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Microbiology of Food – Causes of Food Spoilage and Food Poisoning


Food is an indispensable item for all living organisms. All food items are associated with microorganisms in one form or other. Foods get contaminated during handling, harvest, transport and storage. Foods also get contaminated due to the methods of food collection, cooking and preparation. Food forms an ideal culture medium for the growth and multiplication of microorganisms. Some of the microorganisms particularly pathogenic forms causing dreadful diseases and food poisoning by their secretions. There are some microorganisms, which are useful for the preparation of different types of food and beverages. The microorganisms themselves play an important role in formation of food (eg.) single cell protein and mushrooms.

Common food items Foods may be classified as 

  • a) fresh foods, 
  • b) preserved foods, 
  • c) canned foods, 
  • d) processed foods, 
  • e) fermented food products.
Common food items as follows: 
  • Fruits and vegetables 
  • Milk 
  • egg 
  • Meat 
  • Fish 
  • Poultry 
  • Bread 
  • Pickles 
  • Syrup 
  • juices Products 
  • milk, 
  • vegetables 
  • fruits
Sources of microorganisms in foods 
Foods receive the population of microorganisms from soil, plants, cooking vessels, by the use of contaminated water for washing and cooking and also due to unhygienic habitats of food handlers, intestinal tract of humans and animals, animal feeds, animal hides, air and dust.
Factors that influence the growth of the microorganisms Many factors that influence the growth of the microorganisms in food. Some of the factors are intrinsic and some others are extrinsic.
  • a) Intrinsic factors: The intrinsic factors include pH, moisture content, oxidation-reduction potential, nutrient status, antimicrobial constituents and biological structures
  • b) Intrinsic factors: The intrinsic factors include pH, moisture content, oxidation-reduction potential, nutrient status, antimicrobial constituents and biological structures
  •  pH : It has been well established that most of the microorganisms grow best at pH values around 7.0, while few grow below 4.0. Bacteria grow at more pH than molds and yeasts. 
  • ii) Moisture content: The preservation of foods by drying is a direct consequence of removal of moisture, without which microorganisms do not grow. The water requirements of microorganisms should be defined in terms of the water activity (aw) in the environment. Water activity is defined by the ratio of the water vapour pressure of food substrate to the vapour pressure of pure water at the same temperature (aw=P/po). The aw of most fresh food is above 0.99. The minimum value of aw for the growth of the microorganisms in foods should be around 0.86. 
  • iii) Oxidation reduction potential: The O/R potential of a substrate may be defined generally as the ease with substrate gains electrons. When an element or compound loses electrons, the substrate is said to be oxidized, while a substrate that gains electrons becomes reduced. 
  • iv) Nutrient content: In order to grow and function normally, the microorganisms of importance in foods require water, source of energy, source of nitrogen, vitamins and related growth factors and minerals. 
  • v) Antimicrobial constituents: The stability of foods against attack by microorganisms is due to the presence of certain naturally occurring substances that have been shown to have antimicrobial activity. Some species contain essential oils that possess antimicrobial activity. Among these are eugenol in cloves, allicin in garlic, cinnamic aldehyde and eugenol in cinanmon.
  • b) Extrinsic factors: These include those properties of the storage environment that affect both the foods and their microorganism. The following extrinsic factors affect the growth of microorganisms: Storage temperature, pH, presence and concentration of gases in the environment.
Causes of food spoilage 
Food spoilage refers to the process where the food is made useless, bad and unfit for eating. It alters the chemical proportion appearance, texture, colour, taste, flavour, odour and stability of the food. The altered food is called spoiled food. Food is spoiled by many factors such as 
  • a) microorganisms, 
  • b) insects, 
  • c) rough handling, 
  • d) transport, 
  • e) improper storage, 
  • f) enzyme activity, 
  • g) unhygienic conditions.
Causes of food poisoningFood poisoning refers to the toxicity introduced into food by microorganism and their products. Food poisoning is caused by various factors as follows: 
  • a) Poisons derived from plant and animal sources 
  • b) Such standard chemicals added to the food. 
  • c) Excess use of preservation in food. 
  • d) Presence of higher population of microorganisms in food 
  • e) Toxins produced by various types of microorganisms
Types of food poisoning 
There are two types of food poisoning. 

1) Food intoxication. eg) Botulism, Staphylococcal food poisoning.
2) Food infection. eg) Shigellosis (Bacillary dysentry), Enteropathogen Escherichia, Cholera, Brucellosis

Food borne diseases 
The common food borne diseases are Botulism, Staphylococcal food poisoning, enterococcus food poisoning, Traveller’s diarrhoea, Mycotoxicosis, Sligellosis, Enteropathogenic Escherichia, Cholera, Brucellosis, Tuberculosis and Tularemia. 
  • i) Botulism : Botulism is a food borne disease due to exotoxin produced by the bacterium Clostridium botulinum. The main sources of this disease are canned food and preserved foods. This disease affects the nervous system so it is called neurotoxin.
  • ii) Staphylococcal food poisoning or staphylococcal enterotoxemia : The causative organism for the disease is Staphylococcus aureus. The main sources for the disease are potato salad, cream-filled bakery goods and dry skim milk. The disease is characterized by sudden nausea, vomiting and diarrhoea. 
  • iii) Enterococcus food poisoning : The causative organism for this disease is Strepcococcus faecalis. It is frequently found in the intestinal tract of human and animal. The disease is characterized by nausea, frequently vomiting, colicky pain and diarrhoea.

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