NANOCELLULOSE: A prospective solution for food packaging
Plastic! Plastic! And Plastic! At present, it seems an inseparable part of our daily life due to its availability, cheaper price, and easy manufacturing process. Apart from its easy useability, it creates a significant and unpredictable impact on the environment. Most of the plastic material ends up as wastage and returns to the environment like a landfill, blocking the waterways and ultimately polluting the ocean. According to National Geography research, the worldwide production of plastic-made packaging is around 78 million metric tons per year which are about 45–50% of total plastic usage.
As food packaging based plastic materials and other plastic materials don’t affect or degrade by nature, it adversely threatens human beings, wildlife, marine creature. Another shocking news was published by National Geography on March 18, 2019. They reported that Darrell Blatchley, working as a marine expert as well as a curator at the D’bone collector museum, Davao city, Philippines, found more than 88 pounds of plastics into a dead whale belly. Can you imagine that plastic-based materials can even kill the world’s largest animal! Due to this environmental problem, nowadays scientists are trying to find out biodegradable, natural polymer to replace plastic-based materials. In this regard, nanocellulose is a potential solution. Here in my write-up, I am trying to shade light upon one of the most promising biodegradable materials-nanocellulose that can be an emerging solution for replacing plastic-based food packaging materials.
What is Nanocellulose?
Nanocellulose is a very tiny, lightweight, non-toxic material and it contains about 95–97% water when it is made in the lab. Nowadays, nanocellulose has been able to draw the special attention of many researchers because it has greater mechanical strength, larger surface area to volume ratio and so many outstanding properties that can even make the world better. The size of nanocellulose material is very important because the smaller the size of nanocellulose, the higher will be surface area to volume ratio. And averagely, it is considered that diameter size of nanocellulose shouldn’t be more than 100nm and several micrometers in length. Mainly, nanocellulose is obtained from cellulose which is the most available natural polymer. Cellulose is mostly found in plant and plant-related materials like softwood, hardwood, sugarcane bagasse, rice husk, coconut coir, pineapple leaf, banana fiber, etc. which is also known as lignocellulosic biomass.
Chemical structure of plant cell wall: Cell wall structure of lignocellulosic biomass is a very complex structure. Different carbohydrates are located in a cell wall. Among them, cellulose is the main component. Besides, there is also hemicellulose (polysaccharides), pectin. Pectin is also in 3 types- prospecting, pectin, and pectic acid. Moreover, lignin suberin, cutin, wax are also attached near to cell wall structure. Though there are different chemical components in lignocellulosic biomass, on average, lignocellulosic biomass can be divided into 3 categories-
- Cellulose: Cellulose is one kind of inert, tasteless polysaccharide. It is available in large quantities in a cell wall. But the composition of cellulose may vary for different biomass. For example, there are 90% cellulose in cotton, 60% in wood. Cellulose plays a vital role in building plant infrastructure. Generally, cellulose is formed by joining the β-1–4 glycosidic linkage of many glucose monomers. (figure-3).
2. Hemicellulose: Hemicellulose comprises of 20–35% lignocellulosic biomass by its weight. But it may vary for different plants like there is 50% hemicellulose in wheat straw, 20–35% in barley straw, 24–40% in hardwood, and so on. Hemicellulose is a polysaccharide. Xylan, mannan, glucan, galactan can be found in a plant cell wall structure in the form of hemicellulose. Generally, hemicellulose is linked with cellulose fibrils with short lateral chains (H-H bond and van der walls bond).
3. Lignin: Lignin (phenolic polymer) acts as a supportive layer between hemicellulose and cellulose polysaccharides. Lignin consists of phenylpropanoid monomers: p-coumaryl alcohol, coniferyl alcohol, sinapyl alcohol. The chemical structure of these monomers is shown in fig-4. These monomers are bonded together with β-O-4, α-O-4, β- β, 4-O-5 and so many bonds (fig-5). Lignin makes the plant cell wall a rigid structure and also protects from any microbial attack.
Lignocellulosic biomass pretreatment: As there are many chemical components in a plant cell wall, it is necessary to separate cellulose from biomass so that nanocellulose can be made. To achieve this purpose, scientists apply different biomass pretreatment methods like physical treatment (ball milling, grinding), chemical treatment (acid hydrolysis, alkali treatment, oxidation treatment), enzymatic treatment. For simplification, here I mention only chemical treatment method.
1. Alkali and oxidation treatment method: For alkali and oxidation treatment, NaOH, Ca(OH)2, and H2O2, C2H3O4 are used. An oxidation agent is used to enhance the alkali performance. This treatment method is applied to remove lignin and hemicellulose portion. This method mainly performs in two ways-
- Attack the glycosidic ether bond (R-O-R`) of lignin structure and
- Attack structural ester linkages (R-C=O-O-R`) between lignin and hemicellulose.
Fig-5 demonstrates the basic mechanism of alkali and oxidation treatment. In this purification stage, some researchers also use sodium chlorite as a bleaching agent and acetic acid as a buffer solution to maintain pH value.
Johar et al., 2012 used both bleaching and alkali treatment methods to isolate cellulose from rice husk. They showed that, after alkali treatment, the composition of cellulose, hemicellulose, and lignin was 57%, 12%, 21% respectively. But after the bleaching process, cellulose composition was 96% while hemicellulose and lignin are totally removed.
