Biopolymer surface activation

An alumina matrix may be prepared with high pore density (more than 60 %) and pore diameters ranging from 5 to 250 nm. Ruiz-Hitzky et al. [214] immobilized GOD in nanoporous alumina membranes with regular hexagonal arrays of highly ordered cylindrical pores aligned perpendicularly to the membrane surface. GOD was anchored in the membrane by the highly hydrophilic chitosan biopolymer. Full activity was maintained for at least 50 hours. 

Adsorption of (bio)polymers occurs ubiquitously, and among the biopolymers, proteins are most surface active. Wherever and whenever a protein-containing (aqueous) solution is exposed to a (solid) surface, it results in the spontaneous accumulation of protein molecules at the solid-water interface, thereby altering the characteristics of the sorbent surface and, in most cases, of the protein molecules as well (Malmsten 2003). Therefore, the interaction between proteins and interfaces attracts attention from a wide variety of disciplines, ranging from environmental sciences to food processing and medical sciences. 

Information on the chemical potentials of components in a solution of biopolymers can serve as a guide to trends in surface activity of the biopolymers at fluid interfaces (air-water, oil-water). In the thermodynamic context we need look no further than the Gibbs adsorption equation,... 

There is now a solid body of available knowledge to indicate that the general features of biopolymer self-assembly in bulk aqueous solutions can account for various detailed aspects of the stability, rheology and microstructure of oil-in-water emulsions (and foams) stabilized by the same kinds of biopolymers (Dickinson, 1997, 1998 Casanova and Dickinson, 1998 Dickinson et al., 1997, 1998 Semenova et al., 1999, 2006 van der Linden, 2006 Semenova, 2007 Ruis et al., 2007). In particular, the richness of the self-assembly and surface-active properties of the... 

The presence of a thermodynamically incompatible polysaccharide in the aqueous phase can enhance the effective protein emulsifying capacity. The greater surface activity of the protein in the mixed biopolymer system facilitates the creation of smaller emulsion droplets, i.e., an increase in total surface area of the freshly prepared emulsion stabilized by the mixture of thermodynamically incompatible biopolymers (see Figure 3.4) (Dickinson and Semenova, 1992 Semenova el al., 1999a Tsapkina et al., 1992 Makri et al., 2005). It should be noted, however, that some hydrocolloids do cause a reduction in the protein emulsifying capacity by reducing the protein adsorption efficiency as a result of viscosity effects. 

The presence of a thermodynamically favourable interaction between protein and polysaccharide is commonly associated with a marked decrease in protein surface activity at the air-water or oil-water interface (see Figures 7.5b and 7.15). There is a slower decay in the surface tension for complexes in comparison with the pure protein, and also higher values of the tension in the steady state. Data establishing these trends have been reported for the following biopolymer pairs in aqueous media legumin + dextran and legumin + maltodextrin (Antipova and Semenova,... 

Figure 7.15 Effect of thermodynamically favourable interactions between biopolymers on protein surface activity at the planar oil-water or air-water interface. The surface pressure n reached after 6 hours is plotted against the polysaccharide concentration ( ), legumin (0.001 wt%) + dextran (Mw = 270 kDa) at / -decane-water surface at pH = 7.8 and ionic strength = 0.01 M, (Ay = -0.2 x 105 cm3 mol1) (Pavlovskaya et ah, 1993) ( ), legumin (0.001 wt%) + maltodextrin (MD6, Mw = 102 kDa) at air-water surface at pH = 7.2 and ionic strength = 0.05 M (Ay = - 0.02 x 105 cm3 mol-1) (Belyakova et ah, 1999) (A), legumin (0.001 wt%) + maltodextrin (MD10, Mw = 45 kDa) at air-water surface at pH = 7.2 and ionic strength = 0.05 M (.1 / = - 0.08 x 105 cm3 mol-1) (Belyakova et ah, 1999).

It is important to understand the characteristic interactions involved at an interface containing each of the main types of surface-active molecules, i.e., biopolymers (proteins, polysaccharides) and low-molecular-weight surfactants (lipids). But that is not the whole story. In real food systems there are almost always mixed ingredients at the interface. So it is necessary to understand what sorts of mixed interfacial structures are formed, and how they are influenced by the intermolecular interactions. 

In the case of a protein + polysaccharide mixture, whether the adsorption process is competitive or cooperative in character will depend on the concentration and surface activity of each adsorbed biopolymer species, and on the nature and strength of the protein-polysaccharide interactions (Murray, 2002 Baeza et al., 2005 Dickinson, 2008). In addition, the... 

Figure 8.12 illustrates the effect of complex formation between protein and polysaccharide on the time-dependent surface shear viscosity at the oil-water interface for the system BSA + dextran sulfate (DS) at pH = 7 and ionic strength = 50 mM. The film adsorbed from the 10 wt % solution of pure protein has a surface viscosity of t]s > 200 mPa s after 24 h. As the polysaccharide is not itself surface-active, it exhibited no measurable surface viscosity (t]s < 1 niPa s). But, when 10 wt% DS was introduced into the aqueous phase below the 24-hour-old BSA film, the surface viscosity showed an increase (after a further 24 h) to a value around twice that for the original protein film. Hence, in this case, the new protein-polysaccharide interactions induced at the oil-water interface were sufficiently strong to influence considerably the viscoelastic properties of the adsorbed biopolymer layer. 

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

In Part Four (Chapter eight) we focus on the interactions of mixed systems of surface-active biopolymers (proteins and polysaccharides) and surface-active lipids (surfactants/emulsifiers) at oil-water and air-water interfaces. We describe how these interactions affect mechanisms controlling the behaviour of colloidal systems containing mixed ingredients. We show how the properties of biopolymer-based adsorption layers are affected by an interplay of phenomena which include selfassociation, complexation, phase separation, and competitive displacement. 

