Coalescence, polysaccharides

Protein-polysaccharide conjugates can also act as the stabilizers of multiple emulsions. Fechner et al. (2007) reported that, under acidic conditions, conjugate-containing water-in-oil-in-water emulsions were more stable to coalescence than the corresponding emulsions made with just sodium caseinate. They also observed that the extent of vitamin B]2 release from the inner aqueous phase of the conjugate-based system was significantly lower. This result could be useful for preparing double emulsions with variable release behaviour. 

Emulsions are dispersions of one fluid into another. Both oil-in-water and water-in-oil emulsions are encountered. Foams are similar to emulsions, but the dispersed phase is a gas. Emulsions are everywhere some examples of products that are based on emulsions are salad dressings, mayonnaise, egg yolk, milk, margarine, cream, ice cream, waterborne paints and bitumen. Emulsions are generally not stable, so they need to be stabilized against coalescence. One can use surfactants for that, or polymers, such as proteins and polysaccharides, or particles. 

Long-term stability of emulsified systems is usually provided by proteins or polysaccharides. The role of a good emulsion stabiliser is to keep the droplets apart once they have been formed. This protects the emulsion against processes such as creaming, flocculation and coalescence during long-term storage. 

Figure 2 Schematic representation of cellulose synthesis from Acetobacter xylinus (not to scale). Microfibrils of cellulose are secreted into the fermentation medium via terminal complex transmembrane synthetic sites. In the extracellular medium, a number of elementary microfibrils coalesce to form a flat, twisting and highly persistent ribbon of cellulose. The presence of polysaccharides in the fermentation medium allows interactions to occur both before and after the assembly of microfibrils into ribbons. The right angle bend at the point of ribbon assembly is purely schematic...

Indeed examination of many soil bacteria in situ reveals them to be heavily encapsulated. The synthesis of bacterial polysaccharides appears crucial to the maintenance of soil structure and ultimately soil fertility. The coalescence of mineral particles to form aggregates and the resistance of such aggregates to stress is a measure of a soil s stability. Many soil bacteria are able to bind soil particles into aggregates, through the production of extracellular polysaccharides, or by charge interactions. Addition of Azoto-bacter chroococcum or Pseudomonas sp. promotes the stabilisation of sterilised silt loam soils. 

Covalent protein-polysaccharide conjugates are sometimes used to avoid any flocculation and phase separation that is produced with weak non-specific protein-polysaccharide interactions. An example of such conjugates is that produced with globulin-dextran or bovine serum albumin-dextran. These conjugates produce emulsions with smaller droplets and narrower size distribution and they stabilize the emulsion against creaming and coalescence. 

Although the above sugar surfactants found many applications, particularly in cosmetics and personal care products, they are seldom very effective in stabilization of disperse systems against flocculation and/or coalescence. This is due to the reversible nature of adsorption of these molecules at the solid/liquid or liquid/liquid interfaces. For that reason we have developed a polymeric surface-active molecule based on inulin (which is extracted from chicory roots). Inulin is a polydisperse polysaccharide consisting mainly, if not exclusively, of j8(2 1) fructosyl fructose units with normally, but not necessar-... 

Figure 4.9 shows the creaming behaviour of an emulsion whose continuous phase contained 0.02% of a polysaccharide, hydroxyethylcellulose (HEC). This emulsion had the same primary droplet size distribution as the previous emulsion and was also stable to coalescence. The creaming behaviour, however, showed two distinct populations of droplets, one fraction moving rapidly to the top of the container and leaving behind about half the oil which creamed in a manner more similar to the polymer-free emulsion described above. ... 

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