CBSE Class 7 Science Advanced Biology Food and Food Preservation Notes Set 01

Food and Food Preservation

Food and Nutrients

Any substance that a living thing takes in to get nourishment is food. It may come from plants or animals and holds the nutrients needed to keep life going. Nutrients are the materials that nourish the body, help it grow and keep it alive.

Nutrients are grouped by the quantity the body needs. Macronutrients are needed in large amounts and are carbohydrates, fats and proteins. Micronutrients are needed in small amounts and are vitamins and minerals. Water and dietary fibre (roughage) are also part of a healthy diet.

Nutrients present in food: carbohydrates, proteins and fats are macronutrients; vitamins and minerals are micronutrients; roughage and water are also needed.

Why Food Matters

Staying healthy depends on eating the right food, in the right combinations, at the right times and according to age. Eating too much and eating too little can both make a person ill. Too much food leads to weight gain and can bring heart trouble, diabetes and high blood pressure. Malnutrition is the state in which what a person eats does not match what the body needs.

Healthy Food and Junk Food

Food can be sorted by its quality. Healthy food supplies all the nutrients needed to stay fit and lets the body work well, for example milk, fruits and vegetables.

Junk food is tasty but gives few useful nutrients and contains extra oil and salt, for example burger and pizza. Eating a lot of it adds body weight and causes dietary problems. One such problem is obesity, in which too much fat collects in the body and harms health.

Milk is called a complete food because it holds all the main nutrients. It is not, however, a major source of vitamin C.

What milk contains: fat gives extra energy, sugar is the fuel, vitamins reduce vitamin deficiency, water reduces water deficiency, proteins build muscles, and minerals (calcium and phosphorus) help form bones.

Balanced Diet

A balanced diet is one that supplies the body with all the nutrients it needs to work properly. Most of the day's energy should come from fresh fruits, vegetables and whole grains. It contains vitamins, minerals, proteins, carbohydrates, fats and enough water.

Composition of a balanced diet. Rice and potatoes give vitamins, minerals and fibre; fruit and vegetables give vitamins, minerals and plant chemicals; meat, fish, eggs and beans give nutrients for health and upkeep; milk and dairy foods keep bones strong; herbs and similar foods add vitamins and minerals.

Importance of a Balanced Diet

1. It gives energy and supports growth.
2. It helps us lead an active and healthy life.
3. It keeps us free from disease.
4. It repairs the body.
5. It supports body functions such as breathing, digestion and keeping the body warm.
6. It prevents deficiency diseases.

Deficiency Diseases

A deficiency disease results when the diet lacks a particular nutrient, mainly a vitamin or mineral. These are also called insufficiency diseases. Examples are beriberi (lack of vitamin B), night blindness (lack of vitamin A), kwashiorkor (lack of protein) and marasmus (lack of protein and calories).

Table: Kwashiorkor and marasmus compared

Anaemia occurs when the number of red blood cells or the amount of haemoglobin in the blood falls. Goitre is a swelling in the neck caused by enlargement of the thyroid gland, which lies in the neck.

Table: Nutrient deficiency diseases

Food Spoilage

Food is said to be spoilt when it has become contaminated enough to be unfit to eat. Spoilt food changes colour, smells foul and tastes bad. The causes are:

1. Time: food may change by itself as time passes.
2. Microorganisms: bacteria, viruses and fungi grow best at room temperature and attack food left uncovered.
3. Enzymes: enzymes (biological catalysts) in the food itself can spoil it, such as a banana turning from green to yellow to brown, with changes in taste and texture.
4. Environmental and chemical factors: food reacts with air and changes chemically.
5. Insects, rodents and pests: they physically damage the food.

Factors causing food spoilage: microorganisms, time, enzymes, environmental factors, chemical reactions, and insects and rodents.

Ethylene gas, made naturally by plants, makes fruits ripen.

Effects of Eating Spoilt Food on Health

The harm from spoilt or expired food can range from nothing at all to severe. A change in look, taste, texture or smell warns that the food is unsafe. Such food can cause food poisoning, which is due to bacteria, with symptoms like nausea, vomiting, diarrhoea and stomach pain.

Food Preservation Methods

Food preservation is the science of stopping food from decaying or spoiling so that it can be stored in good condition for later. The food should stay safe from microbes, insects and pests and should keep its colour, texture, flavour and nutritive value. The methods are:

1. Drying: the oldest method; sun or wind removes water, and microbes cannot grow without moisture. Examples: dry fruits, dry meat.
2. Freezing: storing in the cold slows the growth of microbes. Example: frozen peas.
3. Smoking: exposing food to the smoke of burning wood, which works against microbes. Example: smoke roasting.
4. Packing: sealing in air-tight bags and bottles keeps microbes out. Example: dry fruits.
5. Oiling and salting: salt dehydrates microbial cells and stops them growing; oil is also used. Example: pickles.
6. Sugar: used for foods such as fruits and jams.
7. Chemicals: vinegar (acetic acid) and sodium benzoate are artificial preservatives.
8. Heating and boiling: most microbes cannot survive extreme temperature.

Food preservation methods: drying, freezing, smoking, packing, oiling, salting, sugar, chemicals, pasteurization, and heating and boiling.

Pasteurization

Pasteurization heats food to destroy disease-causing bacteria and make it safe. The French scientist Louis Pasteur first showed it. The food is brought to a high temperature for a few seconds and then cooled suddenly. Milk, for example, is pasteurized at 72°C for 15 seconds. Milk, canned drinks and cheese are pasteurized.

Food Storage

Food storage lets food be kept for a long time. On a large scale, crops are kept in granaries, big storehouses often raised above the ground so that pests and animals cannot reach the grains. Huge quantities of farm grain are kept in silos, tall tower-like structures on farms for bulk material. At home, food is stored in refrigerators, tins, cans and air-tight containers, which keep microbes and pests out.