Common salt has many useful properties. In food preservation it acts as an antiseptic, killing germs. It absorbs water thus deydrating food and depriving parasites and germs of the mosture they need. Dry salting has been used for centuries to preserve meat, ish and other foods. Cured meats such as salami and ham are prepared by methods relying on salt as a preservative. Salt solution consisting of about 10% of salt dissolved in water is used for preserving many vegetables suc as onions, carrots and turnips.
Drying is a very old method of preservation. Placing foods in the sun to dry them has been used by man for thousands of years. this deprives parasites of moisture. In industry hot air is passed over solid foods to dhydrate them while liquid foods are dried by spraying them in a heated chamber. Another method is the use of air at very low temperatures which is called freeze drying.
Smoking reduces the growth of microbes and the oxidation of fats. It is maily used to preserve herrings (which are then named kippers) and with certain cheeses to give them a particular flavour.
Pickling is a traditional method of preserving food, especially vegetables using vinegar which stop the spread of bacteria.
Oil prevents contact with air thus preventing the spread of those micro-organisms which multi-ply in the atmsphere. Preliminary cooking is often necessary to kill those microbes which can survive without air. It is used for many kinds of vegetables and some fish such as sardines. A solution of 70% sugar dissolved in water is often used to preserve fruit. It prevents the spread of microbes and has the advantage of conserving most vitamins althoughsome vitamin c is lost. Candied fruit are obtained by boiling several times in sugar solution and then allowing it to cool and evaporate slowly til it crystallizes.
Alcohol is an excellent antiseptic and therefore makes a good preserving agent. However it is very costly which means it is used only for cocktail fruits or other expensive delicacies.
Blanching is the technique of immersing food, especially vegetables, in boiling water for a few seconds. this blocks the increase in enzymes and helps to preserve vitamins.
Canni bottling are excellent methods of presrving many fodstuffs. In canning the food is sealed in an airtight tin so that it is completely isolated from germs and bacteria. It is then heated to a temperature between 100°C and 120°C dependng on the type of foodstuff. this technique completely sterilizes the contens. In bottling, the hot food is placed in the jar and the steam above the food condenses on cooling to form a vacuum sterilization is the steaming or boiling of food to kill bacteria.
Cold storage at temperatures from 0°C to10°C (refrigeration) does not prevent deerioration of foodstuff but slows it down. the lenght of time foods may b conserved depends on the type of food.
Deep freezing is a modern , efficient way of preserving foods. The very low temperature prevents the growth of micro-organism. It has the advantage of preserving the original flavour and the food's nutritional value is unaltere. Freezing should be so rapid (within half an hour to -4°C) so that only small crystals of ice are formed. Industrial freezers lowers the temperature to -29°C. Domestic freezers must maintain the food below - 18°C.
Irradation is a method used to kill microbes using low levels of radiation but it is a controversial method not very widely used.
Puffing is a method of preparing cereal grains such as wheat, oats and rice. The grain is put under very high pressure and then released into air at a normal pressure. This causes it to swell and expand making it very light in weight.
Ohmic heating is the use of an electric current to heat and sterilize foodstuffs before they are packed. It is similar to microwave heating as the electrical energy is transformed into heat throughout the whole mass of food and not just on the surface.