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126.96.36.199 Morphology and ultra-structure of starch grains
188.8.131.52.1 Morphology of starch granules
A native state, amylose and amylopectin are associated with the level of entities called semi-crystalline granular starch grains, whose size (1 to 100 microns), morphology (spherical, lenticular, reniform …), the composition (amylose / amylopectin), position of hilum (point of departure for the growth of grain starch) depend on the botanical origin (Figure 2.9). Observation by polarized optical microscopy light shows the starch granules are birefringent and show a Maltese cross whose branches converge at the hilum (Biot, 1844). The birefringence is positive, implying a radial organization of macromolecular chains in the grain.
Potato Pea Cassava
Figure 2. 9: Grains of different starches observed in scanning electron microscopy SEM (magnification × 280) (Fellow, 2005)
184.108.40.206.2 Ultra-structure of starch grains
The starch granules are entities formed semi-crystalline layers formed concentric rings of crystalline and amorphous. The size and number of these rings depend on the botanical origin of starch (BeMiller & Whistler, 2009). These layers correspond to alternating crystalline and amorphous lamellae whose thickness varies from 120 to 400 nm (Yamaguchi, Kainuma & French, 1979). The existence of these zones was confirmed by X-ray diffraction at small angles (SAXS) (Oostergetel & Bruggen, 1989).
Jenkins and Donald (1995) proposed a model of organization of the starch grain containing different structural scales (Figure 2.10). The thickness of repetitions semi-crystalline (910nm) corresponds to the cluster structure of amylopectin. The crystalline lamellae are composed of short chains of amylopectin Dp 15, while the amorphous lamellae are composed of branch points.
Figure 2. 10: The different levels of grain starch (Jenkins and Donald, 1995)
However, in recent decade, studies by atomic force microscopy (AFM) on the surface of starch grains (Baldwin, Adler, Davies & Melia, 1998) and in their internal structure (Baker, Miles & Helbert, 2001; Ridout, Gunning, Parker, Wilson & Morris, 2002; Ridout, Parker, Hedley, Bogracheva & Morris, 2003) have strengthened the concept of blocklets proposed by Gallant et al(1997) (Figure 2.11) based on observations by scanning electron microscopy (SEM), transmission (TEM) and atomic force (AFM). In this model, the strips are arranged in blocks spherical (“blocklets”) with a diameter ranging from 20 to 500 nm according to botanical origin and location in the grain starch. The size of wheat starch blocklets (crystallinity type A) varies from 10 to 50 nm (Baldwin, Adler, Davies & Melia, 1998) while the smooth pea starches (crystallinity type B) form blocklets much more important with size ranging from 200 to 300 nm. Amylopectin would support the crystal structures. The short chains of amylopectin have enough DP to form double helices, associated themselves together to form clusters. The clusters are organized to form lamellae of about 9-10 nm thick, characterized by alternating regions of crystalline (double helices) and amorphous (branch points).
Amylose would link the blocklets and improve the resistance of grain starch (Ridout, Parker, Hedley, Bogracheva & Morris, 2003). The ability of AFM to determine hardness on the local section of grain, and the wetting and swelling localized to these sections, helped locate a crystalline and amorphous material in the starch grains (Ridout, Gunning, Parker, Wilson & Morris, 2002). The detailed analysis of AFM on the internal structure of starch granules of pea suggested that blocklets are distributed evenly and that the amorphous growth rings result from localized defects in the grain growth (Ridout, Parker, Hedley, Bogracheva & Morris, 2004).
Figure 2. 11: Organization of starch grains in “blocklets” (Carvalho, 2008; Gallant, Bouchet & Baldwin, 1997)
220.127.116.11 Semi-crystalline structure of starch grains
18.104.22.168.1 Crystalline phase
The semi-crystalline nature of native starches has been shown by X-ray diffraction (XRD). The native starches can be classified into three groups according to their diffraction pattern (Figure 2.12): A, B and C. The Type A general characteristic of cereal starches (wheat starch and maize). Type B characterizes starches of tubers and grains rich in amylose. Finally, type C is characteristic of starches of legumes. It corresponds to a mixture of both crystal types A and B.
