Starch has a granular structure that describes the arrangement of starch molecules in a starch granule. These granules are round and made up of tightly packed starch molecules. When starch granules absorb water, they swell and dissolve through a process called starch gelatinization. This creates a thick, sticky solution of swollen starch molecules that can be used to thicken or emulsify in a range of food and non-food applications.
Granular structure
Starch is found in particles called granules that can vary in size and shape depending on the source of the starch. Granules can be as small as 1 µm or as large as 100 µm and have different shapes. Starch granules are made up of two types of glucose polymers, amylose and amylopectin, with different structures. Amylose is made up of units of D-glucose linked through α–D–(1–4) linkages, while amylopectin is a branching polymer composed of α–D–(1–4)-linked glucose segments containing glucose units in α–D (1–6) branches.
Amylose is a linear polymer made up of glucose units, but some studies suggest that it might have some branching. There is a type of starch called “intermediate material or amylose-like” that is found in 4-9% of the population and is considered a part of normal and high amylose starches. Amylose can also form a complex with fatty acids, glycerides, alcohols, and iodine, thanks to its hydrophobic helix structure.
Amylopectin is a larger molecule than amylose and its chains are classified into small chains and large chains. The arrangement of amylopectin molecules in the starch granule contributes to its crystalline nature, while amylose is distributed randomly in the granule. Under polarized light, the starch granule displays a Maltese cross pattern, which shows its birefringent structure. The radial packing of the amylose and amylopectin is confirmed by x-ray diffraction.
Starch gelatinization
Starch gelatinization happens when starch granules melt in water as the temperature increases. As the granules swell, water moves into the starch components, amylose and amylopectin.
When starch is heated to 60-70°C, its granules lose their structure and become less crystalline, resulting in an increase in viscosity and solubility. This process is called gelatinization. After heating, when the starch is cooled, the chains tend to reassociate in an ordered structure, leading to another increase in viscosity, called setback. The previously gelatinized starch does not show an increase in viscosity when exposed to changes in temperature.
Native starches have different swelling properties due to factors like granule size, crystallinity, and the way the starch chains interact. Amylose and amylopectin have distinct properties that affect how starch behaves when cooked.
To study starch structure and suggest new applications for starch, it’s important to understand how starch behaves when heated and its rheological properties.