with phospholipids, proteins, etc., prove to be very suitable in the study of surface forces, since they could model the interacting biological membranes in aqueous medium. 

Indeed, a direct relationship between the lifetimes of films and foams and the mechanical properties of the adsorption layers has been proven to exist [e.g. 13,39,61-63], A decrease in stability with the increase in surface viscosity and layer strength has been reported in some earlier works. The structural-mechanical factor in the various systems, for instance, in multilayer stratified films, protein systems, liquid crystals, could act in either directions it might stabilise or destabilise them. Hence, quantitative data about the effect of this factor on the kinetics of thinning, ability (or inability) to form equilibrium films, especially black films, response to the external local disturbances, etc. could be derived only when it is considered along with the other stabilising (kinetic and thermodynamic) factors. Similar quantitative relations have not been established yet. Evidence on this influence can be found in [e.g. 2,13,39,44,63-65]. 

The latter, however, do not have the capacity, once adsorbed, to stabilise the foam, it is well established that pure liquids do not foam. Transient foams are obtained with solutes such as short-chain aliphatic alcohols or acids which lower the surface tension moderately really persistent foams arise only with solutes that lower the surface tension strongly in dilute solution - the highly surface-active materials such as detergents and proteins. The physical chemistry of the surface layers of the solutions is what determines the stability of the system. 

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. 

Let us now discuss mixtures of proteins with low-molecular weight surfactants. These mixtures are of great practical importance for the stabilisation of emulsions and foams, and play an important role in biological systems. Let us consider equilibrium ideal (with respect to the enthalpy) solutions of proteins and surfactants. If such a solution contains i different surfactants (or proteins) which exist in j different states at the interface, the following system of equations from (2.26) and (2.27) may be formulated [24,25] ... 

Many food items contain emulsions and foams, which are often stabilised by proteins forming a protective membrane at the interface. By preparing the food, adsorption of the available proteins, - by virtue of their surface activity -, is performed at the liquid/air (foams) and/or at the liquid/liquid (emulsions) interface. One way to study the interfacial behaviour of food proteins at those interfaces is to follow the interfacial tension decay accomplished by the adsorption of the proteins. 

Proteins and proteoglycans are important stabilisers of emulsions and foams in many food and non-food applications [3]. The interactions between protein molecules, either when adsorbed at interfaces or when present in the bulk, are very complex and are connected with changes in the conformation of the folded polypeptide chains. These changes have a wide range of characteristic time-scales [4]. When these particular features of proteins are superimposed on the above picture of a deformed interfacial film it is seen that the task of understanding the mechanism of action of proteins as surfactants is a daunting task. 

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