Polysaccharides in starch and their significance

By
Dr. Asha Jyoti
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Polysaccharides in starch

Introduction

Starch is an essential and ubiquitous carbohydrate present in most of the food. Its complex nature, composed of intricate polysaccharides, serves as a cornerstone in our understanding of energy storage and plant biology. The following article is about polysaccharides in starch and their significance

Starch and its role as a carbohydrate in human life

Starch is a form of polysaccharide that is largely present in the plant kingdom. The most common sources are cereals, grains, potatoes, carrots, millet, and legumes. Carbohydrates are also known as saccharides as sugars are the main component. 

Carbohydrates are one of the compounds largely present in nature. They perform the functions of storage and energy production and act as one of the membrane constituents.

Polysaccharides in starch and their importance

When hydrolyzed, polysaccharides possess twelve molecules of monosaccharide units. 

The molecular formula is (C6 H10 O5 )n. 

Polysaccharides are classified into two types

1. Homopolysaccharides

  • Polymers with similar monosaccharide units. They are glucose units. 
  • Homopolysaccharides are named after monosaccharide units. For example: Glucan consists of glucose units. 

2. Heteropolysaccharides

Polymers of different monosaccharide units or their derivatives like mucopolysaccharides. 

Starch

Basic composition of starch

The basic composition of starch is glucose. It is composed of two units, namely amylase and amylopectin. 

Sugars, animal starch, plant starch, and cellulose are the commonly known carbohydrates. Starch and glycogen are called macromolecules. 

Importance and availability of starch as plant-based food

The carbohydrates are formed by the green cells of algae, plants, and many protists. The latter utilizes water and carbon dioxide in the presence of solar light to form sugar and molecular oxygen. 

Animals, fungi, and many bacteria including protists get carbohydrates from plants. Hence, all organisms depend on nutrition in photosynthesis which is considered the source of food production. 

Amylose

Structure

The structure of amylose is a long helical-coiled unbranched chain of glucose molecules connected by alpha,1-4  linkage. The molecular weight of amylase is between 10,000 and 1,00,000. Amylose reacts with iodine to produce a deep blue color. 

  • Amylose has a low molecular weight (~10^6 ) polymers. 
  • It possesses maltose as a repeating unit. 
  • It is formed due to (1—> 4)-alpha-D linkage. 
  • The shape of amylose varies from helical to coiled tube shape. 
  • It is semi-crystalline in nature. 
  • Molecular interaction results in a deep blue coloration when reacted with Iodine. 

Amylopectin 

Amylopectin is insoluble in water. The molecular weight of amylopectin varies from 50,000 to 10,00,000. Amylopectin is composed of branched chains of glucose and joined by alpha, 1-6 linkages. 

Branching occurs through a (1 → 6) linkage between two glucose units of neighboring chains. The branching occurs at 25–30 a-D-glucose units. Amylopectin reacts with iodine and gives a violet color.

  • Amylopectin is a comparatively larger polymer than amylose. 
  • Branching of (1→6 ) linkage reoccurs every ten maltose units. 
  • Amylopectin possesses both the polymers of maltose and isomaltose. 
  • It is globular-shaped. 
  • It is non-crystalline in nature. 

Properties of starch

  • Starch comprises two polymeric substances namely amylose (20%) and amylopectin (80%).
  • Starch is a polysaccharide obtained from plants that is present naturally and is considered the main source of carbohydrates, hence adding fuel to the human diet.
  • Carbohydrate polymers, like glycogen and starch, are energy-storage materials.
  • They are produced by the higher plants and are stored in the form of granules in the roots, tubers, and rhizomes as energy reserves.
  • Most of the starch production is done from maize and alternative sources include potatoes and bananas. 
  • Levoglucosan is a high-quality starch produced by the pyrolysation of potato starch. 
  • Waxy starch is an example of amylose-free starch, produced through engineering techniques. 

Influence on starch solubility 

The starch solubility is determined by a number of factors

  • Amylose is a linear polysaccharide and its presence affects the starch solubility in water. 
  • The degree of granule particles is directly proportional to their solubility. Smaller and broken particles tend to be more soluble.
  • Factors like pH levels and temperature play an important role in determining starch solubility.
  • Higher temperatures increase the solubility whereas lower pH levels tend to decrease the starch solubility.

Digestion of Starch

Digestion of starch begins in the buccal cavity by the action of amylase, produced by the salivary glands. It breaks glucose molecules into maltose and other simple sugars. The half-digested starch passes into the stomach, is further broken down by HCL and pepsinogen. 

Pancreatic enzymes in the small intestines break down glucose molecules to form monosaccharides that are absorbed through intestinal walls. 

The remaining undigested material goes through the large intestine where bacteria start fermenting before elimination through the body. 

Interesting facts

  • Levoglucosan is a high-quality starch produced by the pyrolysation of potato starch. 
  • Amylose is a long helical-coiled unbranched chain of glucose molecules connected by alpha,1-4  linkage.
  • Amylopectin is composed of branched chains of glucose and joined by alpha, 1-6 linkages.
  • Amylopectin reacts with iodine and gives a violet color whereas amylose gives a deep blue color.

