Creatures such as mice, rats, insects and microorganisms (bacteria, virus, yeast and moulds) cause food spoilage. Enzymes, also, cause food spoilage.
The metabolism of bacteria mostly take place in foods that have organic nitrogen. Examples of these foods include protein foods (Milk, meat, eggs, beans and peas are some of the examples).
Moisture has to be present for the biological mechanism to take place. In cases of frozen, dry, sugary or salty food, metabolism will not take place. Sugar and salt have higher concentration than the bacterial cytoplasm. This may lead to osmosis on the bacterial cytoplasm leading to the bacteria becoming dehydrated.
*freezing, adding salt and sugar and drying are forms of preserving food. The freezing and drying methods are, however, reversible once moisture is available for the bacteria. The reason behind this is the spores of some bacteria remain dormant under conditions of dehydration.
Bacteria do survive under a wide range of temperature. They are divided into three groups depending on their optimum temperature:
• Psychrophilic bacteria: they cause damage to refrigerated food because they survive and grow at a temperature of below 20° C.
• Mesophilic bacteria: they grow at a temperature of 37° C. Affects food such as milk and meat.
• Thermophilic bacteria: they grow in temperature up to 60°C. They cause little spoilage to food.
A temperature below the range of a group will slow down the rate of metabolism of a bacteria but will not destroy it.
A temperature above the range of a group will kill/destroy the bacteria. An example, the vegetative bacteria are killed are killed by temperature above 100° C.
Some bacteria are either:
• Aerobic in that they need oxygen to metabolise.
• Anaerobic in that they do not need oxygen to metabolize.
Enzymes control the metabolic activities of bacteria and they function at a certain pH and food poisoning need a pH of 7.
This includes the presence of poisonous chemicals or bacteria.
Poisonous chemicals include additives in food, weed killers, pesticides and heavy metals such as mercury and lead.
These foods contain natural toxins and can be dangerous if consumed in large amounts:
1. Onions
They contain anaemia producing ingredient which if taken in large amounts sometimes cause ill effects.
2. Potato
Potatoes which have turned green contain solanine which is dangerous if taken in large amounts.
3. Rhubarb
It has oxalic acid throughout the plant. If stems are eaten they have no effect but its leaves are dangerous because they have higher concentrations of the acid.
4. Cassava
It contains concentrations of cyanides and it can be poisonous when eaten raw. It can still be poisonous when processed if it contains II ppm.
5. Cabbage and cauliflower
They contain goitre producing compounds.

Moulds (mycotoxins) and fungi contaminate some foods.
A fungal disease in in rye, Ergot, produce a powerful toxic chemical in grains. Ergotism develops when the toxic is taken frequently.
A group of toxins produced by mould, Aflatoxins, cause illness in animals fed on imported food stuffs.
• Not all bacteria are bad. Bacteria in yoghurt and cheese are useful to the body.
• Bacterial infections can be prevented by good food hygiene.
• It causes food poisoning when ingested in large numbers by a healthy adult and in much lesser number when ingested by infants, children and the sick.
• Symptoms which include nausea and vomiting, abdominal pain, diarrhoea, headaches, faintness and chills appear 12 to 36 hours of eating and may last for 2 to 3 days.
• Main sources are human and animal faeces, duck eggs, poultry and fish.
• Cows and pigs, mainly, carry salmonellae in their intestines and there is cross contamination during slaughter and in the kitchen.
• Cold cooked meats, low temperature cooked food and mayonnaise spread salmonellae when eaten. Also in this list is water and milk contaminated by sewage and frozen chicken.
• Salmonellae causes typhoid and it can be destroyed by heating food.
• Staphylococcus aureus is the most common
• Symptoms which include salivation, nausea and vomiting, abdominal cramp, diarrhoea, prostration and sweating take half an hour to 3 hours to be manifested
after been eaten.
• Sources are the human throat, nose, mouth and nasal passage, human skin that has open wounds and abrasions, boils, clothing and dust.
• Contamination is caused through cold meat, meat sources, gravies, fish and fish products and dairy products.
• Staphylococcus is heat destroyed. The toxins produced are heat resistant and require more heat to destroy. Also, food should be handled properly and wounds be covered.

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