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Once the spoilage agents are known and their characteristics understood, it becomes easier to apply these knowledge to prevent deterioration and spoilage. From the study of the characteristics of enzymes and micro-organisms, it is clear that their activities are dependent upon several factors and that any drastic change in these factors results in their decreased performance.
The principles of food preservation are based precisely on this knowledge and on the understanding of how chemical reactions, physical changes and pest activities collectively reduce the quality of foods. These principles may be stated as follows:
Principle 1:
Autolysis of foods may be prevented or delayed by the destruction or inactivation of the enzymes and by the treatment and handling of food in such a manner as to inhibit the reactivity of chemically active molecules.
Principle 2:
The spoilage of foods as a result of microbial activity may be prevented or delayed by either prohibiting the entry of micro-organisms into the food, physically removing micro-organisms from the food, hindering their growth and activity or even destroying them, if they are already part of the food.
Principle 3:
Quality defects and losses caused by insects, rodents and birds can be controlled by adequate packaging and by instituting a sustained population control programme of such pests.
Principle 4:
The deterioration of foods caused by the physical phenomena that occur during processing, handling and storage may be reduced by the development of optimal and compatible processing, handling and storage conditions.
THE EXPLANATIONS OF THESE PRINCIPLES:
A). PREVENTION OR DELAY OF AUTOLYSIS:
Enzymes require specific temperature range, PH range and the presence of moisture in the environment in order to catalyse the life reactions.
- Temperature:
Some fruits and most vegrtables are normally subjected to around 100°c for a very short time(few seconds to few minutes) in an operation called blanching.
- Another procedure employed is the use of low temperature(4°c -10°c). E.g. inside the refrigerator.
- Some foods can be preserved by merely altering the PH environment of the food. E.g., in biological preservation of vegetables called pickling, PH of the materials is changed through the activities of lactic acid bacteria or by the addition of preformed organic acids. Some meat and milk products are likewise preserved as a result of changes in the PH.
- Removal or binding of moisture: The moisture in the food may be removed by sun-drying or dehydration or it may be bound with the help of some chemicals such as sugar, sorbitol or glycerine, generally known as humectants.
B). PREVENTION OR DELAY OF MICROBIAL ACTIVITIES:
While enzymes are part and parcel of fresh foods, micro-organisms are invaders.
- Micro-organisms are kept out of foods by ensuring a proper protective covering over the food- a technique called ASEPSIS. This protective covering can be in the form of a containers(bottles or cans). Some foods are given a coating of wax or other inert materials. E.g. coating of cheeses and eggs with wax or similar substances.
- Filteration: Some liquid foods such as clear fruit juices, wines and beer, the micro-organisms can be picked up and thrown out with the help of special filters.
- Centrifugation: In milk, centrifugation is applied to separate cream or the fatty portion from the skim milk. During this process quite a number of micro-organisms are separated from the milk fraction and removed with the cream.
- Exhausting: Creation of anaerobic conditions will prevent growth of vegetative cells and germination of spores. This condition is usually created in canning of foods by the use of mechanical vacuuming or in an operation called EXHAUSTING.
- Another technique is the removal or binding of moisture.
- Use of chemical substances, generally known as PRESERVATIVES. The use of sulphur dioxide and benzoic acid(added in the form of potassium metabisulphite and sodium benzoate, respectively) in fruit juices protect them from spoilage in sealed containers. While some chemicals may be intentionally added, others may be developed or produced in the food. Lactic acid is a good example. Lactic acid is an organic acid produced by the activities of lactic acid bacteria.
- Temperature is yet another very useful tool in controlling micro-organisms activity. Use of low temperature(chilling or freezing). BLANCHING is used in the food induustry mainly to destroy or inactivate food enzymes. However, it also helps to destroy some of the vegetative forms of micro-organisms. PASTEURIZATION temperatures(65°c to 88°c). STERILIZATION temperatures(100°c and above). The heat treatment applied is a temperature/time dependent phenomenon.
- Use of radiations of various frequency. The radiations from the ultraviolet(uv) rays from the electromagnetic spectrum have germicidal properties. Gamma rays, emitted from by-products of atomic fission are also germicidal and when applied to foods help to preserve them by destroying the micro-organisms. Usually Cobalt 60 and Caesium 137 have been used for most of the experimental work on the applications of Gamma rays in food industries.
C). CONTROL OF PEST ACTIVITIES:
Traditionally, the food industry has controlled insect pests in grains and other dried foods by fumigation with such chemicals as methyl bromide, ethylene oxide and phostoxin.
In developing countries, premium is placed on systematic insect control programmes. A systematic control programme would include both preventive and curative measures. Systematic insect control is not an amaturist activity and should not be undertaken without professional guidance.
RODENTS when uncontrolled or poorly controlled, can be as devastating to our food stores as insects. Like insects, rodents consume foods and contaminate it with urine and droppings. These discharges are sources of some disease bacteria.
A typical systematic control of rodents would include the use of poison baits, rat traps and biological methods to kill off mice and rats.
BIRDS are not as closely associated with stored foods as are insects and rodents. However, birds can consume substantial quantities of grains in the field prior to harvest.
To prevent this, bird screens can be erected over important water supply sources and food handling areas. Also, bird invasion has traditionally been checked by installing noise-making dummys(shaped like humans or animals) at strategic points on the farm to scare birds away. The successful control of birds should always be under proper supervision.
D). REDUCTION OF PHYSICAL DEFECTS:
Added to the defects caused by autolysis, micro-organisms and biological pests, additional quality defects in food can be caused by physical and physio-chemical interactions. These interactions are induced by the series of treatments that accompany food manufacture, handling and storage.
The dessication or drying out observed on the surface of certain foods during frozen storage is an important physical defect that can be controlled by adequate packaging and the maintenance of a steady storage temperature.
In refrigerated storage of fresh foods, moisture loss and surface drying can be minimized by maintaining higher than conventional average relative humidity in the storage chamber. The increase in microbial activity induced by such high relative humidity is kept under check by the use of ultraviolet lamps as storage atmosphere sterilants.
Another common physical defect in food involves the crystallization of sugars from syrups and other sugar-based products. Such defective products develop a distasteful sandy texture. Unwanted crystallization of sugars in foods can be prrventrd by using sugar mixtures instead of a single type of sugar like sucrose. This knowledge is applied to the production of high sugar products like jams and sugar-based jellies.
Other attempts at reducing physical and textural defects in foods during processing and storage can be seen in the process handling and storage controlled applied by the food industry. For example, relative humidity and drying rates are controlled in dehydrators to minimize the incidence of case-hardening of products.
Stabilizers are used to prevent or suppress the collapse of emulsions during handling and storage and freeze-drying is used to remove moisture from heat-sensitive foods to enhance the retention of as much of the natural nutritional and textural properties on dehydration.