Difference between Amylose and Amylopectin

Basis Of

Amylose

Amylopectin
Definition Amylose is a long helical-coiled unbranched chain of glucose molecules connected by alpha,1-4  linkage.  Amylopectin is composed of branched chains of glucose and joined by alpha, 1-4 linkages. 
Starch Constitution 20% 80%
Glycosidic Linkages 1-4 glycosidic linkages 1-6 glycosidic linkages
Hydrolyzation Amylose is hydrolyzed to Beta-galactosidase.  It cannot be completely hydrolyzed. 
Reaction with Iodine Deep-blue coloration Violet coloration
Hot Water Test No reaction Gel formation when reacted with hot water. 

Glycemic Impact Of Starch

The glycemic impact of starch is a key factor in evaluating the health benefits of a food. This index helps us to understand how quickly food when absorbed by the body, can affect blood sugar levels. In short, it measures the post-spike level in an individual. 

Starch is composed of long chains of glucose molecules that are broken into simple sugars. 

Different starches can have distinct impacts on blood sugar levels. Recent studies depict that resistant starches are digested gradually in comparison to the other forms of starch. It resulted in lower postprandial spikes in the blood sugar levels. 

Cooking methods also reduce the glycemic index for starchy foods by slow digestion and by increasing the fiber content of the food. 

What are resistant starchs and their impact on digestion and their potential health benefits?

Resistant starch is a form of complex carbohydrate having 2.5 calories/gm and possesses many health benefits. It helps in easy digestion and aids in weight management. 

 As per its name is concerned, resistant starches do not get digested in the small intestine and they undergo fermentation in the large intestine to promote a good source of bacteria. 

Foods that contain resistant starches

  • Lentil
  • Brown rice
  • Quinoa
  • Cashews
  • Green bananas
  • Oats
  • Potatoes

Health benefits of consuming Resistant Starches

  • Helps in weight management
  • Helps in boosting gut health by promoting good bacteria in the large intestine. 
  • It helps in relieving constipation and hence reduces the risk of possible hemorrhoids. 
  • Boost the immune system 
  • Improves sensitivity towards insulin by regulating the blood sugar levels in case the body is affected by high sugars. Thus, it lowers the risk of getting Alzheimer’s disease, Type II diabetes, and heart complications. 

Nutritional value

The nutritional value of the polysaccharides in starch and their contribution

Starch has 2.5 calories per gram of carbohydrates. The amylose content in starch granules ranges from 11 to 37% w/w.

Higher plants tend to store starch in two different types: large A-type and small B-type. 

Large A-type granules contain 5-10% more amylose than the B-type starch granules. These granules’ ratio in carbohydrates increases with exposure to high temperatures. 

Starchy foods are hence highly beneficial in controlling insulin levels and a potent source of energy as they contain carbohydrates. 

Fiber content in starch helps in easy bowel movement and thus prevents constipation.

Common foods that are rich in starch polysaccharides

Foods such as potatoes, wheat, maize, quinoa, brown rice, bread, grains 

(oats, barley, bulgur) , peas, millets, sorghum, etc are examples of food products rich in polysaccharides.

Importance of starch in the food industry and its role

Polysaccharides are generally known as Glycans. Starch is a key ingredient in the food industry. 

  • Starch helps in providing a texture to foods by binding the ingredients together. 
  • Acts as a thickening agent in restaurants and other food industries for different sauces and gravies, which allows them to achieve their desired consistency in certain dishes.
  • In mayonnaise, starch helps in stabilizing emulsions. 

Q&A

1. What polysaccharides make up starch?

Starch is considered a polymer of glucose and functions as a storage polysaccharide in plants. In the case of animals, it is known as glycogen. 

2. What is the role of polysaccharides in starch?

They are produced by the higher plants and are stored in the form of granules in the roots, tubers, and rhizomes as energy reserves. 

3. What are the 4 types of polysaccharides?

Four types of polysaccharide include:

  • Glycogen – Found in animals and fungi
  • Starch – Made of Amylose and amylopectin. 
  • Cellulose – The plant’s cell wall is made of cellulose. 
  • Inulin – Mostly found in tubers like ginger, cassava, etc. 

4. What is an example of starch polysaccharide?

Potatoes

Summary 

  • Well, Polysaccharides in starch are known as glycans and are largely present in the plant kingdom.
  • The most common sources are cereals, grains, potatoes, carrots, millet, and legumes. 
  • Carbohydrates are also known as saccharides as sugars are the main component. 
  • Starch comprises two polymeric substances namely amylose (20%) and amylopectin (80%).
  • They are produced by the higher plants and are stored in the form of granules in the roots, tubers, and rhizomes as energy reserves.
  • Most of the starch production is done from maize and alternative sources include potatoes and bananas. 
  • Resistant starch is a form of complex carbohydrate having 2.5 calories/gm and possesses many health benefits.
  •  It helps in easy digestion and aids in weight management. 
  • In food industries, starch helps in providing a texture to foods by binding the ingredients together. 
  • Starch also acts as a thickening agent in restaurants and other food industries. 

References 

Written By: Sushmita Mukhopadhyay