Science · Classes 9-10 · Chapter 1 · 1.4

Preservation of Food.

Bread left in the open grows an ash-grey layer within days, yet jam stays good in its jar. Why does food spoil, and what stops it? And why do some sellers use chemicals that turn the food itself into poison?

1. Why food spoils

Several things spoil food: attack by germs and fungi and their quick multiplication, an increase of enzymes in the food, humidity, and a rise of acidity as the temperature goes up. None of these spoils food alone; several together do it.

Germs or bacteria make poisonous substances in food, called toxins. Being affected by some toxins is called food poisoning. Some toxins attack the nervous system and may even cause death.

Yeast is a kind of fungus. It quickly destroys fruit juice, tomato sauce, jelly, sweet pickle and sarbot, and leaves a sour smell. The ash-coloured layer on open bread is mould, fungus like mucor and aspergillus. Oranges, pickles, tomatoes and cheese also rot from this kind of fungus.

So why preserve food? Stored food lets us eat seasonal fruit, crops and fish in another season. Surplus food can be eaten or exported at another place or time, and shortages can be met.

The core idea is simple: food spoils through the multiplication of germs and the enzymes they release, and humidity and heat boost both. Control these and food keeps for a long time. Every method below blocks one of them.

Each method takes away one of the germs' advantages. Pick a method, drag the days, and watch how far the germs grow on a slice of bread.

2. Methods of preservation

Drying is an ancient method. Fungus and bacteria cannot attack dry food, and the activity of enzymes is inhibited.

With refrigeration, vegetables, fruit, cooked food and sweetmeats keep for several days. But the multiplication and activity of germs cannot be held back for a long time.

For longer keeping there is freezing: food is held at 0°F or −18°C or below. Fresh vegetables, fruit, juice, fish, meat, ready-made food and ice cream are preserved this way.

Chemicals used to preserve food are called preservatives. They hinder the growth of fungus and bacteria. They have no nutritional value and must be used in a specific amount; too much harms health.

Vinegar is a common preservative, used in pickles and sauces. A 4%-10% aqueous solution of acetic acid is called vinegar.

Sulfite salts stop the growth of fungus, bacteria and other micro-organisms. The book names sodium bisulfate and potassium meta-bisulfate.

Sodium benzoate is a salt of benzoic acid. It especially stops fungus and is used for fruit juice and fruit pulp. Salts of propionic and sorbic acid are used for yogurt, sweets, cheese, butter and bakery food.

Sugar and salt preserve food too. A salt solution is called brine. Sugar and salt cause exosmosis of the microbes, so the food does not rot. Jam, jelly and marmalade are kept in sugar. Pieces of guava, apple or pineapple can be kept for some days in an airtight concentrated sugar solution.

3. Has it started to rot?

If food changes colour, swells up, grows a white or black layer, or turns slippery on the surface, it has started to rot. There is a risk of food poisoning, so such food should not be eaten. Tap the signs in the book's order, then the action.

4. Chemicals that poison food

Formalin is a toxic chemical, and using it to preserve food is prohibited. Long-term use causes indigestion, diarrhoea and asthma, damages the liver and kidneys or causes cancer, and child-bearing mothers may give birth to malformed babies.

The chemicals Ripen and ethylene are used to ripen mangoes, tomatoes, bananas and papayas quickly. Fruit treated with ethylene should be sold after 7-8 days, but it reaches the market in 2-3 days. So the chemical effect remains and causes disease.

Calcium carbide is also used to ripen fruit. In the presence of air vapour it forms acetylene gas. In the book's words this is then transformed into acetylene ethanol, which is very harmful to health.

There is an opposite trick too. A hormone called Culter is sprayed on mango trees for delayed ripening, so the mango does not ripen quickly and stays on the tree for long. Culter is also harmful to health.

To stop these toxic chemicals, the consumers' right law must be applied strictly. Printed and electronic media must raise awareness, and offenders must be punished. The mobile court and public awareness are effective.

5. Check yourself

Three questions in the exam's style. Start with the first.

The second asks you to recall a method.

The last is about the chemicals.

The whole note on six cards.

Source: Science, Classes 9-10 (NCTB), Chapter 1 “Healthy Life, Better Living”, section 1.4.