2. Acid hydrolysis: After separating cellulose from biomass, it is needed to apply the acid hydrolysis process. The acid may be either diluted acid or concentrated acid. But it is strongly suggested that concentrated acid is not suitable due to its corrosive nature. So diluted acid is the best medium for acid hydrolysis and H2SO4 is the most commonly used acid for this purpose but its concentration should be between 4–5 wt%. After acid hydrolysis, we can get our desired product in the form of nanocellulose. From fig- 6, the biomass treatment process can be demonstrated briefly.
Classification of nanocellulose: Different concern bodies classify nanocellulose into three main categories based on their preparation method.
1. Cellulose nanocrystal: Cellulose nanocrystal (CNC) is also known by the name of nanocrystalline cellulose (NCC) or cellulose nanowhiskers (CNW). CNC can be obtained through different acid treatments like hydrochloric acid, sulfuric acid, and phosphoric acid. Nanocellulose which is obtained via acid hydrolysis method has no amorphous region. Cellulose molecules are orderly packed into its crystalline parts.
2. Cellulose nanofiber: Cellulose nanofiber (CNF) can also be termed as nanofibrillated cellulose (NFC), nanofibrillar cellulose, or nanofibrous cellulose. Cellulose nanofibers can be prepared through mechanical processes like grinding, extrusion, blending, etc. In NFC, cellulose molecules are found in both oriented and non-oriented region.
3. Bacterial nanocellulose: In the nanocellulose category, bacterial nanocellulose (BNC) is another type of nanocellulose. Some bacteria like Acetobacter, Azotobacter, Salmonella, Escherichia can produce cellulose. The preparation process of BNC is different from the other two types of nanocellulose (CNC and CNF). But the chemical composition of bacterial nanocellulose is the same as the plant-based cellulosic materials.
Nanocellulose in packaging: For food packaging, some points should be considered very carefully such as containment, protection (from microorganisms, dirt, dust, etc.), barrier property (O2, CO2, water, and other vapor in the environment), and so on. Nanocellulose based food packaging performs better in these points. In nanocrystalline cellulose, molecules are tightly packed together. And for this tight bonding, nanocellulose acts as a better barrier material. Oxygen barrier property is very important for food packaging applications because food can be damaged when it contacts with oxygen. In this case, the oxygen barrier property of nanocellulose is close to synthetic polymer-based food packaging even sometimes better than synthetic polymer.
For manufacturing nanocellulose based packaging materials, many scientists and researchers develop different techniques. Among them Layer by layer assembly (LBL), electrospinning, casting evaporation, composite evaporation etc. are popular. For simplification, here I explain only layer by layer technique.
Layer by layer method: Through this method, multiple layers of nanocellulose can be obtained with a particular thickness. Nanocellulose is coated on a solid substrate with different polymers like PET, PLA, Poly(ethyleneimine), etc. as a composite form via dip coating, spin coating, spray coating procedure. To make nanocellulose-based bio-composite materials for packaging application through dip coating process, it is necessary to prepare a polymer solution and aqueous cellulose solution. After that, the substrate, on which coating will be applied, is dipped into a polymer solution for 1–2 minutes, rinsed with deionized water, and then drying. This procedure is also applied for cellulose solution. And repeat this procedure simultaneously till the desired layer is obtained. Fig-7 describes the layer-by-layer deposition method via dip-coating process.
Aulin et al., 2013 used this layer-by-layer process to prepare nanocellulose based bio-composite for the gas barrier. They used PLA substrate, polyethyleneimine (PMI) as a polymer solution, NFC, and CMC (carboxymethylated cellulose) as an aqueous cellulose solution. They examined oxygen and water vapor permeability for 20 and 50 layers on PLA substrate and suggested that by increasing layer on the substrate, oxygen, and water vapor permeability will also be increased.
Safety and biodegradability issue: For developing nanocellulose-based packaging material, environmental issues must be considered like cytotoxicity, bio-degradability, and so on. Sometimes it may seem that as nanocellulose is reinforced with a synthetic polymer, the nanocellulose material may not degrade with the environment. Many scientists research this issue and they come into the conclusion that as nanocellulose belongs in the nano (1–100nm) dimension, there is no possibility to lose the biodegradable property. Kummerer et al., 2010 showed that nanocellulose is a better biodegradable material than CNT and Fullerene. Degradation of nanocellulose is increased day by day (51% on day 21, 54% on day 24) while CNT and Fullerene don’t degrade at all. Some researchers also find that nanocellulose is a very low toxic polymeric material.
Future aspects and market analysis: Market demand for nanocellulose and nanocellulose based material is increasing day by day. People are now concerned about nanocellulose based material due to environmental issues. Nowadays many industries take the necessary steps to reduce environmental pollution and for this, they use nanocellulose for food packaging products. John Cowie and co-workers mentioned that the estimation of nanocellulose production in packaging and plastic film applications is about 2 million metric tons and 0.7 million metric tons respectively. Here, figure-8 and figure 9 illustrate the reasonable estimation of nanocellulose-based products in the US market and the global market.
[For these two figures, excel is used but the information is taken from “market projections of cellulose nanomaterial enabled products — part 2: volume estimate”]
Conclusion: Plastic made pollution is not a forecasted issue any longer, it is happening and hitting us very hard continuously. Reckless use of polyester made packaging can bring us to the brink of existence. And it seems that unthinkable and beyond repairable catastrophic events can come to us within no time. It is high time that we should take care of it before it’s been too late. Personal and social awareness can be an effective means of facing these challenges. So, we should use nanocellulose for food packaging that has the ability to reduce CO2 emissions to the environment. We should remember that for our temporary happiness, we can’t make our world sick for our future generation. Hopefully, nanocellulose market demand will be increased significantly and will be able to replace all fossil fuel-based food packaging materials.
