Food Preservation

Food Preservation

Methods Of Food Preservation.
Answer.

Methods Of Food Preservation

• Bacteriostatic Methods
  Which inhibit the growth and multiplication of micro-organisms in food e.g., Freezing, dehydration, pickling, salting and smoking.
• Bactericidal Methods
  In which the microorganisms are killed, e.g., cooking, canning and irradiation.
  • Cold Storage and Freezing
    Refrigeration is widely used both in homes and in commercial plants as a means of maintaining the low temperature necessary for storage of perishable foods. Micro-organisms are much less active at low temperature even though they may not be destroyed by severe cold. Fresh milk, and fish are kept just above the freezing point.
    A refrigerator thermometer is kept in the refrigerator at all times. Left over foods from a meal should not stay out of refrigeration longer than two hours. Certain fruits and vegetables also keep better when cold.
  • Boiling
    Boiling food at 100°C kills all vegetative cells and spores of moulds and yeast but not bacterial spores. Cooking of rice, vegetables, meat etc. is usually done in homes by boiling. Many foods are preserved at home by boiling e.g., Milk. Cooked food can be preserved from 12 to 24 hours at room temperature.
  • Canning
    If the effectiveness of pasteurization and sterilization has to last for a long time, the material thus treated must be protected from fresh contamination by canning. Various foods eg. Fruit juices,milk, baby foods, soups and fish are preserved by canning. The food is first sterilized at temperature above 100°C for a few seconds and then cooked and filled in presterilised containers in a sterile atmosphere. There is some loss of heat labile vitamins during the process of canning.
  • Addition of Salt or Sugar
    Certain chemicals are useful in preserving food, either by retarding or preventing the growth of microorganisms. There may be either added to the product or produced in it by fermentation. Dry salting is used for the preservation of tamarind, raw mango, amla, fish and meat. Pickling of mango, lemon, fish and meat is by addition of 15 to 20% salt. Rosagulla and gulabjamoon are preserved by sugar syrup. The principle is high osmotic pressure produced by salt or sugar.

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• • Jams and Marmalades
    Jams and marmalades are prepared by boiling the fruit pulp or shredded fruit peels with sugar (above 55% by weight) to a thick consistency, firm enough to hold the fruit tissues in position. Later on, they are packed hot into glass jars or tin cans and sealed. The same process is used for jellies except that fruit juices are used in place of fruit pulps. The high concentration ofsugar (68%) binds the moisture making it not available for microorganism to grow and multiply. Anaerobic conditions are obtained by sealing. Application of heat kills most of the moulds and yeast. All these increase the shell life of the products.
  • pH
    Low pH inhibits the growth of many organisms. Vinegar used in pickling is acetric acid. Citric acid is added to many fruit squashes, jams and jellies to increase acidity and to prevent mould growth. Formation of curd from milk is an example of lactic acid produced from lactose. The lactic acid inhibits the growth of bacteria. By adding certain condiments along with salt, certain foods like mangoes, vegetables, meat and fish are preserved.
• Chemical Preservation
  Benzoic acid is used to preserve fruits, fruit juices, squash and jams because it is soluble in water and easily mixes up with food products. Potassium metabisulphite or sodium metabisulphite is used to preserve colourless food stuff such as fruits, juices and squash. These preservatives, on reaction with fruit acids liberate SO2 (sulphur dioxide) which is quite effective in killing the harmful microbes present in food. SO2 is a bleaching agent and cannot be used as a preservative for coloured food materials.