METHODS OF FOOD PRESERVATION
A perusal of the history of food preservation reveals that food preservation had its beginning from time immemorial and could be traced to nearly a thousand years ago.
Salting of meat, fish, and vegetables was the oldest method of preservation and could be traced back to the ancient Egypt and Greek civilisations.
Pickling in salt and vinegar, sun-drying and preservation of fruits and vegetables in sugar and honey were among the other methods used.
Storage of food in frozen conditions was also practiced for centuries in places where freezing temperatures were recorded.
The discovery of canning as a standard technique of preserving foods in sealed containers subject to high temperature was established in 1810 by Nicholas Appert. Around 1860, Louis Pasteur discovered that microbes were the main cause of spoilage and introduced a heat treatment known as pasteurisation to the world.
All methods used for food preservation are based on preventing or retarding the cause of spoilage.
When growth of micro organism is only retarded, preservation is temporary. When spoilage organisms are completely destroyed a more permanent preservation is achieved.
Use Of Low And High Temperatures : Use of Low Temperatures
Microbial growth and enzyme reaction are retarded in foods stored at low temperatures.
The lower the temperature, the greater the retardation. The low temperatures employed can be:
1. Cellar storage temperature (about 15º C)
2. Refrigerator or chilling temperature (0º C to 5º C).
3. Freezing temperature (-18º C to -40º C).
Cellar storage temperatures (about 15º C):
Temperatures in cellars (under ground rooms) where surplus food is stored in many villages are usually not much below that of the outside air and is seldom lower than 15º C.
The temperature is not low enough to prevent the action of many spoilage organisms or of the plant enzymes. Decomposition is however, slowed down considerably.
Root crops, potatoes, onions, apples and similar foods can be stored for limited periods during the winter months.
Refrigerator or Chilling Temperature (0º C to 5º C)
Chilling (refrigerator), temperatures are obtained and maintained by means of ice or mechanical refrigeration.
Fruits and vegetables, meats, poultry, fresh milk and milk products, fish and eggs can be preserved from two days to a week when held at this temperature.
In addition to the foods mentioned above, foods prepared for serving or left-overs may also be stored in the household refrigerator. The best storage temperature for many foods, eggs, for example, is slightly above 0º C.
The optimum temperature of storage varies with the product and is fairly specific for any given food.
Besides temperature, the relative humidity and the composition of the atmosphere can affect the preservation of the food.
Commercial cold storages with proper ventilation and automatic control of temperatures are now used throughout the country (mostly in cities) for the storage of semi perishable products such as potatoes and apples. This has made such foods available throughout the year and has also stabilized their prices in these cities.
Low temperatures chiefly inhibit the growth of microorganisms although freezing may result in the destruction of some microorganisms.
Freezing may preserve foods for long periods of time provided the quality of the food is good to begin with and the temperature of storage is far below the actual freezing temperature of food.
Some microorganisms are destroyed during freezing preservation. The chief preservative effect of freezing temperatures lies in the inability of microorganisms to grow at freezing temperature.
In vegetables, enzyme action may still produce undesirable effects on flavour and texture during freezing. The enzymes therefore must be destroyed by heating before the vegetables are frozen.
Slow freezing process
It is also known as sharp freezing. In this method, the foods are placed in refrigerated cabinets at temperatures ranging from -4º C to -29º C. This method is adopted in home-freezers. Freezing may require from 3 to 72 hours under such conditions.
Quick freezing process
The lower temperatures used -32º C to -40º C freeze foods so rapidly that fine crystals are formed and the time of freezing is greatly reduced over that required in sharp freezing.
The fine crystals formed by quick freezing have a lesser effect on breaking up plant and animal cells than do methods of slow freezing that produce coarser ice crystals.
In quick freezing, large quantity of food can be frozen in a short period of time.
Dehydro 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.
The quality of dehydro frozen fruits and vegetables is equal to that of fruits and vegetables frozen without preliminary drying.
The cost in packing, freezing, storing and shipping of such foods is less because of the reduction in weight and volume of foods during dehydro-freezing .
The following figure depicts the relationship between bacterial growth and temperature. The rapid growth of bacteria occurs in the temperature zone of 60 o F -120 o F
Use of heat or high temperatures:
The destruction of microorganisms by heat is due to the coagulation of the protoplasm. The temperature and time used in heat processing a food depend upon the nature of the food and what other methods are combined with heat.
The various degrees of heating used in preservation of food can be classified into three (a) Pasteurisation, (b) Heating upto 100º C or 212º F and (c) Heating above 100º C.
The time and temperature used in the pasteurization process depend upon the product treated and the method used. In pasteurization, most of the spoilage organisms are killed but a few survive and hence must be inhibited by low temperatures or some other method, if spoilage is to be prevented.
There are two methods of pasteurization - flash method and holder method. In flash method, otherwise called high-temperature short time method, a high temperature for a short time is used, while in the holder method or low-temperature long-time method, a lower temperature for a longer time is used. There are slight variations in the time and temperature used for pasteurizing different foods, like milk, cream, ice cream mix and wines.
(b) Heating up to a temperature of about 100º C
Most methods of cooking come under this. This temperature can be obtained by boiling any liquid food, by immersing a container in boiling water or by exposure to steam. Before the use of pressure cookers and autoclaves, canning was done at 100º C and this killed all bacteria except spores.
(c) Heating above 100º C(212º F)
Temperatures above 100º C are obtained by means of steam under pressure as in a pressure cooker or autoclave. Sterlisation of foods can be brought about at 121º C for 15 minutes under moist conditions.