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# Carbohydrates in Food

Carbohydrates are formed by green plants as a product of photosynthesis and are the most abundant compounds found on earth. Carbohydrates are organically derived compounds composed of carbon atoms combined with "hydrates" (such as water, H2O). All carbohydrates follow the empirical formula CnH2nOn.

What are carbohydrates?

Carbohydrate may be defined as :
"Alcohol or aldehyde derivatives of polyhydroxy alcohols or as compounds that yield these derivatives on hydrolysis"They are the most abundant molecules on earth produced by photosynthesis.

## Functions of Carbohydrates

Carbohydrate are the primary source of the body's energy, supplying 4 k calories per gram. Glucose, a simple carbohydrate, is the body's number one source of energy. Most of the carbohydrates are converted to glucose in the body. They also function as structural molecules in cell walls and nuclei acids.

The main functions of carbohydrates are :
1. Yield energy (ATP) to drive metabolic processes.
2. Energy-storage molecules, e.g., glycogen and starch.
3. Structural component of cell wall and exoskeleton of some organisms.
4. Component found in co enzymes (FAD) and nucleic acids.

The carbohydrate group is divided into five different lists : starch, fruit, milk, sweets, desserts and other carbohydrates and non starchy vegetables :

1. One choice from the starch, fruit or milk list provides approximately 15g carbohydrate, any item from any of these lists can be substituted for another.
2. A dairy-like category is contained in the milk list; a serving from this category may count as more than just one carbohydrate choice because it has sugar or is high in fat.
3. Generally, one choice from the sweets, desserts, and other carbohydrate list also provides 15g of carbohydrate. Many items also provide fat. They lack the vitamins, minerals, and fiber that other carbohydrates provide, so, they should be used less frequently.
4. Three choices from the non starchy vegetables list count as one carbohydrate choice.

## Simple Carbohydrates

Simple sugars include mono saccharides and di saccharides. The chemical names of the six sugars to be discussed, end in "-ose" which means sugar.

### Mono saccharides

Mono saccharides include these simple sugars :

• Glucose
• Fructose
• Ga lactose

The prefix mono means "one" these sugars consist of a single ring of atoms. Mono saccharides are the building blocks of other carbohydrates, such as di saccharides and starch.

Glucose is the most abundant sugar in nature and it is our primary energy source as well. It plays a vital role in proper functioning of the human body. Fructose is the sweetest natural sugar. it is about 1.3 times as sweet as white sugar. Ga lactose is almost always linked to glucose to make milk sugar which is a di saccharides.

### Di saccharides

This is most commonly occurring carbohydrates containing two or more mono saccharides units linked together. They are also called double sugars which includes

• Sucrose
• Maltose
• Lactose

Sucrose is commonly two simple sugar joined together (Glucose and fructose). Sucrose is the chemical name which is called table sugar, white sugar, granulated sugar or simply sugar. Maltose is simply two glucose units connected together. It is fairly abundant in germination of seeds and is produced in the manufacturing of beer. Lactose is called milk sugar which is present in milk and milk products. As per the sweetness is considered lactose is ranked as the lowest sugar. Most carbohydrates are considered as plant products, lactose is one of the few considered exclusively from the animal products.

## Complex Carbohydrates

Complex carbohydrates consist of many mono saccharides bonded together in a variety of bonding patterns. Complex carbohydrates includes Oligo saccharides and Polysaccharides.

### Oligo saccharides

Raffinose and stachyose are the two most common oligo saccharides, however, as the humans lack the enzyme needed to digest these two carbohydrates, they pass undigested into large intestine, where bacteria break them down.

In the body oligo saccharides are components of cell membranes allowing cells to recognize and interact with one another. Oligo saccharides are also made in the breasts, where they are incorporated into human milk. These compounds are part of a complex system that helps protect the infants from diseases causing pathogens, and they are one of the main reasons for which women are encouraged to breastfeed their infants.

