Carbohydrates are polyhydroxy aldehyde or polyhydroxy ketones of compounds thereof or polymers that can liberate those compounds upon hydrolysis. With some exceptions, carbohydrates have general formula CnH2nOn where n refers to any positive integer. Carbohydrates can be represented by (CH2O)n where n ≥ 3. Carbohydrates are major source of energy for man and many animals. In man’s diet, the chief carbohydrate is starch. For child, lactose is major source of carbohydrate.

Fig: chemical structure of carbohydrate

Classification of carbohydrate: Carbohydrates are commonly divided into;

1. Monosaccharides

2. Oligosaccharides

3. Polysaccharides

Monosaccharides: These are polyhydroxy aldehyde or ketones generally with unbranched chain. Example; glucose, fructose, mannose, galactose and many more. Monosaccharides cannot be hydrolyzed into smaller molecules.

Oligosaccharides: These yield 2 to 10 number of monosaccharides on hydrolysis. These are further classified as

1. Disaccharides

2. Trisaccharides

3. Tetrasaccharides

Disaccharides: These yield two monosaccharides on hydrolysis.

Trisaccharides: These yield three monosaccharides on hydrolysis.

Tetrasaccharides: these yield four monosaccharides units upon hydrolysis.

Polysaccharides: it consists of ‘n’ monosaccharides unit where n > 10,000. Hence they are high molecular weight compounds and insoluble or very less soluble in water.

Occurrences of some carbohydrates

SucroseCane sugar, sugar beet, table sugar
MaltoseMalt ( germinating barley )
LactoseMilk and milk products
CellobiosePlants (building block of cellulose )
FructoseFruits, honey
RaffinoseSugar beet, sugar cane
StarchPotato, cassava, rice wheat
PectinPapaya, citrus, apple

Food sources of carbohydrates:

Fruits and vegetables: sweet fruits and fruit juice are the rich source of fructose, which is simple sugar. For example, apple, grapes, papaya, melon etc. Tuber vegetables such as potato, yam, cassava etc. are rich source of starch, a complex carbohydrate. Vegetables also provide dietary fiber which are indigestible carbohydrate but are important in our diet for maintaining our gut health. Example; guard, beans, leafy vegetables, cabbage etc.

Milk and milk products: milk contains abundant amount of lactose and so does milk products like paneer, cheese and milk confections like ice-cream. Milk is chief source of carbohydrate for infants.

Sweet and confections: candies, chocolates, gums, jam marmalades, syrups (corn syrups), chocolate bar etc. provide excess of calorie in our diet in the form of sucrose and lactose (in milk chocolates). Overconsumption of such sweet and confections should be controlled as even a small bite of such food provide relatively large quantities of calorie in our diet.

Sweetened beverages and energy drinks: sugar added beverages like fruit juice, cold drinks, mocktails and energy drinks contains abundant amount of sucrose and glucose as carbohydrate. They give instant energy as they contain simple sugar like glucose.

Cereals, cereal- products and bakery: cereals corn, rice, wheat, millet are rich source of carbohydrate; they contain carbohydrate in the form of starch, pectin and cellulose which are the complex carbohydrates. Starch is digestible while cellulose is not digested by human body and works as dietary fiber. Similarly, cereal products like wheat flour, rice flour, , corn flakes etc. and their bakery products like cake, bread, cookies, biscuits etc. are major source of carbohydrate in our diet.

Poor source of carbohydrate: Fat, oil, meat, fish, poultry are poor source of carbohydrate. They contain less amount of carbohydrate of which most of them are embedded with protein forming glycoprotein r conjugated with lipid to from lipoprotein.

Bad carbohydrate: These mainly include simple carbohydrate that cause rapid rise in blood sugar but contain few beneficial nutrients than complex carbohydrate. Sugary foods such as soda, candies, cookies, cake and other desserts are some of the examples.

Good carbohydrate: Good carbohydrates are usually complex carbohydrates that are usually more nutrient dense. These supply key nutrients such as fiber, protein, vitamins, calcium and iron. Example of complex carbohydrate includes brown rice, whole wheat bread, fruits and vegetables.

Function of carbohydrate in human body.

1. Provide energy ( physiological value ~ 4 Kcal/g and gross energy value 4.1 Kcal/g)

2. Excess carbohydrate changes into fat and protein.

3. Essential for oxidation of fat (fat is burned in the flame of carbohydrate)

4. Indispensable for nervous system.

5. Synthesis of biomolecules.

Recommended Dietary Allowances (RDA):

Institute of Medicine (IOM) –USA, recommends to get between 45% -65% of daily calories from carbohydrates. For example, if you consume roughly 2,000 calories per day, you should get between 900 and 300 calories from carbohydrate that equals to 225 – 325 g/day. But not below 130 g/day, required for proper brain functioning.

Importance of fiber:

Fiber is non- digestible form of carbohydrate. Soluble fiber is found in food like oatmeal, nuts, beans, lentils and apples. These are water soluble and can lower blood cholesterol and glucose level.

Food such as brown rice, legumes, cucumbers and carrots contain insoluble fiber which doesn’t dissolve in water. It help soften and provide bulk to stool, promoting regularity. IOM recommends that adults 50 years and younger consume 38 g of fiber daily for men and 25 g for women. If you are male over 50, consume 30 g daily and 21 g if you are a female over 50 years.

Deficiency of carbohydrates:

When our body lacks carbohydrates, the first sign we gt is lack of energy, followed by constipation, nausea, headache and bad breath. Deficiency for extended period of time leads to hypoglycemia and ketosis which are the severe form of carbohydrate deficiency.

Hypoglycemia: This condition is also known as low blood sugar or low blood glucose. This is the condition when glucose level in the blood drop below normal (100 mg/dL during fasting and 140 mg/dL after meal).

Ketosis: When there is no longer enough glucose to keep brain and muscle functioning, it produces ketones, condition known as ketosis. Ketones helps in breakdown of fat for energy. Ketones are acidic and accumulate in blood and when ketone level in blood rises, it results in ketoacidosis.

The dangers of excess carbohydrates:

Carbohydrates are major portion of our diet but when our body constantly feeds upon carbohydrates more than required amount, the hormone insulin is produced in larger amount from pancreas. Excess insulin results many health problems like high blood pressure, high cholesterol, heart disease, high blood sugar, heart disease and insulin resistance.

Effect of heat on carbohydrate:

1. Caramelization: When is sugar is exposed to heat first of all sugar will melt producing a thick syrup. As the temperature continues to rise, the sugar syrup changes from light yellow color to golden brown and progressively deep brown. This browning process is called Caramelization. Granulated white sugar melts at 160°C and begins to caramelize at 170°C. Caramelization is desirable while preparing sweet and confections and sugar syrups.

2. Maillard reaction: This involves the reaction of simple sugar with amino acid (protein block) in the presence of high temperature. This results formation of brown color in food surfaces and typical aroma to some foods. Example; baked breads, dark beer, roasted meat, nuts, coffee, chocolate (cocoa beans) etc.

3. Gelatinization: Starch, a complex carbohydrate have powerful thickening property. When starch is mixed with water and heated, individual starch compound absorbs water and swells, causing liquid to thicken. This process is known as gelatinization. Gelatinization occurs at different temperatures for different type of starch. Root based starch ( potato, arrowroot) thicken at high temperature but break down more slowly.



About Author

Name : Pratiksha Shrestha

Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.