Starch Polymers

From Genetic Engineering to Green Applications

P. Halley, L. Averous, P. Halley, L. Averous

• Publisher: Elsevier
• Year: 2014
• ISBN:
  9780444537317
• Topic: Biological Sciences
• Subtopic: Biochemistry
• Index: Biological Sciences

This book focuses on starch polymers including starch genetics, biotechnological and chemical modification, nanostructures, processing, characterization, properties and applications. This books topic is in a cutting edge and emerging technology area of biomaterials, nanomaterials and renewable materials, and will involve international experts in diverse fields from genetic engineering to applications.

• Focuses on cutting edge applications of starch polymers, including starch genetics and Rheology
• Contains working examples and provides real problems and solutions in the area of biomaterials, nanomaterials, and renewable materials
• Provides systematic and in-depth coverage and critical assessment of all starch properties and applications from top scientists in the industry

IV

Novel Starch Modified by Chemistry

Chapter 5

Starch Modification to Develop Novel Starch-Biopolymer Blends

State of Art and Perspectives

Ashok K. Shrestha¹; Peter J. Halley^{2 1} School of Science and Health, University of Western Sydney, Hawkesbury Campus, Richmond, New South Wales, Australia
² Centre of High Performance Polymers, School of Chemical Engineering/AIBN, The University of Queensland, St. Lucia, Queensland, Australia

Abstract

Native starches have limited use in industrial applications due to their poor water resistance, pasting, and mechanical properties. These behaviors can be amended by altering the starch structure by introducing ionic or hydrophobic groups in the starch structure. Structural modifications tend to stabilize the starch granules, impart hydrophilicity or hydrophobicity, heat resistance, desirable rheological properties, and increased processability. Physical modifications of starch can be achieved by various drying techniques, treatment with acids and alcohols, extrusion, etc. On the other hand, chemical modification can be achieved by chemical reactions, e.g., conversion, oxidation, dextrinization, cross-linking, and stabilization of starches. This review investigated the synthesis of modified starches and their physicochemical properties and use in various industries such as paper making, gelling agent, food additives, and pharmaceutics. In addition, the current understanding of modified starches as enzyme-resistant starch, biodegradable thermoplastic starch, and encapsulating and control release agent is also discussed.

Keywords

starch

chemical modification

food ingredients

TPS

ERS

1 Starch Structure

Starch is one of the most abundant biomolecule on earth and a major part of plant foods, providing energy to humans. The structure of starch in plant science is quite intriguing as it is formed for the purpose of storage of molecular glucose structures in the form of macroscopic granules. The starch structure dictates the release of glucose into digestive tract during digestion as well as determining various functional properties of starch in other applications.

Starch molecules are composed of hundreds or thousands of D-glucose units, which are joined together by the α-D (1 → 4) glucosidic linkages to form linear polymer chains (amylose) and α-D (1 → 4) linkages with α-D (1 → 6) glucosidic side chains to form branched chains (amylopectin) (Figure 5.1). However, starch in plant is stored in the starch granules, which consists of a complex hierarchical structure (Cameron & Donald, 1992), which is given as the molecules (~ nm), the growth ring (~ 100 s nm), and the whole grain (~ μm) (Figure 5.2). The starch molecules may have small amount of lipids, proteins, phosphorous, and some minerals (Galliard & Bowler, 1987; Tester, Karkalas, & Qi, 2004; Tester, Karkalas, & Xin, 2004). The starches from most plants are composed of around 15-30% amylose and 70-85% amylopectin, although, mutations affecting starch biosynthesis can dramatically affect the amount of both molecules in starch granules (Davis, Supatcharee, Khandelwal, & Chibbar, 2003; Galliard & Bowler, 1987).

Figure 5.1 Structure of amylose and amylopectin (Tester, Karkalas, & Qi, 2004).

Figure 5.2 Hierarchy of starch structure, from the molecular to the granular level.

