Tuesday, August 18, 2009

food science


Carbohydratesare a major source of energy for humans, providing approximately 45% to 80% of the total caloric intake in different income groups. Since they are a relatively inexpensive source of energy compared to fats and proteins, they form the bulk of the diet of humans throughout the world.

They are mainly present in food in the form of sugars, starches, and fibres. A study of the various types of carbohydrates is necessary because the kind and proportion of different forms of carbohydrate present in food have direct bearing on our health.

Three groups of carbohydrates are important in out diet from the nutritional point of view, namely, sugars, starches and fibres. The sugar and starch that we consume is ultimately broken down to glucose in the digestive tract and absorbed into the blood circulation. In the human body, glucose is removed from blood by the tissue cells and used as a source of energy. Some glucose is converted to glycogen, also called animal starch, and stored in the muscle and liver as a reserve store of energy.

Glucose Oxidised in Energy + CO2 ↑ + H2O

Tissues waste products of metabolism

Classification of Carbohydrates

Carbohydrates which are of importance in the diet are classified on the basis of the number of sugar units present in them (Fig. 13.1, Table 13.1). they may also be classified as:

Available Carbohydrates : Carbohydrates which can be digested in the human body and yield energy when they are oxidized in the body.

Unavailable Carbohydrates : Carbohydrates which cannot be be digested because the human body does not contain the enzymes necessary for their breakdown. Unavailable carbohydrates do not provide any energy to the body but are necessary as they perform some important functions in the body such as regular elimination of faecal waste.

Digestion, Absorption, and Metabolism

Complex carbohydrates and sugars are too large to be absorbed through the intestinal wall. They need to be broken down into their constituent monosaccharides can be absorbed into the blood stream. The mechanical and chemical digestion of starch begins in the mouth.

Fig. 13.1 Classification of Dietary carbohydrates

Table 13.1 Classification of Carbohydrates

Sr. No.


Name of Carbohydates


1. Simple carbohydrates or sugars



(single sugar unit)

Glucose (dextrose)

Fructose (levulose)


Fruits, vegetables


On hydrolysis of lactose



(two sugar units)

Sucrose (glucose + fructose)

Maltose (glucose + glucose)

Lactose (glucose + galactose)

Sugar cane, sugar beet,

Sprouted and malted grains, acid hydrolysis of starch.

Milk is the only source.

2. Complex carbohydrates or polysaccharides




Glycogen (animal starch)


Cereals, pulses, roots, tubers, veggies, and under-ripe fruits.

Liver and muscle of freshly slaughtered animals.

Partial breakdown of starch by dry heat or digestion.


Unavailable or dietary fibres


Water insoluble




Structural fibre in whole grain cereals, nuts, wheat bran, figs, vegetables,etc.


Water soluble




Non-structural fibres in apples, citrus fruits, guava, oats, barley, pulses, seaweeds,etc.

*Lignin is not a carbohydrate.


Daily diet should provide up to 50-70% kCal of energy from carbohydrate, which means that the diet of an individual who needs 2,400 kCal should consume 60% of 2,400, i.e., 1,440 kCal or 360g of carbohydrates/day. Carbohydrates are not only an economical source of energy but are also readily available and easy to store as they have a long shelf life.

All foods of plant origin contain carbohydrates in varying amounts. With the exception of milk, animal foods do not contain carbohydrate. Although milk is not consumed as a source of carbohydrate, some milk products, such as khoa and milk powder, contain significant amount of carbohydrate lactose.

The important sources of carbohydrates in the diets of Indians are cereals and millets, roots, tubers, pulses, sugar and jiggery. (table 13.2)

All sugars provide 4 kCal/g of energy. the carbohydrate and calorie content of a food can be reduced by using sugars which are sweeter than sucrose so that the quantity of sugar required will be less.

Table 13.2 Some rich sources of carbohydrates



Fruits and vegetables

Nuts and oilseeds










All whole grain and dehusked pulses and their by-products,

e.g., rajma, Bengal gram, whole green gram, lentils, and besan.




