Various types of nanomaterials, due to their differing construction material, shape, size, and integration, have been exploited for a wide variety of applications within the food industry. Applications include acting to improve the integrity of food items and as sensing agents in “smart packaging” applications. Some of the challenges surrounding the implementation of nanotechnology into food products and packaging will be discussed in more detail.
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Providing Integrity to Food Packaging
Packaging integrity is vital to food safety and must optionally provide a barrier to atmospheric gasses, water vapor, invasive microbes, and many other contaminating agents, depending on the foodstuff. Many types of nanomaterials have demonstrated enhanced strength and thermal durability as compared to their standard polymer counterparts utilized in food packaging, such as carbon nanotubes.
The surface of carbon nanotubes, and similar materials, can be modified with a wide range of chemical agents that provide further antibacterial, antioxidant, and potentially sensing functions.
Interestingly, titanium dioxide nanoparticles have shown some capacity to slow fruit ripening when incorporated into food packaging, reducing the observance of browning and decay. This is because TiO2 nanoparticles can act as a catalyst for removing ethylene gas in the presence of UV-light, a plant hormone that plays an important role in initiating the ripening process simultaneously for fruits in the vicinity.
Antimicrobial Protection to Food Products
Many types of nanomaterials, such as nanoparticles constructed from silver or copper, demonstrate innate antibiotic properties, typically explained by the slow release of charged metal ions in the vicinity of the bacterium, either directly damaging it, or assisting in the generation of reactive oxygen species that go on to do the same. Direct catastrophic distortion of the bacterial cell wall and membrane by close contact with the high surface energy of metallic and metal oxide nanoparticles has also been observed, all of which are difficult to adapt resistances towards compared with conventional antibiotics. These materials have been incorporated into food packaging to delay microbial spoilage, including bacteria, yeasts, and molds.
As will be discussed in more detail, there are concerns surrounding the repeated ingestion of metal and metal oxide nanoparticles owing to evidence of bioaccumulation. Nanoencapsulation of nanomaterials by other more biocompatible nanomaterials is one potential solution to their innate tendency towards bioaccumulation and toxicity. However, many such materials may also concomitantly limit the antimicrobial properties of the nanomaterial by blocking surface access between it and the bacterium.
Films or porous materials with silver nanoparticles embedded may still allow the steady egress of silver ions and generated reactive oxygen species while holding the much larger nanoparticle in place, and this method of separating nanomaterials from the food product while still providing antimicrobial protection has been utilized extensively in research.
Sensors in Food Packaging
As briefly mentioned, nanomaterials constructed from plasmonic materials, namely gold and silver, demonstrate unique optical properties at the nanoscale not observed in the bulk material. Localized surface plasmon resonance occurs in the vicinity of nanoparticles constructed from these materials when incident light is in resonance with the oscillation of electrons around the nanoparticle body, meaning that a specific wavelength of light, and a narrow Gaussian distribution around it, are very intensely absorbed by the nanoparticle.
As suspensions of the same-sized nanoparticles in some medium, such as water or solid gelatin, lack absorption throughout the remainder of the visible spectrum due to surface plasmon resonance, they are strongly colored, gold nanoparticles with resonance around 520 nm, for example, appear red.
The wavelength of surface plasmon resonance is influenced by a number of parameters, including nanoparticle material, size, shape, proximity to other particles, and the chemical environment within the vicinity of the surface. Physical and chemical changes made to the nanoparticle dispersion can cause color change, engineered to be indicative of some specific parameter.
For example, Tseng et al. (2017) produced paper-based plasmonic sensors, wherein metallic silver, gold, and gold-silver alloy nanoparticles were embedded into flexible paper using nanoimprint lithography, which serve as gas sensors in the detection of volatile biogenic amines originating from spoiled food. Specifically, this sensor operates via the change in refractive index surrounding the nanoparticle induced upon the adsorption of target molecules, causing a red shift in surface plasmon resonance wavelength. Unfortunately, at this stage, the shift generated by such a minor change to nanoparticle properties is relatively small, and may be undetectable by the human eye alone.
Safety Concerns Surrounding Nanotechnology and Food
Once packaging has fulfilled its purpose, many types of plastic and other materials from which it is constructed are discarded and may have landfill lifetimes of many thousands of years. Decompostible polymers are constructed from degradable polymers with environmentally friendly “nano-fillers” that help to provide temporary integrity to the packaging. There is concern relating to the release of such nanomaterials into the food product itself, and thus, it is crucial to conduct thorough studies relating to the toxicity of said nanomaterials, which may differ significantly from that of the bulk or aqueous chemical.
Many types of nanomaterial have been shown to accumulate within specific organs of the body upon ingestion, typically the renal and hepatobiliary systems, and may or may not have demonstrated associated deleterious effects, mainly signs of local tissue inflammation. Generally, organic nanoparticles such as proteins, starch, lipids, or chitosan are considered to be easily degraded and excreted by normal mechanisms, though these materials rarely provide significant functionality, as metal and metal oxide particles do. Both may act as carriers to other functional molecules, such as antibiotics or enzymes, and may be encapsulated by more biocompatible materials, such as polymers to aid quicker excretion, should they be ingested.
References and Further Reading
Singh, R., et al. (2023). Future of Nanotechnology in Food Industry: Challenges in Processing, Packaging, and Food Safety. Global Challenges, 7(4), 2200209. doi.org/10.1002/gch2.202200209
Maneerat, C. & Hayata, Y. (2006). Efficiency of TiO 2 photocatalytic reaction on delay of fruit ripening and removal of off-flavors from the fruit storage atmosphere. American Society of Agricultural and Biological Engineers, 49(3). doi.org/10.13031/2013.20467
Tseng, S., et al. (2017). Food Quality Monitor: Paper-Based Plasmonic Sensors Prepared Through Reversal Nanoimprinting for Rapid Detection of Biogenic Amine Odorants. ACS Applied Materials & Interfaces, doi.org/10.1021/acsami.7b00115
Ashfaq, A., et al. (2022). Application of nanotechnology in food packaging: Pros and Cons. Journal of Agriculture and Food Research, 7. doi.org/10.1016/j.jafr.2022.10027