Author: Izma Khan
Introduction
Polysaccharide
Polysaccharide, also known as glycan, is a long chain of monosaccharide units bound together by glycosidic linkages. Polysaccharides can be a straight chain of monosaccharides, known as linear polysaccharides, or they can be branched, known as a branched polysaccharide.
According to Merriam-Webster polysaccharide can be defined as:
a carbohydrate that can be decomposed by hydrolysis into two or more molecules of monosaccharides
Structure of Polysaccharide
The overall chemical formula for polysaccharides is (C6H10O5)_n, with “n” indicating the quantity of monosaccharide units present in the structure. Polysaccharides are carbohydrate compounds that consist of extensive chains of monosaccharides connected by β → 1–4 glycosidic bonds. When these bonds undergo hydrolysis, they yield their individual monosaccharides and oligosaccharides.
Functions of polysaccharide
Various polysaccharides serve distinct roles in humans, plants, and animals. Some function as energy sources, while others are responsible for energy storage in the body for future use. Additionally, some polysaccharides assist in the movement of food through the digestive system, and others enable certain animals to form tough outer shells or exoskeletons.
Characteristics of polysaccharides
Polysaccharides have some key features that include:
- They lack sweetness.
- A lot of them don’t dissolve in water.
- They tend to be hydrophobic.
- They don’t crystallize when dried out.
- They can be extracted into a white powder.
- They are carbohydrates with a high molecular weight.
- Their composition includes hydrogen, carbon, and oxygen, with a hydrogen-to-oxygen ratio of 2:1.
Types Of Polysaccharides
Polysaccharides are categorized into two types:
1. Homopolysaccharides: A polysaccharide that contains the same type of monosaccharides is known as a homopolysaccharide.
2. Heteropolysaccharides: A polysaccharide that contains different types of monosaccharides is known as a heteropolysaccharide.
Common Polysaccharides
There are many polysaccharides. Some common polysaccharides are:
A. Cellulose
Cellulose is a complex carbohydrate made up of oxygen, carbon, and hydrogen. It’s chiral, has no taste, and is odorless. The first person to identify cellulose was a French chemist named Anselme Payen in 1838. This organic compound is both water-soluble and biodegradable. The cellulose polymer consists of cellobiose units, which are glucose dimers arranged in a way that each monomer is rotated 180° relative to the next. As a key component of plant cell walls, cellulose is the most abundant organic polymer on the planet.
It features long chains of anhydro-D-glucopyranose units (AGU), with each cellulose molecule containing three hydroxyl groups per AGU, except at the ends. While cellulose is insoluble in water and most common solvents due to strong hydrogen bonds between the chains, it still finds use in a variety of applications.
Uses of Cellulose (C6H10O5)n
- It is used in the diet as a fibre supplement
- It is used to produce paperboard and paper products
- It helps as an additive in various food items
- It is used in the production of rayon
- It is used as a preservative in cheese as it plays the role of an anti-clumping agent
- It is used in making explosives
- It is used in the manufacturing of nitrocellulose
Sources
It can be obtained from a vast number of sources, e.g. cell walls of wood and plants, Jute, kenaf, ramie, sisal, and bamboo, Bagasse, Barley, oat, rice, rye, and wheat straw.
B. Starch
Starch is a tasteless, fluffy white powder that is insoluble in cold water, alcohol, and other solvents.
Structure of Starch
Starch is a type of polysaccharide formed by 1,4 linkages between glucose molecules. Its basic chemical formula is (C6H10O5)n, where ‘n’ indicates the number of glucose units in the structure. Also known as amylum, starch is a homopolymer made up of D-glucose units connected by α-(1→4) glycosidic bonds. These bonds are created by starch synthases. Starch can also be referred to as glucosan or glucan. Enzymes like α and β-amylases specifically target and help break down these α-1,4 linkages. When starch is hydrolyzed, it breaks down into dextrins, and eventually into maltose and glucose units, thanks to the action of amylase enzymes. Starch consists of two main components: amylose, which is water-soluble and makes up about 20-30%, and amylopectin, which is water-insoluble and accounts for 70-80%. Both components are found within starch granules.
Amylose: Amylose is a straight-chain molecule made up of 200 to 1,000 α-D-glucopyranosyl units connected by α (1-4) glycosidic bonds. It has an amorphous structure. When heated, amylose can leach out of granules, allowing water to penetrate and cause swelling. When starch interacts with iodine, amylose produces a blue color. It has lower solubility in water and doesn’t form a gel when hot water is added. A common source of amylose is hybrid corn, which contains between 50% and 70% amylose. Other sources include rice and potato starches (up to 30%), as well as quinoa, beans, bananas, and lentils.