Amylose is a long-chain molecule with a typical molecular weight of about 10⁵-10⁶ and degree of polymerization of 500-5000 (Yoshimoto, Tashiro, Takenouchi, & Takeda, 2000). Approximately 25-55% (molecular basis) of the amylose have secondary chains connected through intermittent α (1 → 6) branch points ranging from 2 to 11 depending on the botanical origin (Morrison & Karkalas, 1990). Compared to amylose, amylopectin is a very large and more frequently branched molecule with 95% α-D (1 → 4) and 5% α-D (1 → 6) glucosidic linkages and molecular weight of 10⁷-10⁹ (Tester, Karkalas, & Qi, 2004; Tester, Karkalas, & Xin, 2004). Although the degree of polymerization of amylopectin molecules ranged from 700 to 26,500, the individual chains are relatively shorter and more complex compared to those of amylose molecules (Tester, Karkalas, & Qi, 2004; Tester, Karkalas, & Xin, 2004). These short chains in amylopectin are not distributed randomly throughout and instead form a highly organized three-dimensional structure in the form of clusters (Hizukuri, 1986).

Starch structure is also known as “the other double helix” (Hancock & Tarbet, 2000) due to its helical structure, similar to DNA strand (Figure 5.2). The maltose structure indicates the torsion angles across the α-D (1 → 4) glucosidic bond between two glucose molecules are slighted twisted. If a series of glucose units is connected maintaining these torsion angles across the glycosidic bonds, a left-hand helix is obtained. Individual helices are intertwined to give the double helix (Figure 5.2) that forms the basis of crystalline structure of A, B, and C, as shown by recent crystallographic studies (Imberty, Buleon, Tran, & Perez, 1991). It has been proposed that at least 10 glucose units are required to form double helices within the ordered arrays of amylopectin (Imberty et al., 1991). The “A” type is commonly found in cereals such as maize, wheat, and rice; “B” type in tubers such as potatoes; and “C” type, which is a combination of A and B types, in legumes such as smooth peas, e.g., the center part exhibiting the B form and the outermost regions the A type (Gidley, 1987). A less prominent starch polymorph in starch granule is V type, which is characterized by the presence of internally absorbed small molecules such as iodine, DMSO, n-butanol, or lipids in the single helical structure of amylose (Tester, Karkalas, & Qi, 2004; Tester, Karkalas, & Xin, 2004).

It is now well established that amylopectin, a highly branched molecule with its side chain branches, intertwined to form double helices forming the basis for crystals. The amylose molecules are known to exist inside the granules as amylose-lipid inclusion complexes or free amylose (Morrison, 1988). The role of amylose in starch granules is not well understood.

According to “cluster model,” the amylopectin molecule made up of three broad classes of glucose chains, A, B, and C. The outer chains (A) are glycosidically linked at their potential reducing group through C-6 position of glucose residue to an inner chain (B). B chains bind to other B chains or to a C chain that has a single reducing terminal end (Hizukuri, 1986). The amylopectin side chains are incorporated into alternating crystalline and amorphous lamellae, 9-10 nm in size (Jenkins & Donald, 1996). The side chain clusters with double helices form the crystalline lamellae, whereas the branch points form the amorphous lamellae. Figure 5.2 showed these alternating structures form a blocklet structure, representing one segment of lamellar growth ring in the starch granule. A number of these lamellae combine to form crystalline growth rings, separated by amorphous growth rings in concentric, onion-like granule architecture of ~ 100 nm size. These growth rings are in fact the result of multiple concentric shells (or lamellae) of increasing diameter extending from hilum toward the surface of the granule. Every variety of starch granules has its own characteristics size and shapes. Appearance of birefringence or Maltese cross when granules are observed under the polarized light gives some assumption about the correlation between molecular axis and principal axis that the underlying packing of the molecule is radial (Banks, Geddes, Greenwood, & Jones, 1972).

2 Starch Functionality

Starch has ability to impart a broad range of functional properties in food and equally in industrial products (BeMiller, 2007). The functionalities of the starch are largely influenced by the properties of the granular as well as supramolecular assembly of the starch, processing conditions, and modification of structure (Donald, 2004). Starch structure-functionality relationships have been interlinked in numerous characterizations of different starches (Cooke & Gildley, 1992; Jenkins & Donald, 1998). Development of various starch-bas...