Food preservation is the act of keeping food in good condition to prevent decay and be made readily available for consumption when needed. Decay is caused by the growth of microorganisms such as yeast, moulds and bacteria which are present in most fruits and vegetables. All foods decay sooner or later, but some do so much faster than others.
Food preservation is mainly done to achieve the following:
- Destruction of microorganisms
- Prevention of their entry into the food.
- Arrest of the action of food enzymes
- Arrest or prevention of purely chemical reactions.
The above principles can be achieved by employing different preservation methods
Reasons for Food Preservation
Foods are preserved for the following reasons:
- To have varieties of food available, in case of an emergency situation.
- To conserve food and prevent wastage in times of plenty.
- To avoid waste of money by purchasing food when they are expensive.
- To allow us buy and eat foods that are not in season.
- To prevent spoilage.
- To prolong its shelf life.
- To avoid wastage, especially when they are in season.
- To be able to take care of emergency situations.
Preparations to be Made before Preserving Food
- Some of the preparations that should be made before preserving our foods include the following:
- Foods to be preserved by freezing should be well packaged to prevent absorption of odour from other food stuffs in the fridge or freezer.
- Foods like onion, garlic, etc. should not be refrigerated with other food stuffs to prevent odour transfer.
- Expose as much surface areas as possible to facilitate drying.
- Examine drying equipment routinely to make sure they are in good condition.
- Use good quality chemicals for drying; always look out for expiry dates.
- Foods to be preserved should be properly cleaned or washed before preservation.
- Canned or bottled foods should first be heat treated to make them sterile.
- Care should be taken and precautions observed while preserving foods by irradiation.
- If possible, avoid the use of antibiotics as a preservative to prevent resistance to these drugs.
- Ensure that the food is of good quality i.e. wholesomeness of the food.
- Package food appropriately and label if need be.
Methods of Food Preservation
There are different methods of preserving foods. However, the type of methods used depends on the nature of the food, length of preservation required, the facilities available and the type of dish to be prepared. The different methods of food preservation are discussed below.
This is the process by which the water content (moisture content of the food) is reduced to create an unfavourable environment for the action of food enzymes and microorganisms, hence preventing spoilage.
Types of Drying
- Sun drying or solar drying e.g. cassava, maize, melon, cereals, legumes, etc.
- Oven drying e.g. melon, legumes, yam, etc.
- Roller drying e.g. vegetables.
- Vacuum drying e.g. milk.
- Tunnel drying e.g. vegetables.
- Freeze drying e.g. vegetable, fish, meat.
- Spray drying e.g. milk, egg white, cocoa beverages.
- Roasting on fire/hot oven e.g. meat, fish.
LOW TEMPERATURE TREATMENT
This is keeping the food at low temperatures by freezing below 0oC in the freezer and chilling at a higher temperature in the refrigerator e.g. fish, meat, milk, fruit, vegetables, etc.
HIGH TEMPERATURE TREATMENT
This is the application of heat to food stuffs to destroy the food enzymes and microorganisms. It could be done by the following methods:
- Pasteurization – to destroy only the pathogenic microorganisms with minimum effect on the nutritional properties of the food, usually at a temperature below 100oC e.g. milk.
- Sterilization – to destroy all the microorganisms present in the food stuff at a very high temperature e.g. tomato puree, canned meat and fish, etc.
- Blanching – to kill insects and their eggs, pathogenic microorganism to inactivate the food enzymes. The process involves dipping food stuff into boiling water for 3 to 5 minutes e.g. vegetables. It is used with other preservative methods like freezing.
USE OF CHEMICALS
Chemicals are added to food to achieve the following:
- To inhibit the action of food enzymes.
- To create an unfavourable environment for the growth of the microorganisms.
- To prevent chemical reactions from taking place in the food.
- Tieing up the moisture present in the food.
Examples of natural chemical preservatives are vinegar, salt, sugar, phenols and aldeheydes.
CANNING AND BOTTLING
This is keeping food especially in cans or bottles. The food is sterilized, sealed in a vacuum or in the presence of inert gases and then put in cans or bottles e.g. fruit juices, meat, fish, fruit purees, beverages, etc.
Irradiation involves the use of radioactive elements like cobalt to destroy microorganisms and food enzymes e.g. potato, yam tubers and onion.
Fermentation involves the decomposition of some of the food components to produce acids that make the food unfavourable to microorganisms, e.g. locust bean seeds, melon seeds and wines.
Smoking is the drying of food stuffs over a fire for different reasons. Some of these reasons may be that the food is not needed immediately or it is purchased in excess and cannot be completely consumed.
The nature of the food will determine the storage method to adopt. However, most leftover cooked foods are stored in the refrigerator (where available) or in the food cupboard. Cereals and legumes are usually stored in containers with tight fitting covers, in dry and airy places. Onions are usually stored in open, dry airy places. Tubers e.g. yam are also stored in open, airy places on raised platforms.