Mechanism of Degradation of Starch, a Highly Branched Polymer, during Extrusion

Abstract

An investigation of the mechanisms of degradation of a branched polymer in extrusion was performed using starch as substrate. Starch has the advantage that the distribution of degree of polymerization of individual branches can be readily obtained using a debranching enzyme and also that it does not undergo any reaction except scission during extrusion, thereby aiding mechanistic interpretation. Various starches, containing a range of the highly branched amylopectin component and the much less branched amylose component, were extruded in the presence of water and glycerol as plasticizers with extruder barrel temperatures ranging from 50 °C at the hopper zone through to 140 °C near the die exit. Analysis by size-exclusion chromatography of both whole and debranched samples subject to various levels of extrusion showed that the extrusion degradation process involved preferential cleaving of larger molecules, while causing the size distribution to narrow and converge toward a maximum stable size. This is analogous to a similar effect of shear degradation of droplets in emulsions. It was also found that the susceptibility of polymer molecules to shear degradation is not only dependent on the size of the molecule but also extensively influenced by the branching structure. High branching density and short branch length were associated with higher susceptibility to shear degradation. This is explained by the hypothesis that a short-chain hyperbranched polymer has a relatively inflexible structure, leading to a higher susceptibility to shear scission. The degradation process is not significantly selective toward the length of individual branches when the polymer is in a molten state but it preferentially breaks longer branches when the starch polymer is in a semicrystalline granular form. These inferences are generally applicable and use the additional information from the branch length distribution and absence of side reactions, which is generally not available for synthetic polymers.

Understanding the structural features of high-amylose maize starch through hydrothermal treatment

“…High-amylose starch has some special properties, such as its heat resistance [12], which is reflected by high gelatinization temperature and the retainment of granules in boiling water [13][14][15], and its digestion resistibility [16][17], due to which this starch has been used as resistant starch and in drug delivery systems. In addition, high-amylose starch is especially suitable for producing thermoplastic materials [18][19], because amylose as a linear molecule can provide better mechanical properties resulting from easier formation of crystallites and entanglements.…”

Characterization Methods for Starch-Based Materials: State of the Art and Perspectives

“…The most straightforward is simple reduction in size of the whole molecule. Studies on the evolution of both Level 2 and Level 1 structures during these processes [94,95] show first that amylopectin is preferentially degraded compared with amylose, which is unsurprising because of its larger size. In addition, it is found that this degradation brings the starch down to a maximum stable size (which depends on the mechanical conditions).…”

An improved approach for evaluating the semicrystalline lamellae of starch granules by synchrotron SAXS

“…There are two kinds of starch polymers, i.e., amylose and amylopectin (Jiang, Gao, Li & Zhang, 2011;Liu, Halley & Gilbert, 2010). These two biopolymers are organized on multiple scales in the starch granule to form its semicrystalline structure, including the whole granule, the growth rings, the semicrystalline lamellae and the crystallites (Buleon, Colonna, Planchot & Ball, 1998;Luengwilai & Beckles, 2009;Perez & Bertoft, 2010;Pikus, 2005;Zhang et al, 2015).…”