Starch

Starch is a polysaccharide made of many glucose molecules linked by glycosidic bonds. Most green plants produce it to store energy. It is the most widely consumed carbohydrate in human diets around the world, appearing in large amounts in staple foods like wheat, potatoes, corn, rice, and cassava.

As a substance, starch is a white, tasteless, odorless powder that does not dissolve in cold water or alcohol. It is made up of two kinds of molecules: linear, helical amylose and branched amylopectin. In most plants, starch is roughly 20–25% amylose and 75–80% amylopectin by weight. The animal energy reserve glycogen is a more highly branched version of amylopectin.

Industrially, starch is often turned into sugars, such as through malting. These sugars can then be fermented to produce ethanol for beer, whisky, and biofuel. Sugars from processed starch also appear in many processed foods.

Mixing most starches with warm water creates a paste, like wheatpaste, which can thicken, stiffen, or glue things. The main non-food industrial use of starch is as an adhesive in papermaking. A similar paste, known as laundry starch, can be applied to textiles before ironing to stiffen them.

The word "starch" comes from a Germanic root meaning "strong, stiff, strengthen, stiffen." Modern German *Stärke* (meaning both strength and starch) refers to its historical use in textiles—sizing yarn for weaving and starching linen. The Greek term *amylon* (ἄμυλον), meaning "not milled," gives the root "amyl," used as a prefix for starch-related carbon compounds like amyl alcohol, amylose, and amylopectin.

Starch grains from cattail rhizomes used as flour have been found on grinding stones in Europe dating back 30,000 years. Sorghum starch grains were found on grind stones in caves in Ngalue, Mozambique, dating up to 100,000 years ago. In Ancient Egypt, pure extracted wheat starch paste was likely used to glue papyrus. The extraction of starch was first described by Pliny the Elder around 77–79 CE. Romans used it in cosmetic creams, hair powder, and to thicken sauces. Persians and Indians used it to make dishes similar to wheat halva. In China, rice starch has been used as a surface treatment for paper since 700 CE. By the mid-eighth century, paper sized with wheat starch was being produced in the Arabic world. Laundry starch was first described in England in the early 15th century and was essential for making the ruffed collars of the 16th century.

Plants produce glucose from carbon dioxide and water through photosynthesis. Glucose provides chemical energy for metabolism and serves as a precursor for organic building blocks like nucleic acids, lipids, proteins, and structural polysaccharides such as cellulose. Most green plants store extra glucose as starch, packed into semicrystalline granules called starch granules or amyloplasts. Toward the end of the growing season, starch accumulates in tree twigs near buds. Fruits, seeds, rhizomes, and tubers store starch to prepare for the next growing season. Young plants live on this stored energy until they find suitable soil. Starch is also consumed at night when photosynthesis stops. Green algae and land plants store starch in plastids, while red algae, glaucophytes, cryptomonads, dinoflagellates, and parasitic apicomplexa store a similar polysaccharide called floridean starch in their cytosol or periplast. When hydrated, glucose takes up much space and is osmotically active, but starch, being insoluble and osmotically inactive, can be stored more compactly. The semicrystalline granules usually consist of concentric layers of amylose and amylopectin, which can be made bioavailable when the plant needs them. Amylose is made of long chains of glucose linked by α-1,4-glycosidic bonds. Amylopectin is highly branched, with a main chain of α-1,4 links and α-1,6 bonds at branch points—the same type of linkage found in glycogen. In contrast, structural polysaccharides like chitin, cellulose, and peptidoglycan use β-glycosidic bonds, which are more resistant to hydrolysis.

Within plants, starch is stored in semi-crystalline granules, and each plant species has a distinctive granule size. Rice starch is relatively small (about 2 μm), potato starch has larger granules (up to 100 μm), and wheat and tapioca fall in between. Unlike other sources, wheat starch has a bimodal size distribution, with both smaller and larger granules ranging from 2 to 55 μm. Some cultivated plant varieties have pure amylopectin starch without amylose, known as waxy starches. The most common is waxy maize, along with glutinous rice and waxy potato starch. Waxy starches undergo less retrogradation, producing a more stable paste. A maize cultivar with a high proportion of amylose, called amylomaize, is grown for its gel strength and for use as a resistant starch in food products.

Plants synthesize starch in two types of tissues: storage tissues (such as cereal endosperm, storage roots, and stems like cassava and potato) and green tissues (such as leaves), where many plant species synthesize it.