One of the most popular alternatives in the last few years is the edible coating - a transparent film that covers the food item and acts as a barrier to humidity and oxygen. Moreover, these films can be used as a host for additives in the conservation of the properties of the product or simply in order to improve its appearance.

This was the subject of the PhD thesis by Navarre chemist, Javier Os�s Fern�ndez, at the Public University of Navarre. In the research, various edible coatings were analysed - all based on proteins extracted from milk serum (whey protein isolate or WPI), mesquite gum and starch - with the objective of evaluating their possible food applications.

The PhD, entitled "Development, characterisation and applications for foodstuffs of edible coatings based on milk serum proteins, starch and mesquite gum".

Mesquite gum, an efficacious and cheap solution

To test the efficacy of the edible coatings compared to synthetic packaging, Javier Os�s prepared a number of samples of milk serum protein films, of mesquite gum and of starch, and stored them for six months at different humidity levels.

The first conclusion from the study is that, depending on the type of plastifier used, the mechanical properties change with the passage of time. Thus, those films with sorbitol plastifier underwent variation in their mechanical properties, i.e. their flexibility; while those containing glycerol did not alter their composition.

Once the ideal plastifier for the coating was determined, it was observed that, in order to improve malleability, it was necessary to introduce a lot of glycerol. However, using high quantities of plastifier in a coating results in an increase in its permeability to water vapour, and its applications are thus very limited. In order to set this deficiency off, mesquite gum was incorporated into the milk serum protein.

This is a polysaccharide that grows in dry and semi-arid regions of the north of Mexico. The result is a compound film that is resistant and that maintains suitable mechanical properties. The use of mesquite gum is an economical and efficacious alternative with a promising future, not only for food conservation, but it could also become an economic resource for indigenous peoples, currently marginalized, as well as having the effect of reducing the desertisation of the soil.

The edible coating of fruit is currently the best-known example of the application. However, it is a packaging system particularly efficacious for conserving foodstuffs that are high in polysaturated fatty acids, susceptible to oxidation, such as nuts, meat and certain fish such as salmon.

In his PhD, Javier Os�s evaluated to what extent the edible films which he had developed were able to protect fatty foods. To this end, he used a model foodstuff - sunflower oil. In a first stage, trials were carried out with WPI coatings.

Results showed that the protection capacity of the film depended on the amount of plastifier in the film, on its thickness and on the relative humidity of storage of the foodstuff. Thus, the most efficient WPI films were those of greater thickness, with less amount of plastifier and that had been exposed to low relative humidity.

The second stage involved a similar experiment, but with films made of starch, known to be an efficient barrier to oxygen, and with which trials were undertaken. In this case, the thickness of the film did not influence the protector effect on the oil, but it was shown that starch films are more effective in ambiences with high relative humidity.

The final part of the PhD outlined the application of edible coatings with WPI on chicken breasts. The aim was to analyse the effect on their aspect and their properties. At the same time, the experiment was used to incorporate nisine, an antioxidant agent that penetrates the foodstuff little by little and thus offers an ongoing protection.

The idea was to extend the conservation time for a chicken breast would go from the current 7 days to 15, but the results were negative. However, a very interesting line of research was opened as it has been shown that the coating formed a second skin on the breast and onto which various additives, such as antimicrobians, can be added

Despite the fact that the coatings did not manage to reduce microbian evolution, it has been shown that the WPI forms a film on chicken breasts which is homogeneous, transparent, has good adhesion and is not easily perceived by the naked eye.

The future of packaging

Currently, the three systems of conservation most used by the food industry are vacuum packing, nitrogen sweeping and aluminium foil. The Pamplona research chemist holds that WPI films are the packaging of the future, although the commercial exploitation of the application is yet to materialize.

There are many possibilities - on the one hand, they will prolong the useful life of foodstuffs in a healthy way and, on the other, they will be very beneficial for the environment given that they will reduce the use of plastics.

Despite the advantages presented by the experts, there exists a number of obstacles to the full development of this alternative system. The main one is that of cost. As edible coatings are still in the research stage, enterprises do not have the technology needed to apply the system.

For the moment, its current use is restricted to products with high added-value. This is why, amongst the most immediate projects of the research team from the Food Technology Area at the University, is the application of edible films to turr�n (sweet nougat), in order to eliminate the aluminium foil currently used to protect it and so that the product would have only one protective coating. This project is still in the study phase.

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