What is Starch?

Starch is a polysaccharide carbohydrate that belongs to the category of polysaccharide carbohydrates.

Carbohydrates

Carbohydrates are organic compounds that are made up of carbon, hydrogen, and oxygen in a $1:2:1$ratio. They belong to one of the most important groups of biomolecules. They are divided into two categories as nutrients: simple carbohydrates and complex carbohydrates. Simple carbohydrates, also known as sugar, are made up of one or two saccharide residues. They're easy to digest and provide a quick source of energy. Complex carbohydrates (such as cellulose, starch, chitin, and glycogen) take longer to digest and metabolise than simple carbohydrates.

Monosaccharides

Monosaccharides are simple sugars with one oxygen atom and two hydrogen atoms for every carbon atom. $\left({\text{CH}}_{\text{2}}\text{O}\right)\text{n}$is the general formula for them. Disaccharides (like sucrose and lactose) and polysaccharides (like cellulose and starch) are made up of monosaccharides . They are soluble in water.

Disaccharides

When two monosaccharides are united by glycosidic linkage, a disaccharide is created. Disaccharides, like monosaccharides, are simple sugars that dissolve in water.

Polysaccharides

Polysaccharides, also known as poly carbohydrates, are the most common type of carbohydrate in diet. They are polymeric carbohydrates with lengthy chains made up of monosaccharide units linked together by glyosidic connections. This carbohydrate can react with water with the help of amylase enzymes, resulting in component sugars. Glucose is found in three main polysaccharides: starch, glycogen, and cellulose.

Polysaccharide carbohydrate ${\left({\text{C}}_{\text{6}}{\text{H}}_{\text{10}}{\text{O}}_{\text{5}}\right)}_{\text{n}}$ contained primarily in seeds, bulbs, and tubers, consisting of a large number of glucose molecules linked together by glycosidic bonds. They belong to one of the most important groups of biomolecules. In our diet, starchy foods are a strong source of energy and the primary source of a variety of nutrients.

As far back as 100,000 years ago, starch was identified and used. It is thought to be used in the preparation of foods such as bread and porridges. This theory is based on stone tools discovered in ancient caves. The tools were most likely used to scrape and grind wild sorghum starch grains. The addition of starch in the prehistoric diet of early humans in the African savannahs and woodlands enhanced diet consistency, according to scientists. The transformation of grains into a staple marked a change in prehistoric diet and is thought to have been a pivotal point in human evolution.  The word starch may be derived from the Old English stearic (“sharp, hard, rough”), which may have a Germanic root, i.e. starch, which means “strong.”

Chemical Formula of Starch

Starch has the formula ${\left({\text{C}}_{\text{6}}{\text{H}}_{\text{10}}{\text{O}}_{\text{5}}\right)}_{\text{n}}$, where n is the number of glucose molecules in the chain. Glucose is the sugar that is connected together in a long chain in the starch structure. Now, if a starch molecule contains ten glucose molecules, the value of n in the formula would be:

$${\left({\text{C}}_{\text{6}}{\text{H}}_{\text{10}}{\text{O}}_{\text{5}}\right)}_{\text{n}}$$

If $n=10$, then ${\left({\text{C}}_{\text{6}}{\text{H}}_{\text{10}}{\text{O}}_{\text{5}}\right)}_{\text{n}}$=${\text{C}}_{\text{60}}{\text{H}}_{\text{100}}{\text{O}}_{\text{50}}$

An example of a starch molecule's chemical formula is indicated above. The iodine test can detect the presence of starch in any compound. The result of the chemical reaction between starch and iodine has a blue-violet tint, indicating the presence of starch.

Two polymers of starch

Amylose and amylopectin are two polymers found in starch. It contains $10-30\%$amylase and $70-90\%$amylopectin when it comes to natural starches.

Amylose makes up one-fourth of the starch molecule and is bound to it by $-\left(1,4\right)$glycosidic bonds. Because of its long side branches and high molecular weight, the amylose molecule takes on a helical form, with six glucose monomers per turn. The presence of a blue-violet colour on chemical reactions of starch with iodine is a characteristic of amylose due to the formation of the amylose-iodine complex.

Amylopectin is a branched-chain of glucose molecules linked linearly with $-\left(1,4\right)$ glycosidic bonds and - $\left(1,6\right)$bonds at intervals of $24$to $30$glucose subunits, while amylose is a linear chain of glucose molecules bound by $-\left(1,4\right)$ glycosidic bonds. Since starch is a polysaccharide that is primarily made up of D-glucose, it is classified as a - glucan.

Amylopectin is more water soluble and digestible than amylose. The many endpoints that can form hydrogen bonds with water contribute to its solubility. Amylopectin makes up $\text{75-80}$percent of starch, while amylose makes up $\text{20-25}$percent.

Short $-\left(1,4\right)$ glycosidic bonds are joined by -$\left(1,6\right)$glycosidic bonds to form branches in amylopectin. Amylopectin is a key component of most popular starches. Amylopectin's helical structure is strongly branched, with three different forms of branch chains. As a result, it has a higher molecular weight, and when reacted with iodine, it produces a less dark reddish-brown hue. Benedict's starch test is the most common name for this test. As a result, a starch made entirely of amylopectin would react with iodine in a reddish-brown Short $-\left(1,4\right)$ glycosidic bonds are joined by -$\left(1,6\right)$glycosidic bonds to form branches in amylopectin. Amylopectin is a key component of most popular starches. Amylopectin's helical structure is strongly branched, with three different forms of branch chains. As a result, it has a higher molecular weight, and when reacted with iodine, it produces a less dark reddish-brown hue. Benedict's starch test is the most common name for this test. As a result, a starch made entirely of amylopectin would react with iodine in a reddish-brown colour rather than a deep blue-violet colour.

Characteristics

A starch is a polysaccharide composed of a large number of glucose units linked by glycosidic bonds. It's a powder that's dry, odourless, and tasteless. It has a molar mass that varies. Alcohol and cold water do not dissolve it.

 Biological Importance of Starch

Starch is present in all plant seeds and tubers in the form of amylose and amylopectin. Starch is used by plants to store excess glucose and, as a result, starch is often used as food by mitochondrial oxidative phosphorylation at night or when photosynthesis is impossible. Plants store excess starch in amyloplasts, which are leucoplasts that mainly act in storing starch granules by polymerizing glucose and transforming these reserves back into simpler sugars (e.g. maltose and glucose) when light is scarce. Chloroplasts, pigmented organelles that are mainly involved in photosynthesis, can also store starch.

Excess glucose is not stored as starch in animals; instead, it is stored as glycogen. Certain species, on the other hand, eat starch-rich foods.

Many staple foods, such as maize, rice, wheat, corn, potatoes, cassava, barley, rye, taro, and yams, contain food starch. It can also be found in cereals, noodles, pancakes, bread, and pasta, among other foods. Per gram of starch, there are approximately kilocalories. Starch can be a significant source of glucose in humans.

Slowly digestible starch is a form of carbohydrate found in starchy foods like grains, legumes, roots, and tubers that releases slowly. The quality of the ingredients as well as food processing conditions such as heat, strain, and moisture play a role in its preservation.

 Context and Applications   

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for, Masters and Bachelors in Chemistry.