### Polysaccharides

They are made up of many mono saccharides bonded together by glycosidic bonds. The type and the arrangements of sugar molecules determine the shape and form of the poly saccharides. For example, some poly saccharides have an orderly linear appearance, whereas others are shaped like branches on a tree. The three most common poly saccharides are :

• Starch
• Glycogen
• Fiber

Glycogen is a storage form of energy in the animal body, starches play that role in plants and fibers provide structure in stems, trunks, roots, leaves and skins of plants. Both glycogen and starch are built of glucose units, fibers are composed of a variety of mono saccharides and other carbohydrate derivatives.

## Carbohydrates in Food List

Carbohydrates are the compounds made of three elements carbon, hydrogen and oxygen, the portion of hydrogen and oxygen being the same as in water. Glucose (C6H12O6), Sucrose (C12H22O12) and Starch (C6H10O5)n where 'n' is a big number are the examples for carbohydrates.

There are three types of carbohydrates in food that we eat. They are :
• Cellulose
• Starch
• Sugar

Starch and sugar are the two carbohydrates which provide most of the energy to our body. During the process of digestion, starch and sugar gets hydrolyzed to glucose. Glucose so produced is absorbed by the blood and transported to various body cells. During respiration most of the glucose is oxidized to produce energy necessary for doing work and carrying out body processes. This can be represented as :

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

Cellulose is not digested by our body. So, it acts as a fiber or roughage in our food. It does not provide energy to our body and hence it is not a food.

## Synthesis of Carbohydrates

Definition & classification of carbohydrates :

Carbohydrates are defined as the optically active polyhydroxy aldehydes or ketones or substances which yield these on hydrolysis.

Carbohydrates are broadly classified as of two types. They are :
1. Sugars
2. Non sugars

1. Sugars

Sugars are sweet crystalline and soluble in water. These are again classified into two types.

Monosaccharides

The sugars which cannot further hydrolyzed to simpler sugars. They may classify into two types based on type of carbonyl group.

1. Aldoses: Having aldehydic group Ex: Glucose, Mannose
2. Ketoses: Having ketonic group Ex: Fructose

Oligosaccharides

These yield two to ten monosaccharide molecules on hydrolysis. And, again these are classified into various types based on the number of hydrolyzed products formed.

1. Disaccharides: on hydrolysis gives two monosaccharides. Ex: Sucrose
2. Trisaccharides: on hydrolysis gives 3 monosaccharides. Ex: Raffinose
3. Tetrasaccharides: on hydrolysis gives 4 monosaccharides. Ex: Stachyose

2. Non sugars

Non sugars are tasteless, amorphous solids and either soluble in water or forms colloidal solutions.

### General Synthesis of Amino Acids

Kilani-fischer synthesis

In this synthesis a higher carbohydrate is synthesized from the lower carbohydrate. The lower carbohydrate (Arabinose) is reacted with HCN to form Cyanohydrin

This cyanohydrin on reduction with H2/Pd gives Imines. And these on hydrolysis form higher carbohydrates (Glucose and Mannose).

## Monosaccharide

Glucose occurs in nature in free as well as combined form. It is present in sweet fruits and honey. Ripe grapes contain ~ 20% of glucose.

### Preparation of Glucose

From sucrose is boiled with dil. HCl or H2SO4 in alcoholic solution, glucose and fructose are obtained in equal amounts.

C12H22O11 + H2O $\overset{H^+}{\rightarrow}$ C6H12O6 + C6H12O6

From Starch

Commercially glucose is obtained by hydrolysis of starch by boiling it with dilute H2SO4 at 393 K under pressure.

(C6H12O5)n + nH2O $\xrightarrow[393K\ 2-3 bars]{H^+}$ nC6H12O6

### Properties of Glucose

Glucose has one aldehyde group, one primary and four secondary hydroxyl groups. It gives the following reactions:

1. Acetylation if glucose with acetic anhydride gives a penta acetate confirming the presence of five hydroxyl groups in glucose.

