Food and Nutrition Notes

Preservation of Foods and causes of food spoilage


Food commodities contain various nutrients which are useful to the human body for the performance of its various functions. Unfortunately this reason also makes food easily susceptible because these nutrients which are useful to man are also useful to micro- organisms which grow on our food, consume them and make the left over unfit for human consumption.

Hence, food preservation is the caring for or treating food in such a way that it will be kept in good condition for a long period of time.


  • To prolong its shelf life.
  • To prevent its spoilage to prolong its shelf life.
  • To avoid wastage especially when they are in season.
  • To allow foods in off- season to be used.
  • To have a variety of food available for an emergency situation.
  • To eliminate the purchase of food when they are most expensive.
  • To introduce variety into the family menu.


Food spoilage usually refers to undesirable changes taking place in food which eventually leads to rejection and therefore to wastage. The following are causes of food spoilage;

  • Actions of micro-organisms e. g. bacterial yeast and moulds.
  • Insect.
  • Food enzymes.
  • Chemical reactions in the food.
  • Physical changes due to freezing burning, drying, pressure and so on.


Food can be classified into three major groups

  • PERISHABLE FOOD OR HIGH MOISTURE FOODS:- These are foods which begins to spoil almost immediately unless preserved in some ways because of their high water content. Examples of such foods are milk, meat, fish, sea foods, fruits and vegetables.
  • SEMI- PERISHABLE FOODS OR INTERMEDIATE MOISTURE FOODS:– These are foods which can be kept for sometimes before going bad. Their water content is low. Examples of such foods are root vegetables and tubers.
  • NON- P-ERISHABLE FOODS OR LOW MOISTURE FOODS:– These are foods which can be kept for a reasonably long period without getting spoilt due to their low water content. Examples are nuts, dried legumes and cereals.

The moist obvious difference between foods in these three groups is their water content which is high in perishable foods. The firm covering foods such as nuts, cereals and legumes also offers protection.


There are different methods of preserving foods. However the type of method 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:-

  1. DRYING: This is the process by which the water content of the food is reduced. Both food enzymes and micro- organisms require water for action and growth, therefore the removal of water from the food creates an unfavourable environment for their actions, hence they are unable to cause food spoilage.

Examples of food preserved by drying are legumes, cereals, meat and fish. There are different types of drying.

  • Solar drying: – This is drying by mere spreading the food stuff in the sun on a mat or concrete floors e. g. cassava, maize, melon etc.
  • Oven drying: – This is drying by putting the food in pre- heated oven or enclosed chamber e. g. melon, legume.
  • Freeze drying: – The water content of the food stuff is frozen and then sublimed under vacuum. Heat is supplied by conduction or radiation. Example of food dried by these methods is, fish, meat and vegetables.
  • Roller drying: – The liquid or slurry to be dried is uniformly applied on a hot rotating stainless steel drum or roller. As the drum rotates, the food dries and is scraped of by a knife. Example are potato flakes, cocoa- oats, yam flakes etc.
  • Vacuum drying: – Water is removed by creating vacuum. Heat is supplied by condition or radiation. Example, milk.
  •  Tunnel drying: – Fried food placed in conveyor belt or perforated trays and passed through a warm air tunnel. Example, vegetables.
  • Spray drying: – Liquid is forced through a small nozzle in the form of a spray into a hot chamber, the spray mixes with the warm air and the water is evaporated while a fine powder is produced. Examples of foods preserved by this method are milk, egg white, bournvita, cocoa beverages.


  • The efficiency of the drying equipment.
  • The method of arrangement of the foodstuff on the drying material. e. g. tray, mat, concrete floor in relation to the heat source or medium.
  • The physical and chemical properties of the foodstuff.
  • The amount of surface area of the foodstuff exposed to the dry air or heat.
  • LOW TEMPERATURE TREATMENT: This is keeping the food at a low temperature by freezing below O C in the freezer and chilling at a higher temperature in the refrigerator. E. g. fish, meat, milk fruits, vegetables.
  • HIGH TEMPERATURE TREATMENT: This is the application of heat to food stuff to destroy the food enzymes and micro- organisms. It could be by pasteurization- to destroy only the pathogenic micro- organism with minimum effect on the nutritional properties of the food. Usually at a temperature below 100oc e. g. milk.
  • USES OF CHEMICAL: This is the addition of chemical to food to inhibit the action of food enzymes, Create an unfavourable environment for the growth of micro- organisms and also prevent chemical reaction from taking place in the food.

There are three types of chemical preservation.

  • Natural chemical
  • Synthetic
  • Antibiotic

The natural chemical preservatives are the chemical that are present naturally in the food substance themselves or in other natural substance that interact with the food. Examples of natural preservatives are vinegar, salt, sugar, phenols and Aldehydes. Examples are synthetic chemical preservatives are benzoic acid; nitrites butylated hydroxy toluene (BHT) Examples of antibiotics are – theabendazole, Nisin and tetracyclines.

  • CANNING AND BOTTLING: This is keeping food aseptically in cans or in bottles. The food is sterilized and scaled in the vacuum or in the presence of inert gases and them in cans or bottles. E. g. fruit juices, meat, fish, beverages, etc.
  • IRRADIATION: This is involves the use of radioactive element like cobalt, to destroy micro-organism and food enzymes e.g. potato, yam tuber and onion.
  • FERMENTATION: This very old method of preserving food is still being practiced. It involves the decomposition of some of the food component to produce acid. The acid so produced will reduce the pH of the food and hence make the food unfavourable to the mirco- organisms. Example of food preserved by fermentation are locust beans seed, (iru) melon paste (ogiri) and wines .Other product of fermentation in the foods are cheese, pitto, fermented fish, fermented plantain, garri and corn paste/ dough.
  • SMOKING: This is drying of food stuff over a fire or heated charcoal, the smoke produced contains some aldeheydes and phenols which have a strong bacteria effect e. g. fish, meat.


  • State and explain the various types of drying
  • State four factors affecting the rate of drying


Write comprehensive note on the use of synthetic and antibiotic methods of chemical drying


  • state the principle underlying food preservation
  • Mention causes of food spoilage
  • Explain perishable foods with examples


mention and explain types of home drying.

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