Agar-lignin based bioplastic film for food packaging material

요약

Last summer, I went to the beach in Jeju island and was shocked by the amount of plastic trash that spread everywhere. During the study, I learned that the petroleum-based plastics deal detrimental damage to the environment. Most plastics are not biodegradable, and the poor recycling infrastructure in most nations cause this non-biodegradable material to end up in our environment (1). 
Upon the research, I found that previously developed bioplastic materials such as polyhydroxyalkanoates(PHA) and polylactic acid(PLA) are biodegradable. However, widespread adoption of these materials has been discouraged by their high price tags (2). So I asked myself, "Is it possible to develop the biodegradable plastics with less expensive materials?" The answer was YES!

I developed a biodegradable plastic film by using agar and lignin, which are much cheaper than PHA, PHB, and PLA. Agar is a gelatinous material that can be found in red marine algae and it is a widely used material to make traditional Korean sweets and desserts (3). Because agar and lignin is a natural polymer that has biodegradability (4), I hypothesized that I could develop agar-lignin based bioplastic. After many failures and over 100 hours of experiments, I successfully developed bioplastic with 1:1 ratio of agar and lignin composition in the final film which yielded promising bioplastic film characteristics: degraded in soil, improved water permeability and food preservation. However, the experiment for the determination of the optimum composition remains to be conducted to improve the tensile strength of the film.

질문/제안

Every minute, one garbage truck amount of plastic is dumped into the ocean, but unfortunately, widespread adoption of previously developed bioplastic materials has been discouraged by their high price tags (2). Due to the many difficulties of developing new bioplastic material, my goal was to find an alternative and cost-effective way to develop bioplastic material that would help our environment.

After studying the previously developed bioplastic materials, I learned that bioplastic materials were developed by renewable resources (5). For example, polyhydroxyalkanoates(PHA) was produced by bacteria, including through bacterial fermentation of sugar or lipids. Also, polylactic acid(PLA) is derived from corn starch.

I, therefore, hypothesized that low-cost renewable materials such as agar and lignin could then be developed into new bioplastic materials. Agar is a jelly-like substance, obtained from red algae. Agar melts at 85oC and solidifies from 40oC. Because this property lends a suitable material to be poured into molds to process it in any shape, I hypothesized that the agar can be used as a base material for bioplastic (3). Because lignin is known to play an important role in conducting water in plant stems by crosslinking of polysaccharides, I hypothesized that crosslinking of agar, which consists of polysaccharides, and lignin could be possible. I further hypothesized that agar-lignin bioplastic could improve water permeability because lignin is more hydrophobic than agar (3). My hope was that agar-lignin bioplastic film could not only reduce plastic waste but also developed as products like fruit and vegetable bags.

연구

Due to an environmental problem with non-biodegradable plastics, the scientific community is endeavoring to develop bioplastics, which are biodegradable and less harmful to the environment. To this day, bioplastics are often mixtures of polyethylene (PE) and organic materials which help the material degrade biologically (7). Although these do reduce the effect of original petroleum-based plastics, even trace amounts of PE still pose danger to the environment when not recycled properly (5).

On the other hand, materials such as polyhydroxyalkanoates (PHA) and polylactic acid (PLA) are both biodegradable and less harmful to the environment. PLA is the first bio-based plastic commercialized on a large scale and can be shaped into films and coatings, but its’ high price limited the adaptation in the market (6).

I interviewed experts and professors about bioplastic, and I learned that polysaccharide film based materials such as starch, starch derivatives, and cellulose derivatives have been studied extensively for the development of food packaging (7). In addition, starch-based materials can be modified by either plasticization and blending with other materials (8).

Through my research, I learned that both agar and lignin have been widely adopted in the production of hydrogels (6) and biofilms (8). I also learned that lignin-agarose hydrogel can be synthesized by chemical crosslinking of epichlorohydrin (ECH) (6). Recently, researchers have discovered that addition of lignin and glycerol on cellulose acetate films increased the tension rupture of the material (8). Therefore, glycerol and lignin both increased the compatibility of cellulose acetate film. Therefore, I considered lignin and glycerol to be viable raw materials for a new bioplastic packaging.

방법/테스트

I learned through many experiments that it is not easy to find the best composition of bioplastic film with agar, agarose, lignin, glycerol, and epichlorohydrin (ECH). Our first step was to make agar film and agar-lignin film. This process was quite straightforward, as I found that agar and agarose films could be easily created from drying the agar hydrogel in 60oC. I performed to create agar hydrogel with water using microwave and heating block from the laboratory to heat the mixture up to 70oC. Then, I pour the heated solution on to the aluminum tray to make the hydrogel less than 50mm thick. When the mixture solidified in room temperature for 1 h, I dried the hydrogel in the heating oven for 4 h. Thin agar biofilms were developed by this method with different agar concentration (0.1~10%). However, agar biofilms were very stiff and easily broken.

