Starch is a polysaccharide composed of numerous glucose units linked together. It is one of the most abundant organic compounds on Earth and serves as the primary energy storage molecule in plants. Found in staple foods like potatoes, corn, rice, and wheat, starch is a fundamental component of the human diet.
The Building Blocks: Glucose
At the heart of every starch molecule is glucose, a simple sugar (monosaccharide) with the chemical formula C6H12O6 that serves as the fundamental building block. In starch, these glucose units are in their alpha-D-glucose form. The way these units are linked together gives rise to the two different types of starch molecules: amylose and amylopectin.
Amylose and Amylopectin: The Two Main Components
Starch is a mixture of two distinct polymers: amylose and amylopectin. The ratio of these two components varies depending on the plant source, which in turn affects the starch’s properties. For example, corn starch contains about 25% amylose and 75% amylopectin, while waxy corn starch is almost 100% amylopectin. Amylose is a linear, unbranched chain of glucose units, while amylopectin is a highly branched polymer of glucose.
The Helical Structure of Amylose
Amylose consists of alpha-D-glucose units joined by α-1,4 glycosidic bonds. Due to the specific geometry of these bonds, the amylose chain does not remain straight, but instead coils into a helical shape. This structure is stabilized by hydrogen bonds between the glucose units. The interior of the helix is relatively hydrophobic, allowing it to form complexes with molecules like iodine, which is the basis for the classic starch-iodine test.
The Branched Structure of Amylopectin
Like amylose, amylopectin is composed of alpha-D-glucose units linked by α-1,4 glycosidic bonds, but its defining feature is a branched structure. These branches are created by α-1,6 glycosidic bonds, which occur every 24 to 30 glucose units along the main chain. This branching results in a large, tree-like molecule that is much larger than amylose, often containing up to two million glucose units. The branched nature prevents the formation of a tight helical structure.
Physical and Chemical Properties of Starch
These properties are important in both its biological role and its industrial applications.
Solubility
The structure of the starch components affects their solubility in water. Amylose, with its more compact helical structure and ability to align, is less soluble in cold water than amylopectin. The larger, highly branched structure of amylopectin prevents it from packing tightly, allowing more water molecules to penetrate and interact with it, making it more soluble.
Gelatinization
When starch is heated in water, the granules absorb water and swell. This process, known as gelatinization, is a property of starch used in cooking and food processing. As the temperature rises, the crystalline structure breaks down, and the amylose and amylopectin molecules begin to leach out and form a viscous gel. Starches with higher amylopectin content tend to form clearer and more stable gels.
Retrogradation
Upon cooling, the starch molecules, particularly the linear amylose chains, begin to re-associate and form an ordered structure again. This process is called retrogradation and is responsible for the staling of bread and the firming of cooked puddings over time.
The Role of Starch in Nature and Industry
In nature, starch is the primary way plants store energy. It is produced through photosynthesis and stored in granules within chloroplasts (in leaves) and amyloplasts (in roots, seeds, and tubers). When the plant needs energy, enzymes break down the starch back into glucose, which can then be used for metabolic processes. This makes starch a vital link in the plant’s life cycle.
For industrial purposes, starch is a versatile raw material with a vast range of applications beyond its use as a food source. Its properties make it useful in many sectors. Some of the most common industrial uses include:
- Paper and textiles: Starch is used as a surface sizing agent to strengthen paper and as a warp sizing agent in textiles to improve weaving.
- Adhesives: Starch-based glues are widely used in corrugated board manufacturing and for wallpaper paste.
- Pharmaceuticals: It serves as a binder and disintegrant in tablets, helping the tablet hold together and then break apart in the digestive system.
- Biofuels: Starch from crops like corn is a major feedstock for the production of bioethanol through fermentation.