AQUEOUS LAYER-7 pm containing In dissolved form Inorganic salts, glucose, urea and surface active biopolymers, proteins and glycoproteins... 

In the mixed systems, the behavior was similar to that observed for surface pressure. In the presence of surface-active PGA (Figure 25.3a and b) at low concentrations in the bulk phase (0.1 wt%), competition between the biopolymers at the interface results in a lower Ed than that expected from the observation of the single components. However, at higher concentrations of PS and long adsorption times, a cooperative adsorption can be deduced. This result could be explained by a concentration of (3-lg at the interface caused by the incompatibility with different biopolymers (that is more evident at higher concentrations). These phenomena would lead to an increase in the protein association in the film with the resultant increase in viscoelasticity. 

Proteins are biopolymers that are encountered in many applications, such as food emulsions, hair conditioners, photographic emulsions, and various medical diagnostic products. Many of these applications are frequently based on the unique surface activity of the proteins, which is reflected in functional properties such as foaming, emulsification, and gelling. The proteins are composed of polymeric chains containing many hydrophobic and hydrophilic domains, often giving the molecules an amphipathic structure somewhat similar to that of polymeric surfactants. 

In more recent studies (51, 52) the biopolymer chitosan was used as an emulsifier in food double emulsions. Chitosan has surface activity and seems to stabilize W/OAV emulsions. Chitosan reacts with anionic emulsifiers such as sodium dodecyl sulfate at certain ratios to form a water-insoluble complex that has strong emulsification capabilities. Chitosan solution was used to form double emulsions of OAV/0 as intermediates fi om which by a simple procedure of stripping the water the authors formed interesting porous spherical particles of chitosan (52). 

Polymer adsorption occurs ubiquitously. Among the biopolymers, proteins are the most surface active. The interaction between proteins and interfaces attracts attention from various disciplines, ranging from soil and food science to medical sciences. 

Electrical surface modification of biopolymers requires surface activation. In the presence of radicals, the two reactive groups —OH and —NH easily lose their protons into the surrounding environment (Fig. 10.4). The resulting radicals, for example, —O and —NH, are able to anchor to ECMs, after which propagation of the ECMs ensues. The intermediate complex of the radical ECMs-biopolymer is called a radical reactivation stage. 

The surface activation pretreatments prior to electroconductive surface modification are similar for polysaccharides and protein. In order to initiate the reaction with monomers, the molecules should be pretreated with alkaline, creating highly polar alkoxide segments. Some highly polar ECMs are however able to break the intermo-lecular bonds of biopolymers at neutral or mild acidic pH [17,19]. Another strategy of pretreatment is preoxidation of biopolymers, creating radicals. The lifetime of the radicals is directly related to environmental physical parameters, such as pH, polarity of solvents, and temperature. 

Polyhydroxy acids are another group of biopolymers. Since polylactic acids, polyglycolic acid, and poly(citric acid) are classified as thermoplastic polyesters (saturated), they lack reactive functional groups for surface reactions. Moreover, any chemical manipulation to create activation sites results in hydrolysis of the ester bonds. The only reported successful methods for functionalization of polyhydroxy acids are blending them with ECPs, or using a plasma polymerization process [29]. Prior to the plasma polymerization process, surface activation or ionization of these biopolymers must be carried out, which is acquired by means of vapor phase deposition, laser deposition, microwave or synchrotron radiation [30], pulsed arc, pulsed combustion, spark, or friction induction [30], electron beams, plasma induction, corona, photons, ion beams, and X-rays [25]. 

Gum arabic is widely used as an emulsifier in the beverage industry to stabilize cloud and flavor emulsions [98]. It is derived from the natural exudate of Acacia Senegal, and consists of at least three high molecular weight biopolymer fractions The surface-active fraction is believed to consist of branched arabinogalactan blocks attached to a polypeptide... 

Rosenberg E, Ron EZ (1998) Surface active polymers from the genus Acinetobacter. In Kaplan DL (ed) Biopolymers from renewable resources. Springer, Berlin, pp 281—291... 

In synthesis of both organic and inorganic nanomaterials in the inverse micellar solutions under mild and high temperatures, the role of organic additives, such as emulsifiers, coemulsifiers, surface active additives, polymers, biopolymers, etc. in the preparation of polymer and metal particles and nanostructured materials is reviewed. In the synthesis of... 

In soils and clays the most generally present biopolymers that naturally occur in the adsorbed state on mineral and clay particles are humic substances , or humic acids these are decomposition products of lignin, whidi is the major non-cellulosic polymer in wood and other plant debris. Humic acids (also called allomelanins (Merck Index, 1989)) are for the greater part polyphenolic compounds, usueJly anionic polyelectrolytes, which can complex metal ions, and are surface active and thus capable, upon adsorption onto mineral particles, to enhance their suspension stability in aqueous media (Chheda and Grasso, 1994). 

The direct demonstration of a surfactant film in the airways is relatively recent (2-4), although a surface-active film had been inferred from physiological (5) and electron microscopic studies (6) many years before. The surface tension in large airways has been measured directly with a bronchoscope from the spreading behavior of oil droplets placed onto the tracheal walls or bronchi of anesthetized sheep and horses (7,8). A surface tension of approximately 32 mN/m has been recorded at the mucus-air interface in these animals. This relatively low surface tension suggests the presence of a surface film in large airways, because proteins, surface polymers of blood cells, polysaccharides, and other biopolymers, all have... 

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