Starch

Starch, also known as amylum, is a polymeric carbohydrate. It consists of many glucose units joined by glycosidic bonds. Plants produce this polysaccharide to store energy for later use. It is the most common carbohydrate in human diets worldwide. We find it in staple foods like wheat, potatoes, maize, rice, and cassava.

Pure starch is a white, tasteless, and odorless powder. It does not dissolve in alcohol or cold water.

Starch is made of two distinct types of molecules. The first is amylose, which forms linear and helical chains. The second is amylopectin, which is a highly branched molecule.

Most plants contain a specific ratio of these two types. Depending on the plant, starch may contain 20% to 25% amylose by weight. The remaining 75% to 80% is usually amylopectin. Some plants produce waxy starches that contain only amylopectin. Examples include waxy maize, glutinous rice, and waxy potato starch.

Plants create starch through a complex biological process. First, they use photosynthesis to make glucose from water and carbon dioxide. This glucose is then converted into starch within specialized structures called plastids. These plastids include chloroplasts in green tissues and amyloplasts in storage tissues.

The process begins when glucose 1-phosphate is converted into ADP-glucose. This step requires energy from ATP and uses an enzyme called glucose-1-phosphate adenylyltransferase. Starch synthases then add the ADP-glucose to a growing chain via α-1,4-glycosidic bonds. To create the branches in amylopectin, starch branching enzymes introduce α-1,6-glycosidic bonds.

Starch is stored in semicrystalline granules to save space. These granules are osmotically inactive, meaning they do not cause cells to swell with water. This allows plants to pack energy very compactly. Different plants produce granules of different sizes. Rice starch granules are small, measuring about 2 μm. Potato starch granules are much larger, reaching up to 100 μm. Wheat starch is unique because it has a bimodal size distribution. This means it contains both small and large granules ranging from 2 to 55 μm.

Human history is deeply connected to the use of starch. Researchers found starch grains from bullrushes on European grinding stones dating back 30,000 years. Even older sorghum starch grains were found in Mozambique from 100,000 years ago.

In Ancient Egypt, wheat starch paste was likely used to glue papyrus. Around 77–79 CE, Pliny the Elder described starch extraction in his work, *The Natural History*. Romans used starch in cosmetics, hair powder, and thick sauces. In China, rice starch was used to treat paper surfaces starting in 700 CE. By the 1500s, laundry starch became essential for making ruffed collars in England.

Modern industry processes massive amounts of starch every year. In 2011, the European Union produced about 11 million tonnes of starch. About 60% of this was used for food, mostly as glucose syrups.

In 2017, the United States produced approximately 27.5 million tons of starch. A large portion of American starch is converted into high fructose syrup and glucose syrups.

Factories use wet grinding, washing, and drying to refine starch from over 50 plant types. Some starch is also processed into ethanol to create biofuel.

Starch serves many practical roles in daily life and science. When mixed with warm water, it creates a paste used for thickening or gluing. This is seen in wheatpaste or as an adhesive in papermaking.

It is also used to stiffen textiles before ironing. Interestingly, starch is similar to the energy reserves found in animals. Animals use a molecule called glycogen to store energy. Glycogen is simply a more highly branched version of amylopectin. This shows how different life forms use similar chemical structures to manage energy.