Multiple and composite emulsions stabilised by the interfacial protein BslA
BslA (Bacterial Surface Layer A) is a protein expressed by Bacillus subtilis bacteria within a biofilm to provide a hydrophobic ?raincoat? at the biofilm surface. Our in vitro studies revealed that BslA is not particularly amphiphilic in solution, but can produce a hydrophobic cap ?on demand? upon adsorption to an air- or oil-water interface. After adsorption, BslA self-assembles to form a flexible, elastic film at the interface. Building on our understanding of how BslA partitions to and stabilises oil-water interfaces, we have studied emulsification using a simple BslA-triglyceride-water system by adjusting three primary parameters: oil volume fraction, BslA concentration and shear duration. Under certain conditions, we found that BslA was able to stabilise multiple emulsions (water-in-oil-in-water [W/O/W]) in a single emulsification step, a highly unusual observation that is unprecedented for a natural folded protein. In addition, shearing the emulsion for longer durations actually resulted in the creation of two phase separated emulsions, a W/O emulsion above an O/W emulsion.
We demonstrate that multiple emulsion formation is dependent on droplet-droplet collisions and that the BslA that stabilises the internal droplets can be ?recruited? from the outer O/W interface of the colliding droplets. This loss of stabiliser from the outer O/W interface can ultimately lead to the destabilisation and coalescence of the multiple emulsion drops and the subsequent formation of a separate W/O emulsion.
Finally, we have studied BslA in composite materials and discovered that rather than being removed from the interface by competing surfactants, BslA is able to function alongside them. As a result, we have created a number of BslA-stabilised multiphase composite materials, including a tasty BslA ice cream!
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