Adsorption of hydrophobically modified starch at oil/water interfaces during emulsification
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The adsorption of starch that had been hydrophobically modified with octenyl succinate anhydride (OSA) at the oil/water interface during emulsification was studied. The starch samples were of waxy barley origin and were varied in molar mass and degree of substitution (DS). The particle size of the emulsions was measured and the adsorbed amount of starch was determined through serum depletion. The results show that adsorption is governed by the relationship between interfacial area and OSA-starch concentration. The surface load of OSA-starch can in some cases become very high, reaching 16 mg/m(2). The emulsification occurs under nonequilibrium and turbulent flow conditions. Under these conditions kinetic factors are likely to play an important role in the adsorption process. Turbulent flow favors transport to the interface of larger molecules over small ones, which could lead to higher surface loads by causing jamming at the interface. A model that treats the adsorption as a collision between particles in turbulent flow has been used, and it shows that the adsorption time of a polymer decreases with increasing polymer radius. It also shows that the time scale of adsorption is shorter than the time scales for configurational changes of macromolecules at interfaces and that emulsion droplet-droplet collisions are of similar time scales as adsorption, which gives further indications that kinetic factors are important during adsorption. The simulation results give a reasonable explanation to why large molecules such as OSA-starch can be efficient as emulsifiers.