Graphene in active packaging

…and how it can help us to solve food waste

Have you ever thrown away some food just because you forgot to eat it until it expires? Everyone’s been in this situation. But according to UNEP around 1/3 of all food produced in the world — approximately 1.3 billion tonnes — is lost or wasted every year.

Fruits and vegetables are among the most easily-spoiled food. According to Business Insider, around 45.7% of all fruit production is wasted every year. The main reason why food spoils is microbial growth. Microorganisms such as bacteria, yeast and molds need water and nutrients for growth, energy and reproduction. With an average water content of 90 percent or more, fruits and veggies grow on the outside of food or within the holes or cracks and spoil quickly.

There’re also some factors, which don’t directly spoil food, but can accelerate the microbial growth:

1. Air & Moisture. Water vapor and oxygen accelerate the oxidation and microbial induced fruit&vegetables spoilage.
2. Light. When fruits and vegetables exposed to light, their outer layers start to spoil in a process known as photodegradation that causes discoloration, loss of flavor, vitamins, and proteins.
3. Temperature. In high temperature microorganisms’ action accelerates, and thefore fruits and vegetables spoil faster.

One possible solution to this problem is active packaging.

Active packaging

Active packaging employs technology that intentionally releases or absorbs compounds from the food or the headspace of food packaging, which extends the shelf life of products by stalling the degradative reactions of lipid oxidation, microbial growth, and moisture loss and gain better than traditional food packaging.

Now the problem comes down only to the choice of right materials for this type of packaging.

example of active packaging

Graphene

You probably heard about this material at least once. It was the first 2d material that humans could actually manufacture. Its discovery was so groundbreaking that it was awarded a Nobel Prize at 2011.

So let’s talk about the structure of this material and its properties:

Graphene is a single layer (monolayer) of carbon atoms (it’s the thinnest material in the world!), tightly bound in a hexagonal honeycomb lattice. It is an allotrope of carbon in the form of a plane of sp2-bonded atoms with a molecular bond length of 0.142 nanometres. Layers of graphene stacked on top of each other form graphite — the first time graphene was created by seperating layers of graphite by a piece of scotch tape!

Structure of graphene

It exhibits such properties as:

1. Heat & Electricity conductivity
2. Optical transparency
3. So dense that it is impermeable to gases (not even helium, the smallest gas atom, can pass through it)
4. The lightest material known (with 1 square meter weighing around 0.77 milligrams)
5. The strongest compound discovered (between 100–300 times stronger than steel)
6. Uniform absorption of light across the visible and near-infrared parts of the spectrum

These superior properties give rise to a lot different application in almost every single field of our life. For example: electronics, energy storage, textiles, medical sensors, and more. And, not surprisingly, graphene can make impact in the field of food packaging as well.

Active packaging made of graphene derivatives

Our main goal with this packaging is to slow down microbial growth and oxidation of food. It means that our packaging should exhibit strong antimicrobial and antioxidant properties.

Graphene itself doesn’t exhibit as strong antimicrobial and antioxidant properties as its derivatives. A derivative, which proved to be the best in this aspect, is graphene oxide (GO).

Structure of GO

GO is produced from the acid oxidation of graphite and contains different oxidative functionalities, such as hydroxyl (-OH), carbonyl (=O), and carboxylic groups (-COOH).

The antimicrobial and antifungal properties of graphene-based nanostructures are based on their capacity to induce cell membrane disruption and oxidative stress that compromise bacterial proliferation and sporulation. According to this research paper, GO shows the biggest bacterial cytotoxicity (ability to kill bacteria cells) and can kill up to 70% of unwanted bacteria. GO is individual nanosheets and thus, its nanoparticles are way smaller than many other graphene derivatives (e.g. graphite oxide, graphite, carbon nanotubes, and more), which explains its high antibacterial activity.

Another great thing about graphene oxide is its ability to form nanocomposites with metals, such as Ag, Zn, Na, and more. This happens because the metal cations can directly attach to the oxygen groups (hydroxyl and carboxyl) on the GO surface by electrostatic interactions. This forms stable metal-decorated graphene nanocomposites (e.g. GO-Ag nanocomposites) with higher antimicrobial activity than graphene oxide exhibits.

So, if we’re able to incorporate GO into the food packaging, we’ll be able to create packaging that will be killing most of the bacteria that spoils our food, so the shelf life of the products will be extended.

One interesting way to do it is dispersing GO nanoparticles into biopolymers, such as chitosan or cellulose, which can also show antibacterial activity. In addition to that, biopolymers along with graphene oxide are biodegradable materials and thus won’t harm our environment as traditional packaging does. The challenge for that to become true is to guarantee the uniform dispersion of graphene materials into a polymer matrix, which might not be easy given the high propensity for self-agglomeration of graphene-based materials, as a result of the strong van der Waals forces and π–π electrostatic interactions between nanosheets or nanotubes, and/or their common weak dissolvability in water during biocomposite fabrication.

Interestingly, graphene derivatives have been widely used as polymer reinforcement and are well known to impact on several properties of the final nanocomposite, namely on its mechanical, thermal, electrical, conductive, barrier, and surface hydrophobicity properties. The improvements in these properties won’t directly kill bacteria, but will slow down its growth.

Here I’ll elaborate on some of the most important properties for our packaging:

• Thermal stability. The high heat resistance of graphene derivatives improves the thermal stability of biopolymers since the introduction of these nanocomposites into biopolymer matrix creates an effective heat resistance which slows own the biopolymers’ thermal degradation.
• Barrier properties. Graphene-based biocomposites exhibit great barrier properties against gases, water vapor, and UV light (this comes from graphene’s impermeability to gases). Resistance to these conditions is key to slowing down the microbial growth.
• Surface hydrophobicity. Surface hydrophobicityof biopolymer-based materials is also enhanced by the incorporation of graphene derivatives. The hydrophilic character of the biobased and biodegradable polymers is one of the main limiting factor for their application. In contact with food, these biopolymers can absorb water or even solubilize and, concomitantly, compromise their protective function as packaging films. The integration of graphene derivatives into biocomposites increases their surface hydrophobicity and decreases their water solubility.

As you can see, graphene and its derivatives provide unlimited amount of possibilities to improve our packaging and shelf life of the products. I would also like to add that humans have just started exploring the intersections of graphene derivatives and food packaging, and a loooot of research is still required to bring this active packaging to the shelves of the stores.