Amylopectin: Amylopectin is a complex, highly branched polymer made up of 50,000 to 500,000 α-D-glucopyranosyl units. These units are connected by α (1-4) linkages throughout, with α (1-6) glycosidic bonds at the branching points. Its molecular weight ranges from 10^7 to 10^8 daltons. Each branch typically contains 20 to 30 glucose units, and a single molecule can have hundreds of these branches. Amylopectin has a crystalline structure, and its branching points help in forming different allomorphs. The bonds in amylopectin are not as strong as those in linear amylose molecules, making it more soluble in water. When hot water is added, it can form a gel and affects viscosity changes due to the presence of water and heat.
Properties of Starch
The characteristics of starch are influenced by the molecular structure and composition of amylose and amylopectin, including their percentage and how they are organized within the starch granules. Here are some key properties:
- It effectively binds with iodine.
- It has a strong swelling capacity.
- It reduces the body’s ability to absorb water.
- It exhibits high viscosity.
- It functions as an emulsifier.
- It serves as an encapsulant.
- It has gelling properties or acts as a clouding agent.
- It undergoes gelatinization.
Uses of Starch
- Confectioneries and cough syrups help prevent crystallization.
- Use in beverages as sweeteners.
- Dairy products function as bulking agents and enhance texture.
- Bakery items provide moisture.
- Soups and sauces work as thickening and binding agents.
- Paper gains strength.
- The corrugated board acts as an adhesive.
- Used in the production of ethanol, polyurethane, and bioplastics.
- Serve as binding agents in glues and adhesives.
- Used in cosmetics as emulsifiers.
- They lower the freezing point in ice creams.
- Contribute to the nutritional value of baby food and clinical nutrition.
There are two categories of starch:
- Native Starch: Native starches are long-chain carbohydrates that represent the pure form of starch. They are derived from plant sources like corn, maize, rice, wheat, potatoes, cassava, and tapioca. When extracted, they appear as white, tasteless, and odorless powders or liquids. These starches do not dissolve in cold water and swell to varying extents based on their type and temperature. This category of starch remains unaltered by physical, chemical, or enzymatic treatments. Although they have been utilized in the food industry for many years, their industrial use is somewhat limited due to their natural characteristics, such as sensitivity to high temperatures and acids and poor thermal stability. Native starches possess excellent properties, including thickening, texturizing, gelling, moisture retention, anti-staling, stabilizing, film-forming, dusting, and dough-binding.
- Applications of Native Starch:
- Bakery mixes
- Frozen cakes
- Sheet snacks
- Brewing adjuncts
- Dry mix soups and sauces
- Pet foods
- Processed foods
2. Modified Starch: A starch that has been physically, chemically, or enzymatically altered to improve its functional properties for industrial and food applications. Unlike native starch, which has limited stability under conditions such as high heat, low pH, or freeze-thaw cycles, modified starch is engineered to withstand such processing environments. Chemical modifications may involve cross-linking, substitution, oxidation, or acid hydrolysis, each designed to enhance specific characteristics like thickening ability, clarity, or resistance to retrogradation. Physical modifications, such as pre-gelatinization or heat-moisture treatment, change the starch granule structure without using chemicals. Enzymatic methods use specific enzymes to tailor molecular size or branching. Modified starches are widely used in processed foods as thickeners, stabilizers, and emulsifiers and also find applications in non-food sectors like pharmaceuticals, paper manufacturing, and adhesives due to their tailored performance and functional versatility.
- Applications of Modified Starch:
- Thickener
- Stabilizer
- Binder
- Filler
- Coating
- Texturizer
- Sizing
- Emulsifier
- Disintegrant
- Adhesive
C. Chitosan
Chitosan is a natural polymer that comes from chitin, which is a homopolysaccharide made up of repeating units of N-acetyl-d-glucosamine linked by β-(1–4) bonds. Chitin, the source of chitosan, is the most common biopolymer found in nature, following cellulose. It’s found in large quantities in various organisms, including insects, crustaceans, and fungi. Chitin consists of N-acetyl-D-glucosamine units, and when it undergoes deacetylation, the acetyl groups are removed, leaving behind a polymer primarily made of β-1,4-D-glucosamine units, known as chitosan. The degree of acetylation (DA) refers to the mole fraction of the N-acetylated units, while the degree of deacetylation (DD) indicates the percentage of β-1,4-D-glucosamine units in the polysaccharide. After cellulose, chitosan ranks as the second most abundant biopolymer.
Sources of Chitosan
When it comes to producing chitosan, shrimp stands out as one of the most talked-about and promising sources. Other potential sources include beetles, various insects, crustaceans, mollusks, and fungi.
Conclusion
Polysaccharide, also known as glycan, is a long chain of monosaccharide units bound together by glycosidic linkages. There are many polysaccharides, but the important ones were discussed above. All polysaccharide holds special properties, and these polysaccharides are used in many things. It helps as an additive in various food items, a binding material, a frozen mixes, in juices, a gelling agent, and much more.
Read More: Proteins: The Ultimate Natural Biopolymer
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