Starch, a common carbohydrate found abundantly in plants, serves as an energy reserve. Starch is a mixture of two polysaccharides: amylose and amylopectin. While both are composed of glucose units, they differ significantly in their molecular structures. These structural differences explain their varied properties and behaviors, especially for amylose.
The Linear Nature of Amylose
Amylose has a linear, unbranched structure. It consists of thousands of alpha-D-glucose units linked together in a long, straight chain. These glucose units are connected by alpha-1,4 glycosidic bonds, forming a continuous polymer. This bonding arrangement allows the amylose chain to coil into a helical structure, contributing to its physical properties.
In contrast, amylopectin, the other major component of starch, is highly branched. It possesses alpha-1,4 glycosidic bonds along its main chains and alpha-1,6 glycosidic bonds at its branching points. Amylose typically constitutes 20% to 30% of total starch, though this varies by plant source.
How Amylose’s Structure Affects Its Behavior
Amylose’s linear and helical structure directly impacts its behavior, especially its interaction with water and digestibility. Amylose is less soluble in water compared to amylopectin. This reduced solubility results from its tightly packed helical conformation, which limits the access of water molecules to its internal structure.
Amylose forms strong gels upon cooling. When starch is heated in water, amylose chains disperse, but as the mixture cools, these linear chains tend to realign and associate with each other. This process, known as retrogradation, involves the formation of new hydrogen bonds between the re-associating amylose molecules, leading to the development of a semi-solid, crystalline gel. Amylose crystallization occurs quickly and is responsible for the initial firmness of starch gels.
Amylose’s tightly packed structure also influences its digestibility. Its ordered arrangement makes amylose more resistant to enzymatic breakdown than amylopectin. This property classifies a significant portion of amylose as resistant starch, which behaves similarly to dietary fiber and can offer various health benefits by reaching the large intestine largely undigested.
Amylose’s Role in Common Foods
The proportion of amylose in a starch significantly impacts the texture and cooking properties of many common foods. Different plant sources contain varying ratios of amylose to amylopectin. For instance, waxy rice, often used in sticky dishes, has a very low amylose content (0-2%), while long-grain rice, which cooks up fluffy, can have a high amylose content (over 25%).
High-amylose starches, such as certain varieties of corn (amylomaize) that can contain 50% to 94% amylose, are often used in industrial applications requiring strong gelling. In foods, a higher amylose content typically results in a firmer and less sticky texture after cooking. This is why understanding amylose is important for predicting the culinary outcome of starchy ingredients. The retrogradation of amylose also contributes to the staling of bread and the change in texture of cooked rice or potatoes upon cooling.