Elsevier

Carbohydrate Polymers

Volume 152, 5 November 2016, Pages 398-408
Carbohydrate Polymers

Plant-crafted starches for bioplastics production

https://doi.org/10.1016/j.carbpol.2016.07.039Get rights and content

Highlights

  • Fundamental crystallinity data guides the design of new bioplastics functionality.

  • Bioplastic prototypes crystallinity and mechanics are affected by water and glycerol.

  • GMO starch allows the production of stronger and elastic bioplastic prototypes.

Abstract

Transgenically-produced amylose-only (AO) starch was used to manufacture bioplastic prototypes. Extruded starch samples were tested for crystal residues, elasticity, glass transition temperature, mechanical properties, molecular mass and microstructure. The AO starch granule crystallinity was both of the B- and Vh-type, while the isogenic control starch was mainly A-type. The first of three endothermic transitions was attributed to gelatinization at about 60 °C. The second and third peaks were identified as melting of the starch and amylose-lipid complexes, respectively. After extrusion, the AO samples displayed Vh- and B-type crystalline structures, the B-type polymorph being the dominant one. The AO prototypes demonstrated a 6-fold higher mechanical stress at break and 2.5-fold higher strain at break compared to control starch. Dynamic mechanical analysis showed a significant increase in the storage modulus (E′) for AO samples compared to the control. The data support the use of pure starch-based bioplastics devoid of non-polysaccharide fillers.

Introduction

Starch consists of two major components, amylose and amylopectin, which differ in their degree of branching and molecular size. Amylose is mostly a linear polymer composed of mainly α-(1-4)-linked glucose residues, with a molecular weight (Mw) of approximately 105–106 g mol−1. Amylopectin is a much larger molecule than amylose (Mw = 107–109 g mol−1) and is a more branched polymer with an α-(1–4)-linked d-glucose backbone and approximately 5% α-(1–6)-linked branches (Blennow et al., 2013). At a higher level of organization starch is arranged in different crystalline polymorphs. These are dependent on the botanical origin and type of organ of the plant and are characterized by two main crystalline structures called A-type and B-type (Pérez, Baldwin, & Gallant, 2009). These complexes, if hydrated, can crystallize into different so-called Vh-type polymorphs, which are classified as type I and type II depending on their melting point and structural stability (Goderis, Putseys, Gommes, Bosmans, & Delcour, 2014).

Being one of the most abundant and versatile polysaccharides on earth, starch is a suitable raw material for the production of new environmentally-friendly bioplastics (e.g. Materbi©, Novamont). Starch is readily transformable using existing technologies and is an abundant and low-cost commodity that can be easily refined. Starch-based plastics can be transformed into edible and compostable products (Ali Shah, Hasan, Hameed, & Ahmed, 2008). Conventional plastic (e.g. polyolefin based plastics), on the other hand, can be a cause of environmental problems related to their low degradability rates. In fact, the degradation of conventional plastic takes place by producing macro- and micro-plastic fragments that persist in both soil and water (Cole, Lindeque, Halsband, & Galloway, 2011). Alternatively, conventional plastic can be mixed with additives that catalyze the degradation of the polymers through light, heat or mechanical stress, leaving pro-degrading additives in the environment but allowing full degradation of the polymer (Ammala et al., 2011). Thermoplastic starch (TPS) is an alternative that can be produced using different existing techniques including casting, thermo-molding, extrusion and injection molding, traditionally used for the processing of synthetic polymers. An interesting advantage of TPS is the modularity of its different properties. For example, the molecular structure (e.g. the molecular size) and amylose/amylopectin ratio influence the final properties of starch-based bioplastic (Gillgren, Blennow, Pettersson, & Stading, 2011). Also minor differences in the amylose and phosphate contents and chain length distribution of the amylopectin, as well as granular organization, can result in major differences in physical properties. The modification of such structures may also result in the extensive alteration of film functionality as has been demonstrated for transgenic starches (Gillgren et al., 2011). The amylose/amylopectin ratio is an important factor influencing the mechanical properties of such materials, especially since amylose affects the degree of crystallinity and entanglement. Increased amylose content is typically related to an increase in tensile strength and a decrease in strain (Alves, Mali, Beléia, & Grossmann, 2007). Low elasticity can be avoided by including a plasticizer, usually glycerol. This enables the material to endure increased strain but leads to an unavoidable decline in the original strength of the material (Lourdin, Della Valle, & Colonna, 1995; Myllärinen et al., 2002). In addition, high amylopectin content is correlated with higher strain as a result of its structural characteristics e.g. high molecular size (Hulleman, Jannesen, & Feil, 1998).

Commercial starch-based plastics are chemically modified or blended with synthetic polymers, such as polycaprolactone (PCL), to enhance the plastic behavior of these materials (Bastioli, Bellotti, Del Giudice, & Grilli, 1993).

Alternatively, TPS may be improved by other methods, such as the use of genetically modified (GM) organism. Starch-modifying enzymes produced by GM microorganisms can modulate starch structure and functionality, or GM crops can be engineered to produce tailor-made starches directly in their starch-producing organs by coordinated expression of starch-modifying enzymes (Hebelstrup, Sagnelli, & Blennow, 2015).

In the present study, a new plant-crafted starch (engineered and crafted in planta) synthesized by an engineered barley-line called amylose-only (AO, Carciofi et al., 2012), composed of 99% of a rather homogeneous amylose-like α-glucan, was used to produce thermoplastic starch-based prototypes. The samples were manufactured by molding and extrusion and finally tested for crystallinity, dynamic mechanical analysis (DMA) and stress and strain at break. The study demonstrates the potential of using GMO plants for the production of new environmentally-friendly polymers as part of a sustainable production of plastics.

Section snippets

Materials

The barley starches used in this study were extracted and purified from two barley lines; a control Golden Promise and amylose only a genetically modified (GM) line, which was generated by RNA interference suppressing all three starch branching enzymes in the Golden Promise background. Briefly, the silencing of all three identified genes of the starch branching enzyme family (SBE1, SBE2a and SBE2b) was achieved by using RNAi suppression. Embryos were isolated from juvenile seeds and used for

Macromolecular structure, crystallinity, melting and phase transitions of starch granules

An overview of the intrinsic characteristics of the native AO and control starches was established by analyzing their molar mass, crystallinity and melting behavior. For molecular size and molar mass distributions, the starches were dissolved and analyzed by asymmetrical flow field-flow fractionation coupled with multi-angle laser light scattering (A4F-MALLS). The solubilization recoveries were higher than 92% for all samples and the elution recoveries were all higher than 90%, providing

Conclusions

Starch-based bioplastic prototypes fabricated from an almost amylopectin-free starch synthesized directly in the barley grain behaved differently compared to most high-amylose systems analyzed to date. Melting conditions for subsequent extrusion cooking was determined from initial DSC and X-ray scattering data. These data demonstrated effects related to interaction between the glycerol and amylose during the heating process. A subsequent thermo-molding screen provided optimized formulations and

Acknowledgements

We would like to thank Lun Li Krusholt for help with the extraction of the starch and Michelle Derbyshire of MD Writing Services, for providing assistance with the preparation of this manuscript. We would also like to thank the Center for Advanced Bioimaging, Faculty of Science, University of Copenhagen for use of their microscopy facilities. This work was funded by The Danish Council for Independent Research Technology and Production Sciences and Carlsberg Foundation.

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