Researchers from the Department of Chemical Engineering of the University of Huelva (UHU) have added essential oils of clove, cinnamon and white thyme to a plastic obtained from wheat gluten protein to make containers with antimicrobial properties.
Experts indicate that these biodegradable materials can extend the shelf life of food by reducing the proliferation of fungi and bacteria that are generated during storage. The research opens the door to the development of plastics obtained from renewable resources or bioplastics ‘a la carte’, intended for the packaging of all kinds of products, says the Discover Foundation in a note.
For the design of these containers with antimicrobial activity, two fundamental components have been needed, a polymer, in this case, the wheat gluten protein, and a biocide, that is, a substance that neutralizes or reduces the development of harmful microorganisms. the man.
In this research, they have selected eight essential oils derived from aromatic plants that are traditionally used as natural preservatives. For example, red and white thyme, lemon, cinnamon, cloves, mint, rosemary, and bergamot, a type of citrus found in some teas.
Added to them are two active principles extracted from oils, cinnamaldehyde, from cinnamon, and carvacrol, from thyme, so they wanted to check whether these substances, isolated, had a greater effect than the oils themselves.
According to the researcher of this project at the University of Huelva, Inmaculada Martínez García, the objective is to produce bioplastics, "one hundred percent natural", as an alternative to plastics derived from oil. And with added value, specifically, with antimicrobial properties that extend the shelf life of packaged products.
To evaluate the antimicrobial capacity of oils against fungi and bacteria, experts have carried out two different studies. In the first, the bioplastic has been put in direct contact with the microorganisms, since, as the researcher indicates, "a culture of the different microorganisms has been made on a plate and, on this, the bioplastic has been placed".
In the second test, the bioplastic has been placed on the lid that covers the culture plate. In this way, it is checked whether an antimicrobial atmosphere is created without the need for the material to rub against the food.
The results have shown that "six of the ten biocides fulfilled their function". Of these, "the most powerful", both for fungi and bacteria, are cinnamon, cloves and white thyme, in addition to the active ingredient cinnamaldehyde. In contrast, researchers have found little activity in lemon and rosemary oils.
The most effective substances also showed their ability to form antimicrobial atmospheres. The biocide is volatile, that is, at room temperature, it evaporates. It is freeing itself, in time, of the plastic. Thus, they have seen that this phenomenon occurs "even when there is no direct contact with the crop, since the oil released would create an atmosphere, which protects the product," says Martínez García.
In addition to the antimicrobial study, which is included in the article 'Development of antimicrobial active packaging materials based on gluten proteins', published in the journal, Journal of the Science of Food and Agriculture, experts have evaluated a series of properties to compare the new material with conventional ones.
“A plastic of natural origin must meet the same viscosity, elasticity or resistance requirements as another derived from petroleum. These properties vary depending on the essential oil that is added in the formulation ”, he assures.
In his opinion, "one of the most important requirements is the reduction of the water absorption capacity of these bioplastics". According to the researchers, a plastic without essential oil retains up to 90 percent of liquid, an amount that they have reduced to 50-55 percent with biocidal agents. Protein-based polymers are characterized by their ability to absorb large amounts of water.
For experts, this quality is "very positive" for manufacturing superabsorbent materials, although, for food packaging, this ability must be reduced to the maximum, and they have achieved it, specifically, with the two active principles.
They have also analyzed the mechanical properties to find out, for example, the weight that a plastic bag made with these bioplastics can support.
Finally, they have studied the rheological characteristics, which indicate the behavior of the material during the processing phase. “The plastic has been formulated with a wheat gluten protein concentrate that has the appearance of a powder, a plasticizer, in this case, glycerin, to give the material flexibility; and the biocidal oil, around ten percent by weight ”, he specifies.
From the mixture of these elements a mass is obtained that undergoes a process to become plastic. In this phase, called ‘thermoforming’, pressure and temperature are applied. "The rheological study tells how the material behaves before these pressure and temperature variables," argues the researcher of the project, funded by the Ministry of Economy and Competitiveness.
According to the researchers, cinnamon, cloves and white thyme are the biocidal agents with the best mechanical and rheological properties.
Based on these results, the experts are working on the design of new bioplastics ‘a la carte’ with two objectives. On the one hand, replace the current essential oils with others without odor, so as not to alter the taste of food, or with new substances such as fertilizers or drugs. And, on the other hand, improve the biocide release system by incorporating nanoparticles of biodegradable materials.