Barrier properties of whey protein coatings increased

Adding an enzyme to whey protein coatings could decrease oxygen permeability by up to a factor of five, according to a study. Researchers suggested this improvement was due to the increased crosslinking of the protein chains.

The enzyme transglutaminase (TG) was used to reduce the permeability of the whey protein-based layers. The use of TG was found to reduce the oxygen permeability, reduce the water vapour transmission rate (WVTR) and reduce the effective water vapour diffusion coefficient of the whey protein layers.

Barrier material potential

Markus Schmid, an author on the study, said that they are not planning to investigate other enzymes but to improve barrier properties they are investigating alternative crosslinking methods. “This barrier material could be used in mainly all applications of flexible packaging which up to now uses EVOH as barrier layer,” he said.

“Examples could be the lid for MAP packed sliced cheese in a tray. This somehow rounds out the story since we take a by-product from cheese manufacturing (namely whey: which is still discarded in huge quantities in Europe) and produce a barrier packaging material from it followed by packaging the cheese with that material. This closes the circle.”

Schmid, who also works in materials development at the Fraunhofer Institute Process Engineering and Packaging IVV, said the processing window is limited due to the enzymatic cross-linking but could be managed by certain equipment.

“The practical use of this additional cross-linking is up to now a problem for the industrial implementation. But we are working on it,” he said.

“Depending on the process and the formulation the Wheylayer barrier material can already perform very well. The enzymatic crosslinking is just an option to potentially tune the performance and is up to now more of scientific than practical relevance.”

He added that industrial implementation is ongoing and tube laminates are produced at near-industrial scale and will be used to pack oxygen sensitive products soon.

Properties studied

Properties measured included oxygen permeability, water vapour transmission rate, effective water vapour diffusion, sorption and permeation coefficients, swellability, light transmission and surface energy. Previous studies have focused on the influence of TG on protein formulations or on the measurement of film properties, which is of little relevance for packaging applications.

Important barrier polymers being used are ethylene vinyl alcohol (EVOH), which is an oxygen barrier, and polypropylene, which is a water vapour barrier – both synthesized from fossil fuels. Whey protein, a by-product from cheese production, seems to be a promising alternative or at least a supplement for these materials and is obtained from renewable resources. An aqueous solution of 10% w/w whey protein and 6.67% w/w glycerol was used as base material for the crosslinking tests and as a reference formulation.

TG concentrations

Transglutaminase in a concentration of 0.1, 1 and 10 units per gram of whey protein was added to the reference formulation and reaction time of TG was varied: 0, 30, 60, 90, 120 and 1440 minutes.

TG concentrations higher than 10 units per gram protein and reaction times greater than 120min must not be exceeded in order to obtain a flowable solution. The oxygen permeability decreased by a factor of five on addition of 10 units of TG to the initial whey protein formulation. The final value was close to the barrier properties of EVOH.

The addition of 10 units of TG reduced the water vapour transmission rate of whey protein films by half. A higher TG content had no effect on the water vapour sorption properties. “However, the effective diffusion coefficient was reduced by the action of TG. This result indicates that the whey protein had a higher degree of crosslinking which reduced the diffusivity of water molecules in the protein matrix,” said the researchers.

Swellability and light transmission rate decreased with increasing TG concentrations however optical properties were slightly impaired by the addition of TG.

Source: Packaging Technology and Science