What do micro-organisms do to fruits and vegetables?

Micro-organisms take from food products the various substances they
need to survive and multiply. Their secreted waste products can have
either a negative or positive effect on the affected food and the humans
who eat it.

The Positive effects of micro-organisms in food

The waste products secreted by some micro-organisms can have a
positive effect on food. Lactic acid bacteria, for example, are used to
make cheese and yoghurt from milk, and sauerkraut from white cabbage.
Moulds are used to make tempeh from soy beans, and yeasts are
used to make beer and bread. These substances influence the taste and
structure of the food products and generally increase their shelf-life.
The products can be kept longer because the desired micro-organisms
decrease the food’s pH level or because they are present in such huge
numbers that other micro-organisms have no chance to grow. This use
of micro-organisms for the preparation of food is called fermentation.

The Negative effects of micro-organisms in food

Sometimes the negative effects of bacteria are clearly apparent, such
as when milk has turned sour and curdled, when meat is covered in
slime, when moulds and gasses have formed, and when food has a
distinctly putrid smell. However, food spoilage is not always this obvious.
There are bacteria whose presence in food does not always
cause a change in its taste or appearance. In any case, it is important to
avoid eating rotten food, because it can make a person seriously ill.
Eating rotten food can cause contamination or poisoning. A food con-
tamination occurs when a person consumes a large number of living
micro-organisms in a meal. These can multiply rapidly in the person’s
gastrointestinal tract and severely disturb the digestive system. The
result is often diarrhoea and sometimes also bleeding. The symptoms
appear between 3 and 24 hours after eating the rotten food. A food
contamination can be prevented by frying or boiling the food thoroughly,
since sufficient heating will kill the micro-organisms.

Food poisoning occurs when a person consumes food containing the
poisonous waste products secreted by the bacteria. Heating the food
does not help in this case: the bacteria will be killed, but the poisonous
waste will remain unharmed. Both food poisoning and food contaminations
can be lethal, but usually they only make a person sick.