In a new study published on June 6, 2013 in the peer-reviewed scientific journal Trends in Food Science and Technology, researchers from the University of Gent review the application of bioplastics in food packaging (Peelman et al 2013). As food packaging is progressively more important in the food industry and a bigger focus is placed on sustainability, bioplastics have attracted increased attention. Bioplastics are plastics based on biobased resources, or biodegradable and/or compostable plastics. The main bioplastics are polyactide (PLA), starch, polyhydroxyalkanoates (PHA) and cellulose. PLA is the most widely used bioplastics with application for fresh foods, dry foods such as pasta and potato chips, fruit drinks, yoghurt, and meat. Starch has been used as an alternative for polystyrene (PS) to package tomatoes and chocolate. Cellulose is used to package dry foods and fresh produce. While all of these materials are biodegradable, their functional limitations have so far restricted their widespread application in food packaging. As outlined by Peelman and colleagues, the main limitations of the four materials is their brittleness, thermal instability, low melt strength, difficult heat sealability, high vapor and oxygen permeability, poor mechanical properties, stiffness and poor impact resistance.
Interestingly, the article does not address migration issues other than those related to nanoclays. Like other food contact materials, substances may migrate from the packaging material into food. In the case of bioplastics, the issue is amplified by the poor functionality of bioplastics described by the authors. With regards to nanoclays used to improve functionality of bioplastics, a studypublished in 2013 in the scientific journal Nanotoxicology found the toxicity of nanoclays to be driven by functional groups (Janer et al. 2013). According to the study, also less toxic pristine nanclay causes cell apoptosis in vitro. The relevance of such findings for human health remains to be elucidated.
Functional improvements
In their study Peelman and colleagues review three processes, which may be used to improve the properties of bioplastics, namely coating, blends and chemical/physical modifications.
a. Coating
Coating comprises the application of a thin biobased or non-biobased layer to the bioplastics. Such coatings can lower the oxygen and vapor permeability, increase tensile strength and result in higher elastic properties.
b. Blending
Blending bioplastics is another approach to improve functionality. Cellulose and other biobased materials may be used to create improved blends. Most bioplastics are immiscible; however the introduction of functional groups, chemical modification or esterification can enhance compatibility. PLA can thereby be blended with PCL and starch, 3-hydroxyvalerate (HV) in PHB and thermoplastic starch with PHA. Blending can reduce brittleness, increasing vapor water barrier properties, flexibility, and tensile strength. However, lowered elongation is a critical point to consider as it results in bioplastics breaking earlier when being deformed.
Alternatively, nanoscale particles can be incorporated into bioplastics, forming nanocomposites. A high affinity between the polymer and the nanoparticle is desirable. The resulting exfoliated structure, where the nanoparticles are well dispersed in the matrix, improves overall barrier properties as well as thermal stability. Hydrophobic nanoclay has been shown to be particularly useful for creating such structures. In order to optimize functionality it is of particular importance to adjust the concentration of the nanocomposite in the bioplastic and to use the proper type of nanoclay.
The authors point out that an issue of particular concern with this approach is the insufficient investigation into the migration of nanomaterials into food. According to the authors, the “current knowledge on migration of nanoclays and their effect on the human health and environment is too limited” to place nanoclay products on the market. Accordingly, the European Food Safety Authority (EFSA) uses a case-by-case approach to evaluate nanocomposite use in food contact materials. In the US, nanomaterials are being used in food contact applications under the GRAS program, with safety assessments usually performed by manufacturers and without FDA oversight (Nestle 2013).
c. Chemical and/or physical modification
The third approach to improve functionality is chemical and/or physical modification. It can be used to enhance compatibility between two polymers or to improve the functional properties directly. Citric acid added to starch films improves water and vapor properties (WVP). Heating can make starch film more water resistant and flexible. Crosslinking cellulose acetate with phosphates improves tensile strength and slows water uptake and degradation. Epichlorohydrin-modified starch has an increased tensile strength and improved elongation. Partially substituting wheat gluten with hydrolyzed keratin or soaking wheat gluten film in CaCl2 and distilled water improves the water vapor and oxygen barrier properties of a wheat gluten derived film.
Peelman and colleagues conclude that using coatings, blending and chemical/physical modification can extend the use of bioplastics in food packaging to a wide variety of food other than fresh produce and dry foods. The authors do not address the issue of chemical migration from bioplastics, other than the migration of nanocomposites. It has been suggested that nanoparticles do not only reduce migration of gasses into food content, but also reduce the migration of other substances from the polymer material (Silvestre et al. 2011). It remains to be investigated whether the proposed approaches to improve functionality may also be used to address concerns related to substance migration from bioplastics into food.
Reference
Peelman, N. et al. (2013). “Application of bioplastics for food packaging.” Trends in Food Science & Technology (published online June 21, 2013).
