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1. Introduction

In the current scenario where plastic packaging and distribution is increasing steadily, the development of an alternative packaging material that is renewable, biodegradable and non-toxic is being focused on by most researchers. The most recent trend is biopolymer which is discussed in reference to sustainability and includes polymers from biomass, microbial sources and chemically synthesized polymers from agri-based sources. Much of the focus on environmentally friendly plastics involves using starch as a food component and a packaging material. Starch blends currently represent about 18% of the global bioplastic production, with 20,000 tonnes of use in the packaging field.

Biodegradable Polymers

2. Thermoplastic starch (TPS) – Sources and properties

Starch is not a true thermoplastic. It combines with plasticizers at high temperatures and shear to form the film. Starch is generally obtained from corn (82%), wheat (8%), potato (5%), rice, barley, oats, tapioca, pea, banana, sugar palm, cassava and soy. It is a polysaccharide that comprises amylose linked with α (1-4) bonds and amylopectin linked with α (1-4) chains and α (1-6) branches. The degree of crystallinity ranges from 15-45% due to amylopectin, affecting biodegradability, solubility and mechanical strength. When amylose content is more, the film is stronger and flexible, while amylopectin leads to poor mechanical properties like brittleness.

TPS films have low permeability to gases (O₂ and CO₂), high hygroscopicity, poor mechanical strength, low density, low thermal property and viscoelastic behavior. It undergoes discoloration or weakening, due to oxidative degradation of light. Starch is physically and chemically modified by participation, blending, cross-linking, esterification, stabilization, pre-gelatinization, derivation and graft copolymerization to get the desired properties.

3. Conversion of starch to TPS

Conversion of Starch to TPS

3.1. Need for Plasticizer in TPS

The plasticizer is a reinforcing agent with low molecular weight, low vapour pressure, low diffusion rate and high boiling point. It is added to starch to increase its stability, flexibility, toughness, dielectric constant and fracture resistance. It decreases viscosity, glass transition temperature and tensile strength. It also reduces internal hydrogen bonding and hence increases free volume and mobility. Commonly used plasticizers are glycerol, sorbitol, ethylene glycol, maltitols, sunflower oil, glucose, fructose, galactose, mannose, citric acid, urea, amines and amino acids. The similarity in structure and presence of hydroxyl groups make monosaccharides more effective than polyols. Glycerol is the commonly used plasticizer in a range of 20-40%, because it has a high boiling point, low cost and is easily available. Lower glycerol content produces difficulties in processing like pressure build-up and higher content leads to exudation. Plasticizers are generally classified into internal, external, primary and secondary groups. Internal plasticizers are inherent, bulky groups that help decrease the elastic modulus, while external plasticizers are low volatile substances that can be lost by evaporation or migration. Primary plasticizers are soluble at high concentrations and do not exude rapidly, while the other has low gelation capacity and compatibility and is blended with a primary plasticizer.

4. Applications of TPS and its blends

Another approach for improving the property of TPS and its blends is by upgrading its barrier, heat resistance and mechanical properties. Also, TPS blends have increased moisture-resistant properties and tensile strength. Blending biodegradable polyesters like PCL, PLA, and PHA with a starch matrix can improve the properties of the starch, thus increasing the scope of its applications in various processing fronts. Due to wider availability and lower cost, the aliphatic polyester PLA is mainly used in food packaging.

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5. Future Trends

Researches have still been focusing on finding out better starch blends and incorporating antimicrobial features into films. The degree of compatibility between biopolymers and starch varies extensively based on composition. TPS offers significant advantages with synthetic blends in terms of cost, mechanical properties and water vapor barrier properties. Although starch/biodegradable blends are good in solving environmental issues, their barrier and mechanical properties have an inverse relationship to their degradability. The incorporation of natural fibers with starch and its optimization of mechanical properties still needs further analysis. Also, various researches intensify TPS’s functionality, pre-treatments and processability in the food packaging industry.

6. Conclusion

The increase in environmental concerns all over the planet has also made us rethink our choices and their consequences. The increase in the use of polymeric materials for packaging solutions has been causing greater waste generation and landfills. Starch is one of the most ubiquitous and cheapest substitutes for petroleum-based plastics. Plasticizers play a vital role in TPS production to break the strong molecular interactions of starch chains. With the help of high temperature and shear, TPS is formed. Starch is blended with different materials (PVA, PLA, PBS) to improve mechanical strength and water vapour barrier properties. According to a few studies, packaging industry contributes significantly to the overall plastic waste generated, but only 7% is recycled. Therefore, enhancing the starch properties by blending with suitable materials provides wide opportunities in the packaging industry.

References:

1. Averous, L. (2004). Biodegradable multiphase systems based on plasticized starch: a review. Journal of Macromolecular Science, Part C: Polymer Reviews, 44(3), 231-274.

2. Kaseem, M., Hamad, K., & Deri, F. (2012). Thermoplastic starch blends: A review of recent works. Polymer Science Series A, 54(2), 165-176.

3. Khan, B., Bilal Khan Niazi, M., Samin, G., & Jahan, Z. (2017). Thermoplastic starch: a possible biodegradable food packaging material—a review. Journal of Food Process Engineering, 40(3), e12447.

4. Singh, A. A., & Genovese, M. E. (2021). Green and Sustainable Packaging Materials Using Thermoplastic Starch. Sustainable Food Packaging Technology, 133-160.

5. Stein, T. M., & Greene, R. V. (1997). Amino acids as plasticizers for starch‐based plastics. Starch‐Stärke, 49(6), 245-249.

About the Authors:
Anupama Harigovindan, Ashwini Anandh, Nikitha Modupalli &          Venkatachalapathy Natarajan
Department of Food Engineering,
National Institute of Food Technology, Entrepreneurship and Management,
(Formerly Indian Institute of Food Processing Technology), Thanjavur, Tamil Nadu.
Email ID: venkat@iifpt.edu.in

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An up-&-coming bloody creative professional passionately involved with both print and digital media; constantly trying to be an irritating perfectionist and surviving solely on inspiration (sometimes from the most inert objects)… Currently, staying busily engaged with producing mouth-watering content for the much anticipated and less explored Indian Food Processing Sector...

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