Household Methods of Preservation and Sterilization
Fruits and vegetables should be washed in clean water to remove dirt, micro-organisms, insecticide etc. Prolonged washing of soaking in water will cause water soluble vitamins and minerals to be lost.
If boiling temperature is maintained for a long period during cooking, this will make food get of bacteria and most of the spores.
In this process, surplus water from the food is removed without reducing taste and nutritive value. It is thought that bacteria cannot survive in the absence of moisture. Some methods of dehydration include sun drying, cabinet dryer etc. Meat, fish, figs apricots, herbs and vegetables are preserved this way. In this method, sugar and fibre concentrates. This method causes severe loss of taste, smell and texture.
Bacteria and enzymes are less at low temperature. The food kept in freezer should be covered as it will prevent them from getting flavor and smell of other foods.
In order to keep foods in the refrigerator, fruits and vegetables should be kept in brown paper or newspaper. Highly perishable foods such as milk should be placed on top shelves as the temperature is lower there. Other (less perishable foods) can be kept on lower shelves. Hot foods can be cooled to room temperature before keeping them in refrigerator. If cooked foods are kept at room temperature for longer hours, they must be re-heated before keeping them in freezer.
Salt and Sugar
The high concentration of salt prevents the water from being available for bacteria to grow. This is due to the fact that concentration of salt in water is higher than that in the bacterial cells. The cellular membrane of the bacteria cannot absorb the water and actually loses water. Principle of preservation with sugar is the same as salt. In case of sugar preservation, mixture must be heated.
The high concentration of sugar solution exerts a high osmotic pressure, withdrawing water from the micro-organism leading to its death. Tamarind, raw mango, lemon, goose berries, bitter gourd, green chillies, fish and meat are preserved this way.
Acetic acid (vinegar) and critic acid (lime juice) are used at homes as preservatives. When the medium of food is strongly acidic, most organisms are unable to survive.
Pickles, sauces, ketchup, fruit squashes etc are preserved this way.
Oil prevents the micro-organisms in the air coming into contact with the food as well as deprives oxygen to the organisms present in the food. Rice and wheat can be stored this way.
Chemical preservation act as either anti-microbial, antioxidant or both. They also reduce the damage to some essential amino-acids and the loss of some vitamins. Anti-microbial prevents the growth of yeast and bacteria whereas antioxidant keeps food from becoming rancid, browning or developing black spots. The most common chemicals used are sulphites, nitrites and citric acid. Grain products, tea, dairy products, meat, juices and salad dressings are stored this way.
It helps in storing food in vacuum environment, usually in an air-tight bag or bottle. The vacuum environment prevents bacteria from having oxygen needed for survival, hence preventing the food form spoiling. Nuts can be stored through this process.
In canning, vitamins are retained and colour and flavour of food item are retained better. It also helps the starches and proteins to be digested easily. Usually this method is used for storing fruit juices, syrups and sauces.
Pasturisation is pressurized heating for a short time. Milk and its products are pasturised. Pasturisation does not change the colour or flavour to any significant level.
It is partial dehydration and freezing. First the food is heated to lose about half its water and then frozen. This method helps in preserving food for a longer period.
The frozen food is placed under vacuum to remove the water and it is then packaged in the presence of an inert gas such as nitrogen. Through this method, the product retains its original volume and shape. Meat products, soups, food mixtures and coffee are subjected to freeze drying.
Cheese, bread, yogurt and soya sauce are fermented. Nutrients’ loss is very less in fermentation and there is an increase in vitamin B, thiamin, riboflavin, niacin, vitamin C, methomine and folic acid. Fermented foods are soft and easy to digest.
Jellying is the process in which, food is preserved by cooking in a material that solidifies to form a jel, such as gelatin, maize and arrowroot flour.