Starch, a crucial carbohydrate, serves as a primary source of biological energy for animals. Derived from various plants such as potato, corn, wheat, and rice, it undergoes hydrolysis through specific enzymes during animal metabolism. This article delves into the intricate compositions and structures of natural starch, shedding light on its significance in the realm of biochemistry.
Compositions of Natural Starch: Amylose and Amylopectin
Amylose and Its Unique Structure:
Starch comprises two key components – amylose, a linear polymer, and amylopectin, a highly branched polymer. While amylose is primarily a linear macromolecule with α[1→4] bonds linking α-D-glucosyl residues, it exhibits a slight degree of branching. The helical structure of amylose, characterized by a left-handed single helix, enables it to form complexes and bind with iodine ions.
Amylopectin: The Highly Branched Counterpart:
Amylopectin, on the other hand, is a highly branched starch component with α[1→4] and α[1→6] bonds. Its branching points are periodically clustered, influencing the inner assembly structure of starch granules. The classification of amylopectin chains into C, A, and B chains adds another layer of complexity, impacting the overall properties of natural starch.
Structures of Natural Starch: A Symphony of Helices and Crystalline Phases
Helix Structures and Crystalline Phases:
Natural starch exhibits helix structures, either left-handed or right-handed. The preferred left-handed form is packed into A-type and B-type crystalline structures. A-type crystals dominate in cereal starch, while B-type structures are prevalent in tuber and amylose-rich starches. Additionally, Vh-type crystalline structures are found in starch complexes formed with various compounds.
Granule Architecture: The Intricate Arrangement:
Polysaccharide chains in natural starch are organized in granules ranging from 1 to 100 μm. The acid hydrolysis reveals a layered concentric shell structure, known as the “growth ring,” consisting of alternating amorphous and semi-crystalline layers.
Transformative Phases: Gelatinization, Glass Transition, and Retrogradation
Gelatinization: The Disruption and Swelling Process:
Gelatinization, a crucial phase, involves the disruption of the crystalline structure and irreversible swelling of starch granules. Affected by temperature and solvent choice, this process is reversible before the gelatinization temperature but becomes irreversible afterward.
Glass Transition and Retrogradation: Influences on Mechanical Properties:
The glass transition temperature (Tg) of dry starch is influenced by water content, impacting mechanical properties. Retrogradation, occurring upon cooling of a gelatinized starch/water mixture, involves the crystallization of starch, demonstrating distinct behaviors between amylose and amylopectin.
In conclusion, the realm of natural starch is a fascinating interplay of complex structures and transformative phases. Understanding its intricacies not only contributes to the field of biochemistry but also unveils the potential applications of starch in various industries.