Thermoplastic Starch

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By Mohini Sain, PhD, Robert Jeng, Bradley Saville, PhD, Chun Bei Huang and Martin Hubbes, PhD
Bradley Saville, PhD

Increasing demand for plastics, which are currently made from non-renewable resources, has negative environmental consequences and may deplete fossil fuel resources. Researchers are therefore seeking alternatives based on renewable materials, predominantly from agri-forest resources.
An increased emphasis on sustainability, eco-efficiency and green chemistry has driven a search for renewable and environmentally friendly resources. Starch — a biodegradable polysaccharide produced in abundance at low cost — possesses thermoplastic behaviour, making it one of the most promising candidates as an alternative to traditional plastics in certain market segments, such as the food packaging industry.
Starch is a complex homo-polymer composed of a-D-glucose units linked together in two different forms: the linear form amylose and the highly branched amylopectin.1 Composition and structure of starch granules varies considerably between different plants, affecting the properties and function of starches from different crops.2
Numerous studies have been conducted to optimize the performance of starch-based plastics.1, 3, 4, 5 These studies show that important properties for evaluating a packaging material include mechanical and thermoforming properties, gas and water vapor permeability, resistance, transparency and availability.6 Designing and engineering a starch-based packaging product that possesses all of these required properties is, however, a significant challenge. Product cost and technical challenges — such as brittleness associated with high loads, and poor water and gas barrier properties — have to be overcome before renewable biomaterials can be commercialized.7 Currently, most research aimed at enhancing the functional properties and inherent bonding strength of starch have focused on incorporating additives, such as plasticizers, to improve the material’s performance.8, 9
Fungi and other micro-organisms present a unique and novel alternative to chemical additives because they produce a variety of enzymes that disintegrate or assimilate organic materials in culture. In addition, micro-organisms can synthesize homo- and hetero-polymers that display unique molecular structures and properties. Ongoing research suggests that biopolymers can be modified by fungi for improved performance.
The purpose of our research program is to develop a commercially viable, economically integrated technology for the mass production of biopolymers for bio-packaging applications, using renewable resources derived from agricultural crops and/or forestry residues.

Starch Modification by Fermentation
Starch is typically fermented with yeast extract and glucose in distilled water. The fungal spores are inoculated into the starch dispersion, and cultures are maintained as a shake culture for several days. Polymer (known as thermoplastic starch) is obtained by solvent precipitation. The molecular weight (MW) distribution of thermoplastic starch is determined by gel-filtration chromatography, and used as a control parameter for the fermentation process. Results from gel-filtration chromatography demonstrate a substantial increase in MW during the modification process. The MW distribution can be controlled by adjusting the duration of the fermentation process and other culture conditions. The MW and MW distribution are important indicators of the quality of polymer cast films, likely because higher MW and branched materials can form an entanglement network during film formation, influencing mechanical strength.
During microbial modification, various botanical resources such as corn kernels, wheat flour, and potato, tapioca, rice and millet starch are used as testing materials. By increasing the amount of starch in the medium, the yield of thermoplastic starch could exceed 85 per cent. As expected, thermoplastic starches derived from each of these starch sources possess different properties, influenced by the proportions of amylose and amylopectin in the original starch, along with its susceptibility to microbial modification. Nonetheless, each of these starch sources could be used to create thermoplastics with properties suitable for packaging applications.
The purified thermoplastic starch developed by fungal treatment is more water soluble than the original starch. Furthermore, water solubility can be controlled through the use of different starch sources, or by blending starches from different sources.

Packaging Film
The extracted thermoplastic starch is combined with glycerol and water, then cast into films using a solution casting process. Mechanical characteristics of films are evaluated by tensile, flexural and modulus testing.
Both pure starches and the thermoplastic starches are cast into film and subjected to mechanical testing. The experimental results show that the thermoplastic starch has better strength properties than the original starch source, and is well suited for use as a packaging material. Tensile tests show that the mechanical strength of thermoplastic starch made from tapioca and potato polymers is superior to that of films derived from others sources, and of films derived from the native starch. Of particular significance is that, compared to the original source starch, a great improvement in the peak stress is obtained, along with a reduction in elongation at the break point. After modification, the thermoplastic starches also exhibit much lower water absorption, which indicates higher moisture resistance, a favourable property in a packaging material application.
The foregoing work identifies the important role of novel fermentation processes in the development of new and novel biomaterials. By identifying and capturing the essential functionality of microbes and microbial enzymes, renewable agricultural and forest resources can be converted into value-added products, including biofilms for packaging and medical applications. Such biomaterials have the opportunity to supplant current materials based on cost and/or superior and unique functionality. Ongoing fermentation research is a key element in this process, as we seek to maximize yield and optimize the functional properties of the biopolymers.

In this particular application, fermentation of starch with fungal micro-organisms successfully modified starch and improved the properties of the starch-based plastic. Tensile tests show that the films made from thermoplastic starch possessed significantly better tensile stress, and improved water resistance properties. Such results represent initial steps towards a future where large-scale production of biopolymers derived from renewable resources is commonplace.

References:
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2.     Jobling S. “Improving starch for food and industrial applications.” Plant Biology 7 (2004): 210-218.
3.     Mali S., M. Victoria, E. Grossmann, M. A. Garcia, M. N. Martino and N. E. Zaritzky. “Mechanical and thermal properties of yam starch films.” Food Hydrocolloids 19 (2005): 157-164.
4.     Fama L., A. M. Rojas, A. Goyanes and L. Gerschenson. “Mechanical properties of tapioca-starch edible films containing sorbates.” Food Science and Technology/LWT 38 (2005): 631-639.
5.     Lawton J.W. “Effect of starch type on the properties of starch containing films.” Carbohydrate Polymers 29 (1996): 203-208.
6.     Weber C. J., V. Haugaard, R. Festersen and G. Bertelsen. “Production and applications of biobased packaging materials for the food industry.” Food Additives and Contaminants 19 Supplement (2002): 172-177.
7.     Lorcks J. “Properties and applications of compostable starch-based plastic material.” Polymer Degradation and Stability 59 (1998): 245-249.
8.     Poutanen K., and P. Forssell. “Modification of starch properties with plasticizers.” Trends in Polymer Science 4(4) (1996): 128-132.
9.     Laohakunjit N., and A. Noomhorm. “Effect of plasticizers on mechanical and barrier properties of rice starch film.” Starch 56 (2004): 348-356.

Mohini Sain, Robert Jeng, and Martin Hubbes are with the University of Toronto’s (U of T) (Toronto, ON) department of forestry. Bradley Saville and Chun Bei Huang are with U of T’s department of chemical engineering.