Custard apple


Green peas





Cashew nuts


Ground nuts

Garden cress


Gingelly seeds








Skimmed milk powder


Carbohydrates have many functions in the body.

1. The chief function of carbohydrate is to provide energy to the body so that it can carry out day-to-day work and maintain body temperature. All carbohydrates except fibre provide 4kCal/g of energy. it is the cheapest source of energy available.

2. Glucose is the only form of energy used by the central nervous system. When blood glucose levels fall, the brain does not receive energy and convulsions may occur.

3. Carbohydrates spare proteins from being broken down for energy and are used for bodybuilding and repair. In carbohydrate deficient diets, proteins meant for bodybuilding and repair are oxidised to meet the most important and first need of the body, i.e., energy.

4. They are required for complete oxidation of fat. In a deficiency, fats broken down rapidly for energy and intermediate products such as ketones are formed in large amounts resulting in a condition called ketosis.

5. Carbohydrates can be converted into non-essential amino acids, provided a source of nitrogen is available.

6. The sugar lactose helps in the absorption of the minerals calcium and phosphorous.

7. Lactose helps certain bacteria to grow in the intestines. This bacterial flora is capable of synthesizing B-complex vitamins in the gut.

8. Dietary fibre plays an important role of increasing faecal mass by absorbing and holding water, stimulating peristalsis, and eliminating faecal waste.

9. Fibre also helps in lowering blood cholesterol levels by binding bile acids and cholesterol.


The daily diet should not contain less than 100g of carbohydrate. Carbohydrate deficiency is uncommon in our country as diets are cereal based. A deficiency of carbohydrate in the diet results in utilization of fat for energy. In severe deficiency, incomplete oxidation of fats causes ketone bodies to accumulate in the blood.

Excess Carbohydrates

1. Excessive consumption of refined sugars could be one of the causes of dental caries or tooth decay.

2. Excessive sugar depresses the appetite, provides hollow calories, and could result in malnutrition.

3. High intake of sugar and refined carbohydrates increase the blood triglyceride levels leading to heart diseases.

4. When excessive carbohydrates are consumed they are converted into fat and deposited in the adipose tissue, whic could lead to obesity, i.e., body weight of 20% or more than desirable weight.

5. Excessive fibre could irritate the intestinal lining causing cramps or bloating due to gas formation.

6. Excessive fibre interferes with the absorption and availability of mineral elements such as iron and calcium.

Role of Dietary Fibre in Prevention and Treatment of Disease

Dietary fibre refers to the total amount of naturally occurring material in plant foods, which is not digested. The terms roughage, bulk, and unavailable polysaccharides are synonymous with fibre. Fibres cannot be digested by human enzymes.

Dietary fibre or roughage does not provide humans with energy but performs many important functions in the body (Table 13.3). Fibre can absorb and hold water thereby increasing faecal bulk. This acts as a laxative and reduces intraluminal pressure in the colon preventing diverticulosis. Insoluble fibre prevents constipation by stimulating peristalsis in the large intestine. The contraction of muscular walls of the digestive tract is stimulated by fibre. Fibre increases water absorption, forming a larger, softer stool that rapidly passes through the colon. Soluble fibre binds bile acids and cholesterol and is beneficial to people suffering from coronary heart disease. Fibre reduces the triglyceride and cholesterol levels in blood.

Table 13.3 Functions and sources of dietary fibre




Food source

Insoluble fibres



1. Insoluble fibre

2. Holds water

3. Increases stool bulk

4. Reduces intraluminal colonic pressure

5. Prevents constipation

6. Binds minerals such as Ca and Fe.

7. Binds bile acids

8. Reduces transit time

Bran, whole grain cereals, specially wheat, rye, apples, pears, tomatoes, cabbage, beans.



Bran, whole grain cereals, specially millets – jowar, bajra, ragi.


