Eight methods used to preserve Food are as follows: 1. Storing food 2. Drying 3. Freezing 4. Adding preservatives 5. Heating 6. Pasteurisation 7. Canning 8. Vacuum-packing.
Why do we store fruit and vegetables in the refrigerator, and cereals, pulses and spices in jars? Dry things like cereals and pulses do not spoil easily, so we can store them longer without refrigeration. That is why they are called nonperishable (perish: die or be destroyed). Milk, meat, vegetables and fruit, on the other hand, go bad easily. So they are called perishable.
What makes food go bad? Usually, the action of bacteria, fungi or insects. You have learnt that bacteria and fungi produce spores. When these spores settle on food, they grow, multiply and decompose the food. They also release substances which are harmful for our health.
There are many ways of preserving food. Most of these involve steps to create conditions in which it is difficult for harmful organisms (especially microorganisms) to grow. These organisms require a moist environment and a range of temperature close to room temperature. Do the following activity to see how temperature affects the growth of the bacteria used for making yoghurt.
Now that you have a general idea of what makes food go bad and how to prevent spoilage, let us discuss some specific methods used to preserve food.
1. Storing food:
Keeping food covered is the most basic way of protecting it from insects and other organisms. But this does not protect it from microorganisms. Storing food in airtight containers is better. It shuts out insects and microorganisms in the air, as also moisture. This method, called dry storage, is suitable for nonperishable food.
Bacteria and fungi need moisture to grow. So drying, or dehydrating, food is one way of preserving it. Sun-drying is a traditional method of preserving food. Wheat, rice and pulses are cleaned and dried in the sun before storing.
Leafy vegetables, like methi (fenugreek) and mint, are sun-dried so that they can be stored longer. So also are potato wafers (chips), papads, and so on. Modern methods of drying food are not dependent on the sun. Hot air is blown over fruit and vegetables to dry them. Other methods are used to dry milk powder, coffee, soup powders, and so on.
Another way of discouraging the growth of bacteria and fungi is to store food at a low temperature. This method, called cold storage, is suitable for perishable food items. You know that fresh fruit and vegetables, as well as cooked food, can be preserved longer in the refrigerator. Frozen food, like peas, meat and fish, can be stored in deep freezers for weeks. The temperature in freezers is kept below -18°C.
4. Adding preservatives:
Preservatives, like salt, sugar and vinegar, are often added to food to prevent the growth of microorganisms. For centuries, people have preserved fish and meat by salting. Rubbing salt over fish or meat, or soaking them in a salt solution draws out water from the food.
Fruit and vegetables are often pickled in vinegar, oil, salt, and so on. Oil and vinegar prevent the growth of microorganisms. In jams, the sugar syrup prevents the growth of bacteria. However, if moisture gets into ajar of pickle or jam, fungi may grow. Citric acid is a preservative added to soft drinks. Chemicals, like sodium benzoate and potassium metabisulphite, are added to squashes and fruit juices to prevent the growth of microorganisms.
Drying, freezing or storing food in airtight containers prevents microorganisms from growing, but does not kill them. Heating does. That is why we boil milk.
Louis Pasteur, a French chemist, came up with a technique of heating wine to about 60° C for a short time in order to kill bacteria, and then cooling and storing it. The process is now used to pasteurise milk, which means heating it to about 62° C for 30 minutes, cooling it quickly, and storing it in sterile bottles and packets.
Food is first heated to a high temperature to kill bacteria and spores, and then sealed in airtight cans or bottles to prevent reinfection by microorganisms.
Tea, coffee, chips and other snacks are vacuum-packed, or packed in cartons, cans or packets from which air has been removed. Sometimes they are packed in nitrogen, which serves the same purpose.