Starch is a polymeric carbohydrate produced by nearly all green plants for energy storage. It is packed into specialized structures within plant cells, efficiently storing excess glucose generated during photosynthesis. As the most common carbohydrate in the human diet, starch represents a major source of dietary energy worldwide. Its foundational structure is a long chain constructed entirely from individual glucose units.
The Glucose Monomer and Glycosidic Bonds
The building block of starch is the glucose molecule, specifically the alpha-D-glucose isomer, which is a simple sugar classified as a monosaccharide. These glucose units link together covalently to form the long polymer chains of starch, creating a glycosidic linkage in a process that releases water. The specific geometry of this linkage dictates the overall shape and properties of the resulting starch molecule.
In starch, two distinct types of alpha-glycosidic bonds connect the glucose monomers. The main chain is formed by alpha-1,4-glycosidic bonds, which connect the carbon-1 of one unit to the carbon-4 of the next. This alpha-1,4 linkage causes the linear chains to naturally coil into a helical shape. Branching is achieved through the alpha-1,6-glycosidic bond, which connects the carbon-1 of a glucose unit to the carbon-6 of a unit already in the main chain, introducing a side chain that sets up the next level of structural complexity.
The Two Major Polymer Forms
The structure of starch is defined by the presence of two distinct polymers that differ based on their branching patterns: amylose and amylopectin.
Amylose
Amylose consists primarily of long, unbranched chains of glucose units connected solely by alpha-1,4 glycosidic bonds. These linear chains coil up tightly into a dense, left-handed helix with six glucose units per turn. This compact conformation affects its interaction with water and enzymes.
Amylopectin
Amylopectin is a highly branched molecule that constitutes the larger portion of most native starches. It utilizes alpha-1,4 linkages for the main chain segments, but also incorporates alpha-1,6 glycosidic bonds approximately every 25 to 30 glucose units to create its extensive branching. This dense, tree-like structure gives amylopectin a significantly higher molecular weight than amylose. The branching disrupts the helical coiling seen in amylose, resulting in a more open and less organized structure.
The relative proportion of these two polymers varies considerably depending on the plant source. Most common starches contain a majority of amylopectin, typically ranging from 70% to 80% by weight. This ratio is a primary determinant of a starch’s functional properties in both food processing and human digestion.
The Semi-Crystalline Starch Granule
Within the plant, amylose and amylopectin polymers are tightly organized into dense, microscopic structures called starch granules. These granules serve as the storage unit and vary in size and shape depending on the botanical origin. The granule’s internal arrangement is described as semi-crystalline, possessing regions of both high order and disorder.
The organized structure results from the specific arrangement of amylopectin. Its short side chains align parallel and pack tightly into double helices, forming the ordered, crystalline regions. These crystalline regions alternate with less ordered, amorphous layers that contain the branching points and most of the dispersed amylose. This radial pattern creates a dense, stable structure that is not soluble in cold water. The tight packing and hydrogen bonding within the semi-crystalline granule protect the glucose polymers from immediate degradation by plant enzymes, allowing for long-term storage.
How Structure Influences Digestion and Cooking Properties
The structure of starch directly affects how it behaves during cooking and how quickly it is digested by the human body. The highly branched structure of amylopectin provides numerous non-reducing ends for digestive enzymes, like amylase, to attack simultaneously. Consequently, amylopectin is typically broken down rapidly into glucose, contributing to the faster-digesting fraction of starch. Conversely, the linear amylose chains coil into tight helices that are more resistant to enzymatic access, meaning they are digested more slowly.
This difference in digestibility is amplified by the integrity of the granule during processing. When raw starch is heated in the presence of water, a process called gelatinization occurs. The heat disrupts the crystalline packing of the granule, allowing water to enter and the polymers to swell and disentangle. This makes the glucose units readily accessible to enzymes, resulting in a far more digestible structure than the native granule.
A second process, retrogradation, occurs when cooked starch is allowed to cool. Upon cooling, the amylose and amylopectin chains begin to re-associate and re-crystallize, forming a more ordered structure that traps water and resists enzyme hydrolysis. This re-organization slows the rate of digestion and increases the proportion of resistant starch, a form that acts more like dietary fiber.