Lignin solution was prepared by the same method that recent research paper provided (6).  Different concentration of lignin stock solution (50g of lignin with 30% NaOH) was added into the agar solution under constant stirring at 150 rpm and maintained the temperature 70 oC with heating block. Agar-lignin solutions were mixed with the 10%(w/v) of the cross-linking agent (ECH). Then, they were stirred at 150 rpm for 20 minutes. 10%(w/v) of glycerol was added on Agarose-lignin mixture. Then, they were stirred for an additional 20 minutes. The hydrogels were solidified and dried in the same manner as agar biofilms were prepared. Different concentration of agar-lignin biofilms was collected by this method. The results were very encouraging as I finally found agar-lignin biofilm become very flexible and increased tensile strength compared to agar film.

The second step was to measure the water absorption of agar-lignin film. Different composition of agar-lignin films was soaked in the water and waited for 10 min. The remaining water was removed completely by the Kim wipe. The weight was measured before and after water treatment using lab scale electronic balance and calculated the percentage of water absorption of the films. The percentage of water absorption was calculated (weight of film before water treatment/weight of film after water treatment x100)

In third step, I measured the biodegradability of the film. The film of agarose-lignin was cut into 2 cm by 10 cm rectangles. The agarose-lignin films were placed in UV chamber at the laboratory in Suwon University. The soils from the local area were collected and agarose-lignin films were buried in the flowerpot for 90 days. Then the mass of the agarose-lignin films was measured with the control samples.

The last step was to measure the food preservation of agar-lignin film. The Kiwis were stored with agarose-lignin film and Polyethylene (PE) based plastic films in room temperature for 21 and 28 days. The Kiwis were wrapped with the films and were seal with tape to minimize the contamination. The picture of the kiwis was taken after 21 and 28 days. Then, the bacteria contamination test was performed with agar plate.

결과

Through many experiments, I successfully developed agar-lignin bioplastic film. I tested various concentration of agar and lignin, all with different outcomes. Initial testing demonstrated that agar cannot be used for bioplastic film because it has very low flexibility. Also, both 8% and 10% agar solution cannot be shaped into a thin film because of its high viscosity. Since 5% agar film shows increased tensile strength than 2% agar film, I keep the condition of 5% agar for later experiments. Through additional trials, I found that the addition of lignin increased the flexibility. However, agar-lignin film the water absorption rate did not decrease as expected compared to agar film. Through more experiments, interestingly, I found the addition of 10% glycerol showed 65% decreased the rate of the water absorption on agar lignin film compared to agar film.

Next, I evaluated the biodegradability of the agar-lignin film with 10% glycerol. Natural degradability for the agarose-lignin film is important in order to overcome the drawbacks of existing petroleum-based plastics. Therefore, the film was exposed to UV and placed under soil for biodegradability testing. UV photolysis, a conventional laboratory experiment for degradability, was used to test for general degradability of the agar-lignin film, while placement in soil tested for film behavior within natural settings. From two identical samples of agarose-lignin film, one was exposed to UV, while another was sequestered from UV. After 30 days, the sample exposed to the UV light was evidently degraded with a decrease in mass and mechanical strength (A).

 

 

 

In order to account for the difference in degradation speed, the sample in soil was buried for 90 days rather than 30 days (B). As a result, the agar-lignin film degraded in soil. The agar-lignin films lose its flexibility and weight (data not shown). These results indicate that agar-lignin film displayed signs of degradation, with physical deformation and decrease in mass.

As a food packaging material, the capability to preserve food by preventing microbial growth is an essential property. Therefore, the agarose-lignin film’s food preservation capability was compared to that of polyethylene (PE). Three kiwis were prepared, each sterilized with ethanol before experimentation. Then, each kiwi was wrapped with either agarose-lignin sheet or PE with the sealing tape. A third kiwi was left unwrapped as a control. The samples were left at room temperature for 28 days. After 28 days, the packaging was removed. Both kiwis which were wrapped in agarose-lignin film or PE packaging displayed no signs of physical deformation or microbial growth, while the control sample displayed signs of microbial growth and moisture loss (C).

 

To ensure that no microbial colonies developed on the kiwi packaged in agarose-lignin sheet, samples were collected using pre-sterilized cotton buds. Then, they were placed on agar gel on Petri dishes to induce bacterial growth. As a result, after 12 hours, E. coli population was only observed in the control sample. This result indicates that agarose-lignin film not only preserved food from outside air but also protected food from bacterial contamination.

