Generate nanotubes from milk protein for encapsulation, gelling applications.

Nanotubes are nanometer-scale wire-like structures. The milk protein alpha-lactalbumin can self-assemble into nanotubes, after the molecule has been partially hydrolyzed by a serine protease. These nanotubes potentially lend themselves for use as new food structures, dairy-derived viscosifiers and as vehicles for delivering ingredients.

The milk protein is already used as an ingredient in infant formula. Now, the unique ability of the protein to form these nanostructures may open up the ingredient to a wider range of uses, such as gelling and encapsulation applications. In gelling applications, the tubes could increase viscosity.

It may be possible to partially hydrolyze alpha-lactalbumin using protease enzymes from Bacillus licheneiformis. Exposing this partially hydrolyzed protein to calcium ions triggers the formation of a linear nanotube. These nanotubes have good stability and can withstand pasteurization treatments as well as freeze-drying treatments.

The nanotubes have a regular helical structure. The nanotubes' self-assembly only occurs in the presence of an appropriate divalent cation, such as calcium or manganese. Dutch scientists used a multidisciplinary approach to study various aspects of the process. The scientists examined the formation conditions, the mechanism and kinetics of self-assembly and disassembly, and the structure and properties of the nanotubes.

The researchers determined the outer diameter of the structures to be 21 nm and the inner diameter to be 8 nm--the cavity. Atomic force miscroscopy (AFM) testing revealed the right-handed helical structure of the tube wall. By performing nano-indentations with AFM, the investigators determined the mechanical properties of the tubes.

The tubes were relatively resilient upon small deformations. The elastic modulus is of the order of 0.1 GPa. The nanotubes could be broken at specific locations. Disassembly could be easily induced, which is of relevance for controlled-release applications.

The researchers also could make stable tubes by cross-linking, which would be a requisite for several other applications. During preparation, the nanotubes showed a weak tendency to associate and form reversible network structures.

Further information. C. G. de Kruif, Van't Hoff Laboratory, Debye Institute, Utrecht University, 3584CH, Utrecht, Netherlands; phone: +31 30 253 91 11; fax: +31 30 253 33 88; email: deKruif@nizo.nl.