When food is available more than the present use, it is preserved for future consumption. Foods such as fruits and vegetables have a short growing season and preservation makes them available for use throughout the year and avoids wastage of surplus crops.
Principles of Food Preservation
Prevention or delay of microbial decomposition.
1. By keeping out microorganisms - asepsis
2. By removal of microorganisms - filtration
3. By hindering the growth and activity of microorganisms - low temperature, drying.
4. By killing the microorganisms - heat or radiation.
Prevention or delay of self decomposition of food -By destruction or inactivation of food enzymes, (eg) blanching.
Prevention of damage caused by insects, animals and mechanical causes.
Methods of Food Preservation
All methods used for preserving foods are based upon the general principle of preventing or retarding the causes of spoilage. - microbial decomposition, enzymatic reactions and damage from mechanical causes insects and rodents.
Preservation by low Temperature
Freezing may preserve foods for long periods of time provided the quality of food is good to begin with and the temperature is maintained in freezers. In slow freezing process or sharp freezing the foods are placed in refrigerated rooms at temperatures ranging from - 4 o C to 29 o C. In quick freezing process the lower temperatures used - 32 o C to -40 o C freeze foods so rapidly that fine crystals are formed and the time of freezing is greatly reduced over that required in sharp freezing.
Of fruits and vegetables consists of drying the food to about 50 percent of its original weight and volume and then freezing the food to preserve it.
Preservation by high Temperature
Pasteurisation is a heat treatment that kills part but not all the microorganisms present and usually involves the application of temperatures below 100 o C. The heating may be by means of steam, hot water or dry heat and the products are cooled promptly after the heat treatment. Usually milk is pasteurized. Three general methods are used now-a-days.
a. Holding or Batch system (Holder method)
This consists in bringing the milk or cream to a temperature usually 65 o C and holding at that point for at least 30 minutes followed by rapid cooling.
b. High Temperature Short Time Method (HTST) (Flash method)
This consists of raising the temperature of the milk to at least 72 o C for 15 seconds followed by quick cooling.
c. Ultra High Temperature System (UHTS)
In this system, milk is held for 3 seconds at 93.4 o C. After pasteurisation the milk is cooled rapidly to 7 o C or lower.
Canning involves the application of temperatures to food that are high enough to destroy essentially all microorganisms present plus air tight sealing in sterilised containers to prevent to contamination. Fruits, vegetables and flesh foods are preserved by this method.
Preservation Using Preservatives
Preservatives has been defined as chemical agents which serve to retard, hinder or mask undesirable change in food. These changes may be caused by microorganisms, by the enzymes of food or by purely chemical reaction. The PFA classifies them as class I preservatives and class II preservatives. Class I preservatives are salt, sugar spices, vineger, honey and edible vegetable oils. Class II preservatives are benzoic acid and its sodium and potassium salts, sorbic acid and its sodium and potassium salts. The preservatives generally used in fruit and vegetable products may be broadly classified as organic and inorganic preservatives.
The organic preservatives are benzoic acid, chlorobenzoic acid or salicylic acid. The only permitted inorganic preservative is sulphur-di-oxide which is generally used in the form of sulphites. The preservatives permitted in fruit and vegetable products in India are sodium benzoate, sulphites and sorbic acid. Sulphur-di-oxide, sulphites and metabisulphites are used in confectionary, fruits and fruit juices and wines. Sodium or potassium metabisulphites are used in the preservation of fruit products. Sodium benzoate is used as a preservative in grape crushes and tomato ketchup.
Preservation by high Osmotic Pressure
By the principle of osmosis, jams, jellies and pickles are preserved.
i. High Concentration of Sugar
Apples, guavas, grapes and pineapples are suitable for making jams and jellies. For making jams and jellies, the fruit should be just ripe because the pectin content is high in such fruits.
ii. High Concentration of Salt
Salt is used to control microbial population in foods such as butter, cheese, vegetables, meats, fish and bread. Spices and other condiments have bacteriostatic effect. In addition to salt and several spices, oils are used in making pickles. Aerobic bacteria and mould growth are prevented by covering the top with oil. Properly prepared and stored pickles can last upto a year or more without spoilage. The important preservative agents in pickles are salt, vinegar, sugar, oil, spices and condiments.
Preservation by Dehydration
Dried foods are preserved because the available moisture level is so low that microorganisms cannot grow and enzyme activity is controlled. Dehydration processes are used commercially for many foods including dried milks, eggs, coffee, tea, fruit drinks, dessert mixes and traditional dried fruits, vegetables, meat and fish. As a result of heat applied during the drying process many of the organisms present in the food are destroyed. Bacteria require 18% available moisture for their growth, yeast 20% or more and moulds 13-16 %. It is therefore essential in the preservation of food by drying to reduce moisture as much as possible without damaging the essential quality of the food.
Food irradiation is a process of food preservation in which food is exposed to ionizing energy - radio isotope cobalt - 60. The potential uses of food irradiation are
To avoid the use of harmful chemical compounds like methyl bromide and ethylene oxide for insect disinfestation in stored products and microbial decontamination of spices.
1. To extend shelf life of meat, poultry and sea foods by killing microorganisms causing their spoilage.
2. To enhance safety of food by killing food borne pathogenic microorganism and parasites.
Food irradiation reduces post harvest storage losses. Irradiation at the appropriate level does not change the flavour, taste, smell, texture and mineral contents of foods. Irradiated foods are safe and wholesome for human consumption.