An overview on starch structure and chemical nature

Chapter An Overview on Starch Structure and Chemical Nature Starch is considered the second most common biomass on earth, as it is produced by green plants as an energy reserve It is found as granules of different morphologies (depending of the botanical source) in plant tissues, mainly seed, roots, tubers, leaves and fruits (Odeku 2013; Pérez and Bertoft 2010; Zia ud et al 2017) On a cellular level, starch is synthesized in two types of plastids, chloroplasts and amyloplasts, through three main pathways: The Calvin cycle, sucrose synthesis and storage starch biosynthesis (Hsieh et al 2019; Tappiban et al 2019) Chemically, starch can be defined as a polysaccharide composed of α-D-glucopyranosyl units that can be linked in either α-D-(1–4) and/or α-D-(1–6) linkages These molecular linkages form to types of molecules: the linear amylose formed by approximately 1000 glucose units linked in α-D-(1–4) manner and the branched amylopectin, formed by approximately 4000 glucose units, branched through α-D-(1–6) linkages, as shown in Fig. 2.1 The union of both amylose and amylopectin forms a semi-crystalline structure arranged as small granules with diameters between 1–100 μm Most of the native starches have amylose percentages that range between 70 and 80% and amylopectin ranging from 20 to 30% Furthermore, some types of starch can have a very high amylose content, such as starch extracted from amylomaize with a 70% of amylose and some can have very low amylose content such as waxy maize starch with a 1% amylose content 2.1  Structural Organization of Starch One of the most accepted models of the starch granules is that of concentric growth rings originating from the hilium of the granules, with alternating amorphous and semi-crystalline regions of 120–400 thickness (Blazek and Gilbert 2011; Donald et al 2001; Vanier et al 2017) Each growth ring is composed of blocklets of around 20–50 nm and each blocklet consist of semicrystalline lamellae, of approximately © The Author(s), under exclusive license to Springer Nature Switzerland AG 2020 C C Villa Zabala, Starch-based Nanomaterials, SpringerBriefs in Food, Health, and Nutrition, https://doi.org/10.1007/978-3-030-42542-5_2 2  An Overview on Starch Structure and Chemical Nature Fig 2.1  Schematic representation of the structure of amylose and amylopectin 9 nm of length, containing amylopectine and amylose chains (Le Corre et al 2010) In brief, the multilevel can be described as follow: the last level is that of the starch granule, preceded by the alternating semicrystalline and amorphous growth rings The next level down is formed by the blocklets, with the smallest ones proposed to be located in the amorphous rings and the largest ones in the semicrystalline growth rings (Baldwin et al 1998; Pérez and Bertoft 2010; Ptaszek et al 2009; Wang et al 2015) Down from the blocklets level are left-handed helices, with an approximated width of 18 nm, and beneath them are the crystalline and amorphous lamellae with a periodicity of 9 nm Finally, the smallest unit of the starch structure are the individual glucosyl units Figure  2.2 shows the structural organization of the starch granule according to the multilevel model One of the most common techniques used in the characterization of the starch structure is X-ray diffraction (XRD), as according to their XRD diffraction pattern starches can be classified into three crystalline types called A, B, and C (Le Corre et al 2010; Magallanes-Cruz et al 2017) The main reason behind the different XDR patterns of starch is the packing configuration of the double helices and their interaction with water molecules In A-type structures, double helices are closely packed with water molecules between each helical structure, as shown in Fig. 2.3 On the other hand, B-type structures have a more open organization, with a central cavity formed by six double helices in which water molecules are located Finally, the C-type starch is considered a mixture of both A and B-types, as their XDR diffraction pattern can be resolved as a combination of the other two types (Bogracheva et al 1998; Imberty et al 1987; Imberty and Perez 1988) Starch from cereals tend to have the A-type pattern, while starches from tubbers and other amylose rich 2.2  Amylose and Amylopectin Fig 2.2  The different levels of starch structure Reproduced with permission from (T. T B. Tran et al 2011) Fig 2.3 Crystalline Packaging of the double helices in A and B-type starches starches have a B-type pattern On the other hand, legumes, roots, some fruits and plantain starches present a C-type pattern (Copeland et al 2009) Figure 2.4 shows typical XDR patterns for A, B and C-type starches Most native starch granules are known to present a Maltese cross when observed under polarized light, indicating a radial orientation of the principle axis of the crystallites (Pérez and Bertoft 2010) 2.2  Amylose and Amylopectin Amylose is a long, linear α-glucan, with a molecular weight of approximately 1x105  – 1x106 and a degree of polymerization of 300–5000, with around 9–20 branch points that are equivalent to 3–11 chains per molecule and the side chains range in length from to over 100 (Ratnayake et  al 2002; Tester et  al 2004) Amylose crystallizes rapidly in solution in the form of left-handed double helices that are packed in parallel fashion forming either A or B-type allomorphs, in a 2  An Overview on Starch Structure and Chemical Nature Intensity (A.U) Corn Starch Potato Starch Banana Starch 10 15 20 25 30 35 40 2q Fig 2.4  XRD patterns of corn starch (A-type); potato starch (B-type) and banana starch (C-type) phenomenon known as retrogradation (Ratnayake et al 2002) Amylopectin, on the other hand, is a more complex structure, with molecular weights in the range of 1x107 – 1x109 and a polymerization degree of 9000–16,000, with an average chain length of 20–25 It’s been reported, that amylopectin molecules contain several types of chains, classified as either A, B or C chains The A chains are unbranched and linked to B chains; the B chains carry one or more A and/or B chains and the C chains, which contains the reducing end of the molecule The distribution of the different types of chains depend on the botanical source of the starch (Ratnayake et al 2002) As mentioned before, the ration of amylose and amylopectin content in the different starches depends on their botanical origin, as Table 2.1 shows the amylose content of different types of starch 2.3  The Starch Granule The size and morphology of starch granules are dependent of their botanical source, as their shape can vary from spherical, oval, polygonal, lenticular, and kidney shapes and their size can range for