Alain Buléon
Unité "Biopolymères, Interactions, Assemblages"
Institut National de la Recherche Agronomique, BP 71627, 44316 Nantes Cedex 3
buleon@nantes.inra.fr
Starch is the major energy reserve of a large variety of higher plants. It is the predominant carbohydrate in the major foods used by man and is also used extensively in non food industries such as those of papers, pharmaceutics, adhesives, packaging. All starches are biosynthesized as semi-crystalline granules containing densely packed polysaccharides (amylose and amylopectin) and relatively little water. The granules that have dimensions ranging from 1 to 100 mm are often compared to positive polymer spherulites with radial orientation of the starch polymer chains. Two main distinct types of X-ray diffraction pattern are commonly observed (A and B) for native starch. Starch properties strongly depend on the crystalline ultrastructure of starch, which is not fully known despite several decades of investigation. Starch granules have been shown to be made of stacks of amorphous and semi-crystalline growth rings (120-400nm thick). The semi-crystalline shells are composed of alternating crystalline and amorphous lamellae with a 9nm repeat (as determined by small angle X-ray scattering), which is found to be remarkably constant between different cultivars. These semi-crystalline stacks are separated from each other by an amorphous growth ring. Some structural models have been determined for crystalline areas of starch from lamellar or fibre crystals of pure amylose.
Starch melting behaviour is linked to both the water content present during heating (water being plasticizer, constituent of the crystalline structure and also solvent of starch at high temperature) and its intrinsic characteristics (internal structure, crystalline type, chain length involved in the crystalline domains…). After disruption of the granular structure, amylose and amylopectin behave as a mixture of incompatible polymers in water. Depending on the water content and temperature, semi-crystalline gels,, semi-crystalline solids or amorphous glass can form. These products are plasticized by water and behave as synthetic polymers in terms of glass transition and physical ageing. These properties are widely used in food industry and starch materials processing.
Amylose has also the remarkable property to form complexes with a variety of molecules as fatty acids, alcohols, iodine and other hydrophobic small molecules. Such interactions induce a single helical conformation of amylose with characteristics depending on the nature of the complexing molecule, which can be either included in the single helix or between helices in some crystalline forms. This complexing ability has major applications in the field of encapsulation and controlled release, and also a strong impact on food properties (amylose-lipid and amylose-flavours complexes) or mechanical properties of starch materials (amylose-lipid and amylose-plasticizer complexes).
Enzymatic hydrolysis by a-amylases of native and transformed starch is directly linked to structural parameters as porosity, morphology, crystalline type, crystal size or chain length. Various enzymatic and hydrothermal treatments have been optimized for the manufacture of resistant starch. Resistant starch is the starch fraction which is incompletely degraded in the upper gut and fermented into the colon. It is extensively studied due to its health potential (low glycaemic index and beneficial effects on pathologies of the colon). The knowledge of the relationships between the structural features and the susceptibility to enzymatic hydrolysis is also necessary for optimizing the biofuel production form starch or the making of biodegradable starch materials.
The main structural characteristics of starch and their involvement in its hydrothermal or enzymatic modification are presented and discussed in relation with the main techniques used (X ray scattering, electron microscopy, solid state NMR, calorimetry…) .
