In the last years, new food packaging trends have appeared: Companies have the tendency to collect and recycle as much plastic as possible, rather than replace it by sustainable packaging solutions.
However, for this trend to become a reality, more incentives should be available to research institutions and companies to accelerate the transition from conventional to environmentally friendly packaging systems. To move in this direction, two Portuguese scholars, M. C. Gaspar and M. E. M. Braga, from the University of Coimbra and the Polytechnic University of Leiria (
2023) wrote a review on the use of bioplastics and biopolymers as innovative solutions to replace conventional packaging systems.
The term bioplastics is often used as a synonym for biopolymers, but technically the 2 types of materials differ. Bioplastics are biodegradable and compostable and can be obtained from renewable plant sources and non-renewable fossil sources. Some examples include polybutylene succinate, polylactic acid, polybutylene adipate terephthalate and polyhydroxyalkanoate.
Bioplastics
The market for bioplastics is increasing, partly because they can be produced with the machinery already existing in industrial plants, albeit with higher production costs. Unlike bioplastics, biopolymers are not derived from petroleum and can only be of natural origin, i.e. they can be extracted from agrifood residues. Examples of biopolymers are starch and gelatine, thus biodegradable, non-toxic and antimicrobial. Although the production of biopolymers is not economically viable because it is not feasible with existing packaging machinery, their use is constantly increasing in the food packaging sector, in particular to produce edible films and coatings.
This area has become the subject of intensive study in recent years, so much so that the number of scientific papers published on this topic is now about four times more than it was 10 years ago. In fact, the idea of using edible films and coatings as packaging is not new; for example, wax applied on fruits and vegetables and lard on meat and fish have been used for centuries to protect them and extend their shelf-life. In addition, however, there are some specific edible applications that have been studied in recent years.
These include:*Sachets made of polysaccharide (soybean, gelatine, etc.) for soups or beverages.*Gelatine-pectin wraps used to reduce the moisture content of ricotta cheese.*Green tea extract to preserve fresh sausage, due to its intrinsic antimicrobial activity.*Zein-based films and coatings for extending the shelf life of wheat bread. In this case, the incorporation of sunflower extract into zein decreased the water vapour permeability and promoted a plasticizing effect.*Chitosan and alginate films have extended the shelf-life of fruits and vegetables.*Pectin-based coatings improve the preservation of fruits and vegetables as they act as a barrier on the surface of these products, allowing for better moisture retention.
In addition to all these possibilities, the incorporation of prebiotics and probiotics in edible films/coatings is another recently introduced strategy as a means of improving the added value of packaging materials. One example was the development of edible films containing sodium alginate and chitosan with probiotics able to preserve cheese from spoilage.
Despite this wide range of promising possibilities, biopolymers also have some disadvantages:*Edible films usually have limited mechanical and barrier properties. However, these properties can be improved by using multi-layers or resorting to nanotechnology.*Given that even natural compounds can be toxic, it is also possible that a material considered safe may turn into a toxic substance due to unwanted reactions. Therefore, any appearance of toxic characteristics should always be monitored over time.*The hydrophilic nature is also a delicate aspect that makes these materials sensitive to water and moisture.
However, this problem can be effectively addressed by adding hydrophobic components. In particular, lipophilic compounds, such as some essential oils (thyme, lemon, oregano, etc.) may be incorporated into polysaccharide- and protein-based films to improve moisture barrier. In addition to their hydrophilic nature, these essential oils present interesting antimicrobial properties: For example, a chitosan-gelatine coating, loaded with lemongrass oil, was used to coat cherry tomatoes, since it inhibited fungal contamination for 20 days.How can biopolymers be produced for use as films and coatings?
The production of films takes place by solvent casting, i.e. the biopolymer dissolution in a solvent, followed by a drying step. Despite the low temperatures that are usually applied in the film casting method, long drying times are needed, impairing its use for commercial applications. Hot melt extrusion is another method that can be used to obtain edible films, but in this case high temperatures are needed since the material melting is necessary to obtain the film. Therefore, only materials that tolerate heat may be used in this process. Regarding coatings, they are usually applied by dipping, spraying, brushing, as well as fluidized bed processing techniques, and the viscosity of solutions is an important factor to take into consideration.
In conclusion, it can be said that food residues are a valuable source of biopolymers, as well as bioactive molecules, which can be valorized and used in edible films and coatings. These systems can be much more than packaging because, in addition to being biodegradable, they also provide additional functions such as antioxidant, antimicrobial and nutritional properties, However, taking into account the associated technological challenges to increase the production of these systems, additional efforts are needed, namely more research and incentives, to achieve the best solution, in an environmental and economic sustainable way.
References
M. C. Gaspar and M. E. M. Braga, of the University of Coimbra and the Polytechnic University of Leiria (current opinion in Food Science 50, 2023, 101006.
