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General Microbiology

Lecture 14

Food Microbiology

Micro-organisms play many roles in food some desirable and helpful and some undesirable and detrimental

Good effects of Microbes in food

Food Fermentations

Microbes are deliberately grown in food to produce required changes. Microbes that can not be reproduced synthetically produce these changes. The changes are usually complex and the microbes can often be grown economically.

Alcoholic fermentations
  Beer, wine, saki etc.

Dairy fermentations
  Cheese, yoghurt, butter,
  Sauerkraut, Tempe, Soy sauce

Incidental Fermentations

Some food processes do not use microbes for the main changes but fermentations may contribute incidentally to flavours
 Coffee, Cocoa, and Tea

Single Product Production

Fermentations can be used to produce ingredients used in food such as Citric Acid, Vitamin C, Vitamin A, and Enzymes.

Microbes as Food:

Microbes can be grown as food. This usually involves converting some cheap waste or biproduct of another process in to microbial cells that can be user as human food or fed to stock to convert it to meat or milk.
 Eg Singe Cell Proteins
  From Waste petroleum material
   Paper production waste (sulphite liqueur)
   Molasses (waste from sugar processing)

In some areas it is cheaper to grow a high yielding carbohydrate crop like cassava and the convert it to SCP. This produces more grams of protein per acre than growing a high protein crop like soybeans.

Bad Effects of Microbes on Food

Food Spoilage

This is when microbes produce a negative effect on food that make that food less desirable. The changes produced may be called undesirable in one culture but considered desirable in another. It may or may not make the food dangerous to eat. But is always obviously changed so it is rarely eaten. In this way spoilage as a beneficial effect by stopping us from eating food that might be dangerous
If food is cooked the food spoilage organisms are usually killed. This allows
Food poisoning bacteria to grow with out us knowing. This is because food poisoning organisms donít make obvious changes.

There is many different type of food spoilage but they are usually caused by saprophytes such as Pseudomoni and fungi. Most often plant material is spoiled by fungi

Food Born Disease

Food Poisoning

Diseases can be caused by microbes in food producing a toxin. In most the organisms must grow in the food while they make the toxin. These include e bacteria, fungi. Toxins can accumulate in the bodies of sea food these are be caused by algae that the fish eats in the sea.

Bacterial food poisoning:

 Staphylococcus aureus
 Clostridium botulinum
 Clostridium perfringens
 Bacillus cerus

Fungal food poisoning

 Aflatoxin   Aspergillus flavis
 Ergot        Claverceps purpurea
 Yellow Rice Disease   Penicillium

Algae Food Poisoning

 Red Tide    Dinoflagellates
 Physteria ( Physteria piscicida)

Food Borne Infections

With infections the organism must grow in the human to cause an infection. This may be confined to the intestine to cause gastro-enteritis or they may invade the rest

Food Borne Infections that require a Large Inoculum

These diseases are coursed by week pathogens that require large numbers to cause a disease and must grow in the food before it is eaten

Eg Salmonella, E. coil, Vibrio Paraheamolytica , Listeria
Food Borne Infections that require a Small Inoculum

There are many parasites that can cause infections with only one or two cells
These do not need to grow in the food and can be transmitted by Just a Touch from an infected individual
Many of these are Zoonoses ie disease of other animals and include bacteria, viruses and parasites


Typhoid  Salmonella typhi
Cholera    Vibrio cholerae
T.B.    Mycobacterium tb.
Q- fever     Coxiella burnetii


Hepatitis A



Tape worms
Cryptosporidium parvum