OHC - (CHOH)4 - CH2OH $\overset{[CH_3CO]_2O}{\rightarrow}$ OHC - (CHOCOCH3)4 - CH2OOCCH3

2. Glucose reacts with hydroxylamine to give monoxime.

HOH2C-(CHOH)4-CHO + HONH2 HOCH2-(CHOH)4-CH=NOH

Glucose adds a molecule of hydrogen cyamide to give a cyanohydrin.

HOH2C-(CHOH)4-CHO + HONH2 HOCH2-(CHOH)4-CH=NOH

These reactions confirm the presence of a carbonyl group in glucose.

3. Glucose reduces ammoniacal silver nitrate solution (Tollens reagent) to metallic silver and also Fehlings solution to reddish brown cuprous oxide and itself gets oxidized to gluconic acid. This confirms the presence of an aldehydic group in Glucose.

HOCH2-(CHOH)4CHO + Ag2O HOCH2-(CHOH)4-COOH + 2Ag

4. On oxidation with nitric acid, glucose as well as gluconic acid both yield a dicarboxylic acid saccharic acid. This indicates that presence of a primary alcoholic group in glucose.

HOCH2-(CHOH)2-CHO $\overset{HNO_3}{\rightarrow}$ HOOC-(CHOH)4-COOH

5. Glucose on prolonged heating with HI forms n-hexane suggesting that all the 6 carbon atoms in glucose are linked linearly.

HOCH2-(CHOH)4-CHO $\overset{HI}{\rightarrow}$ H3C-CH2-CH2-CH2-CH2-CH3

6. D-glucose reacts with phenyl hydrazine to give glucose phenyl hydrazine which is soluble. If excess od phenyl hydrazine is used, a dihydrazone known as osazone, is obtained.

7. On heating with conc.solution of NaOH, glucose first turns yellow, then brown and finally resinifies. However, with dilute NaOH, glucose undergoes a reversible isomerisation and is converted into a mixture of D-glucose, D-maltose and D-fructose. This reaction is known as Lobry de Bruyn-van Ekenstein rearrangement. Same results are obtained if maltose or fructose are treated with alkali. It is probably on account of this isomerization that fructose reduces Fehling's and Tollen's reagent in alkaline medium although it does not contain a -CHO group.

D-glucose D-maltose D-fructose

8. Dehydration
When treated with concentrated sulfuric acid glucose undergoes dehydration and results in the formation of hydroxy methyl furfural.

### Cyclic Structure of D-glucose

The open chain structure of glucose proposed by Bayer explained most of its properties. However, it could not explain the following

1. Despite having an aldehydic group, glucose does not give Schiff's test and it does not react with sodium bi-sulphite and ammonia
2. The penta acetate of glucose does not react with hydroxylamine indicating absence of -CHO group
3. Mutarotation

When glucose was crystallized from a concentrated solution at 30oC it gave a form of glucose (Melting point 146oC) whose optical rotation is 111o. The b form (Melting point 150o) obtained on crystallization of glucose from a hot saturated aqueous solution at a temperature above 98oC has an optical rotation of 19.2o. These two forms of glucose are called anomers.

## Oligosaccharides

Oligosaccharides are classified according to the number of monosaccharide units (or monomers) they are composed of, into disaccharides, trisaccharides, tetrasaccharides, pentasaccharides and hexasaccharides. Among these, disaccharides are most abundant in the living system.

They are carbohydrates formed by the joining of 2 to 6 monosaccharide molecules. The bond formed between monosaccharides is called glycosidic bond.

1. Maltose is a disaccharide, formed by two glucose units. It occurs in cells as a breakdown product of starch. It is also formed in the seeds during germination. It is commonly called malt sugar.
2. Lactose is a disaccharide found in milk and hence commonly called milk sugar. It is formed by condensation of a glucose molecule and a galactose molecule.
3. Sucrose is disaccharide found extensively in plants. It is commonly called cane sugar. It is formed by a condensation of a molecule of glucose and a molecule of fructose.
4. Raffinose is a trisaccharide formed by condensation of a molecule each of glucose, fructose and galactose. It occurs in plants.

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