Lignin (non-carbohydrate source)

Whole grain cereals, pears, peaches, plums, mature vegetables.

Soluble fibres



1. Soluble fibre

2. Binds cholesterol and bile acids

3. Holds water

4. Fermented in the colon to volatile fatty acids and gas by the normal bacteria flora of the colon.

Guava, apple, citrus fruits, wood apple, berries, carrots and green beans.



Oatmeal, pulses, and beans, dinkache ladoo, processed foods.


Mucilages, seaweeds and algae.

Thickener in food products, stabilizer, gelling agent in puddings.

Fibre is beneficial to people on weight reduction regime. It provides satiety value to the meal because of more chewing required and at the same time does nto add to the calorific value of the meal.

It helps in lowering blood sugar levels in diabetic individuals by slowing down carbohydrate absorption and lowers the insulin requirement. Regular intake of fibre may prevent cancers of the colon and rectum.

Although fibre is not a nutrient, because it cannot be digested by humans, it is nutrionally important. Foods such as whole grain cereals, fruits, and vegetables, specially when the peel and seeds are edible, are rich sources of fibre.

The fibre content of the daily diet should be approximately 30-40 g/day (Table 13.4)

Recommended dietary intake for adults

Fibre – 40g/day is desirable


Minimum – 100g

Maximum – less than 70% of total calories from carbohydrates.

Artificial sweeteners

These are also known as non-nutritive sweeteners. Artificial sweeteners are 100-350 times as sweet as sucrose and provide no or very negligible calories. A wide variety of sweetening agents are available in the market and are used for low-calorie products such as diet coke. These processed products are specially manufactured for obese individuals, weight watchers, and diabetic patients. They are used quite successfully in bakery items such as cakes, biscuits cookies, Indian sweetmeats, confectionery products, beverages, puddings and chewing gum. Saccharin, aspartame, sodium cyclamate and sugar are some of the commonly used substitutes for sugar. Their use is not recommended in soft drinks and other food consumed by children as these foods may be a substitute for essential.


Ethyl alcohol is produced by yeast fermentation of carbohydrates under anaerobic conditions. Different carbohydrates are used to manufacture alcoholic beverages.

Alcoholic beverages do not supply necessary nutrients but contribute significant amount of energy (Table 13.5). alcohol contributes 7kCal/g or 5.6kCal/ml and in people who consume alcoholic beverages, up to 10% of total energy needs may be derived from alcohol. Some chronic alcoholics may consume insufficient food and suffer from malnutrition while the reverse may be observed in the case of social drinkers who consume large amount of high-calorie foods such as starters, nuts, and wafers along with their drinks. These high-calorie snacks are rich in carbohydrates, fats, and sodium. Alcohol is absorbed rapidly, directly into the blood stream. Drinking on an empty stomach increases the alcohol level in blood twice as fast as on a full stomach. It is a good rule to have some light snacks along with alcoholic beverages.


C6H12O6 2C2H5OH + 2CO2

anaerobically (ethyl alcohol)

Percentage of alcohol is proof divided by 2

86 proof whiskey = 43% alcohol

Calorie content of one peg or 30ml of whiskey = 43 x 30 x 5.6


= 78 kCal

If alcohol is taken along with antidepressants or tranquilizres, it prolongs the sedative effect of these medicines.

Excessive consumption of alcohol accompanied by decreased intake of other nutrients can lead to malnutrition and serious liver disorders such as cirrhosis of the liver.

Table 13.5 Calories supplied by alcohol




Alcohol (%)

Lager beer



4 – 15




4 – 12.5

Gin 80 proof







42 . 8

Whiskey 86 proof




Wine red



12 – 15

Wine dry



12 – 15




18 – 22




12 - 13


  1. There is growing opportunity in the use of lignin as a substitute for fossil-based raw materials which are used in the manufacture of a wide range of products such as plastics, chemical products, and carbon fibers.

    Global lignin market