Figure 2. 12: X-ray diffraction diagram for crystalline starch type A, B and C.
The XRD diagrams of starch grains show large peaks and an amorphous important contribution. The degree of crystallinity of starch ranges from 15% to 45% depending on the botanical origin(Zobel, 1988). It is known that water is an integral part of the crystalline structure of starch type A and type B. Below 10% moisture content; the diffraction patterns are poorly defined.
Figure 2. 13: Crystallinity of potato starch: influence of water content on the resolution of the diffraction pattern of X-rays (Buléon, Bizot, Delage & Multno, 1982)
The intensity of diffraction peaks increases and the resolution of diffraction patterns of type B increases from a water content of 10% to a maximum of 33% (Buléon, Bizot, Delage & Multno, 1982; Cleven, Berg & Plas, 1978). The newest models on the structures of type A and B offer a parallel arrangement of double helices with left parallel strands (Imberty, Buléon, Tran & Péerez, 1991; Imberty, Chanzy, Pérez, Bulèon & Tran, 1988). The most stable conformation for amylose is a double helix with 2 × 6 glucose units per turn. Each double helix is ready parallel to its neighbor with a lag of a half-step along the axis of the double helix.
This structure is stabilized by interactions of Van der Waals and hydrogen bonds.
The main difference between the two main polymorphs A and B is the stacking of double helices in the crystal lattice and the amount of water between the double helices.
In the structure of type A, with a mesh monoclinic (a = 2.124 nm, b = 1.172 nm, c = 1.069 nm and γ = 123.5 °), each double helix has six neighbors, forming a dense structure (Figure 2.14) (Popov et al., 2009; Popov, Burghammer, Buléon, Montesanti, Putaux & Riekel, 2006). The compactness of the structure does not allow moisture by only 4 water molecules per cell. Type B is characterized by a hexagonal (a = b = 1.85 nm, c = 1.04 nm and γ = 120 °) (Figure 2.14). Each double helix has three neighbors. The helices are arranged around a central cavity containing water molecules (36 per cell) (Hsein-Chih & Sarko, 1978; Imberty, Chanzy, Pérez, Bulèon & Tran, 1988; Takahashi, Kumano & Nishikawa, 2004).
Figure 2. 14: Crystalline arrangement of double helices of amylose type A and B(Hsien-Chih & Sarko, 1978)
The three crystalline types observed for native starches, we should add the type characteristic of crystalline V-amylose complex ligands for starches after heat treatment (lipid unicycles, some flavorings …) (gelatinized V, ie observed on cooked starches).
22.214.171.124 Thermal transitions
126.96.36.199.1 Gelatinization – Starching (hydrated medium)
The heating an aqueous suspension of starch at temperatures above about 60 °C, excess water causes irreversible swelling of starch grains. At a given temperature called gelatinization temperature, starch grain loses its semi-crystalline structure (disappearance of the Maltese cross observed in polarized light) and inflates very quickly over a limited temperature range (1 to 1.5 °C). This phenomenon is also observed by differential thermal analysis (DTA) and is characterized by a melting endotherm. The endothermic peak corresponds to the gelatinization temperature. These phenomena depend on the botanical origin and type of crystalline.
At higher temperatures in the swelling of starch grains, the amorphous amylose was solubilized in the medium. This is the stiffening phase leading to the award of a starch. This paste is a suspension in which swollen starch granules form a dispersed phase and the macromolecules of amylose solubilized form the continuous phase. The thermodynamic incompatibility between amylose and amylopectin leads to phase separation in hot, 70 ° C (Kalichevsky, Orford & Ring, 1986; Kalichevsky & Ring, 1987). Increasing the volume fraction of swollen starch granules causes an increase in viscosity. The grains of potato starch have a high degree of swelling of starch resulting in a significant apparent viscosity. However, the starch grains (crystalline type A) are characterized by an initial stage of swelling limited to the gelatinization temperature, followed by a second inflation to 90 ° C during which a marked dispersion of macromolecules occurs. This explains the low consistency of corn starch compared to starches from tubers (Doublier, 1981; Thebaudin, Lefebvre & Doublier, 1998). If the heating of starch continues, the residual grains burst and disperse but solubilization is rarely complete.