 

 

 

결론

In this study, agar was chosen over other types of starch-based materials due to certain desirable traits. According to the previous studies, agar gels have a more regular texture than starch gels do; starch gels tend to have imperfections such as microscopic cracks, which are rare in agar gels (). Therefore, using agarose for the packaging material would prevent pollution of internal contents, which may occur with starch-based materials.

Properties of agar gel are not suitable for use as commercial packaging material. Incorporation of lignin is a method often used to reinforce mechanical strength and increase antibacterial resistance. The agarose-lignin film showed improved firmness and tensile strength compared to pure agarose hydrogel (). In addition, lignin-based hydrogels demonstrate retention of antibacterial properties, which indicate the likelihood of antibacterial property retention in agarose-lignin film (). In addition, lignin is the waste product of the Kraft process, which produces wood pulp for paper production. Thus, the price of Kraft lignin is lower than that of other bioplastics such as PHA and PLA. Moreover, because lignin is an organic material, it is naturally biodegradable.

The agar-lignin film degraded in the soil in a similar manner to those of PHA and PLA, which makes it a suitable replacement for the existing bioplastics materials. The degradability in soil is extremely beneficial since the degradation in soil requires less maintenance than complex recycling systems.

As a food packaging material, the film performed similarly to existing PE packaging. It prevented microbial growth to the same extent as the PE packaging in comparison to a sample not packaged in either material. In addition, the food’s decrease in mass due to moisture loss was comparable to that of the food packaged in PE. Overall, as a packaging material for fresh food, the agarose-lignin film proved to be a viable replacement for existing PE packaging.

In the present study, 5% agar 5% lignin with 10% glycerol in the final film yielded promising results. However, the experiment for the determination of the optimum composition remains to be conducted. The deformation of the material during the drying process provides a challenge for commercial mass production, thus requiring further research for mass production. Agar-lignin film dramatically decreased its water permeability compared to agarose film. However, the problem of water permeability on agarose-lignin still remains. Interestingly, addition of 10% of glycerol, which contains both hydrophilic and hydrophobic characteristic, improved water permeability dramatically. Therefore, a new candidate of hydrophobic materials should be investigated and tested in the future.

내 정보

건강 및 안전

Suwon University (Biosystem Lab)

Mentor/Lab manager: Dr. Lee, Woo Rin

email: gosyber@suwon.ac.kr

phone number: +82) 10-2021-7064

 

All safety instructions were provided by Dr. Lee, who is a lab manager. I always conducted the experiment when Dr. Lee was presented in the lab. I also took many health and safety precautions throughout the development of agar-lignin plastic. I also followed the safety instruction manual provided by Dr. Lee.

참고문헌, 참조자료 및 감사의 말

Reference

1.    Xanthos, D. and T.R. Walker, International policies to reduce plastic marine pollution from single-use plastics (plastic bags and microbeads): A review. Mar Pollut Bull, 2017. 118(1-2): p. 17-26.
2.    Jambunathan, P. and K. Zhang, Engineered biosynthesis of biodegradable polymers. J Ind Microbiol Biotechnol, 2016. 43(8): p. 1037-58.
3.    Lee, W.K., et al., Biosynthesis of agar in red seaweeds: A review. Carbohydr Polym, 2017. 164: p. 23-30.
4.    Brzonova, I., et al., Production of lignin based insoluble polymers (anionic hydrogels) by C. versicolor. Sci Rep, 2017. 7(1): p. 17507.
5.    Prieto, A., To be, or not to be biodegradable... that is the question for the bio-based plastics. Microb Biotechnol, 2016. 9(5): p. 652-7.
6.    Awadhiya, A., D. Kumar, and V. Verma, Crosslinking of agarose bioplastic using citric acid. Carbohydr Polym, 2016. 151: p. 60-67.
7.    Jariyasakoolroj, P., P. Leelaphiwat, and N. Harnkarnsujarit, Advances in research and development of bioplastic for food packaging. J Sci Food Agric, 2018.
8.    Phan, T.D., et al., Functional Properties of Edible Agar-Based and Starch-Based Films for Food Quality Preservation. Journal of Agricultural and Food Chemistry, 2005. 53(4): p. 973-981.

 

Acknowledgements

I would like to thank Dr. Woo Rin Lee, my mentor from Suwon University Biosystem Lab for his assistance and guidance in this project. He provided all chemical reagents and experimental equipment for this project. I would also like to express my gratitude to the Suwon University for their generous support of my research. I also thank Dr. Ha and Dr. Kim, who provided crucial assistance with water absorption test.