Milk protein nanotubes offer encapsulation potential

While the protein is already used as an ingredient largely in infant formula, with Danish firm Arla the leading alpha-lactalbumin producers, the unique ability of the protein to form nanotubes may open up the ingredient to a wider range of applications.

Food manufacturers are increasingly turning to encapsulation technologies as a way of achieving much-needed differentiation and enhancing product value. Tapping into key and emerging consumer trends with innovative techniques is becoming increasingly important for food manufacturers.

While the majority of focus has been on micro​encapsulation, the authors of a new review in the journal Trends in Food Science and Technology​ (Vol. 17, pp. 196-203), J. Graveland-Bikker and C. de Kruif suggest that the special properties of alpha-lactalbumin may see the milk protein become a key player in nano​encapsulation.

Under appropriate conditions, alpha-lactalbumin can be partially hydrolysed by protease enzymes from Bacillus licheneiformis​. When this partially hydrolysed protein is exposed to calcium ions, the formation of a linear nanotube is triggered.

These nanotubes, said to be the only food protein nanotube, have good stability and can withstand pasteurisation conditions (72 degrees Celsius for 40 seconds) as well as a freeze-drying treatment, said the authors.

"The most special feature of the alpha-lactalbumin nanotube is perhaps its cavity,"​ wrote Graveland-Bikker, formerly with NIZO food research, and de Kruif, from Utrecht University.

"Because of their cavity, the alpha-lactalbumin nanotubes could well serve as a vehicle for encapsulated molecules, such as for example vitamins and enzymes, or protect or mask encapsulated compounds,"​ they said.

Indeed, Professor de Kruif told FoodNavigator.com: "The most exciting application would be, using the nanotubes as carriers for pharmaceuticals and encapsulation. The pore size of the tubes is about 8.7 nm and allows other molecules to enter the tube and bind to the proteinwall."​

The alpha-lactalbumin nanotubes, with an eight-nanometre cavity, can also be formed with open or closed ends. Recent research has also reported that lipid caps can be used to close the tubes, said Graveland-Bikker and de Kruif., "a process that forms a basis for controlled release applications."​ (Proceedings of the National Academy of Sciences of the United States of America​, 2005, Vol. 102, pp. 11167-11172)

The protein itself is said to have important nutritional values since it is calcium ion binder and a rich source of the amino acid, L-tryptophan. The amino acid is mostly used in the animal feed industry but it also has a number of food applications, including clinical nutrition and supplements designed for bodybuilders.

But the properties of the nanotubes are not limited to encapsulation, said Graveland-Bikker and de Kruif, with alpha-lactalbumin nanotubes also showing significant potential for increasing viscosity and in gelation.

The high protein density and linearity of alpha-lactalbumin could provide industry with "an alternative thickener"​, they said, and can also form gels that can be reversibly broken down by changes in pH to acidic values. Another point to note is that the gel formed is transparent, which may be of particular interest to certain food applications.

"We aim at developing a full dairy viscosifier with potentially new structures and textures as the nanotubes show a remarkable rheology,"​ Prof. de Kruif told this website. "The more so as the gels and dispersions are almost transparent.​

"From as of today there is no application on the market, but there is interest for various food applications and contracts for one or two applications may be signed this fall,"​ said Professor de Kruif.

"The alpha-lactalbumin nanotubes show that it is possible to create interesting nanostructures based on food proteins,"​ concluded the reviewers.

"They illustrate the promising applications of nanotechnology could have in food science."​