What are micro-organisms, and what factors affect their growth?

There are three types: bacteria, moulds and yeasts. Bacteria and yeasts
cannot be seen with the naked eye, but moulds are often visible because they form
visible thin threads (filaments) or a solid cluster. Just like humans,
micro-organisms require certain minimum living conditions. They
cannot survive without:

* sufficient water
* oxygen
* the right degree of acidity
* nutrients
* the right temperature
Water is essential for maintaining many physical processes. Where
there is a shortage or lack of water micro-organisms cannot grow, such
as in dried legumes. Drying is therefore one way to prevent spoilage.
Meat and fish do not have to be 100% dry in order to preserve them.
By adding salt, the remaining water becomes unsuitable for microorganisms.
The same effect can be achieved by adding sugar to fruit.
Enzymatic spoilage is also inhibited by drying.

Most micro-organisms need oxygen. If there is a shortage of oxygen,
it is difficult for bacteria to survive, let alone multiply. But there are
always a few that manage to survive. As soon as the oxygen supply is
increased, these remaining bacteria will again grow and multiply.
Some types of micro-organisms even thrive in an oxygen-poor environment.

Bacteria grow best in an environment that is not too acidic. Less
acidic products are therefore especially susceptible to bacterial spoilage.
Examples of such products are meat, eggs, milk and various types
of vegetables. Beer, yoghurt, wine, vinegar and fruit are less sensitive
because they are more acidic. Adding acidity to products slows down
the process of microbial spoilage. The degree of acidity is measured as
a pH level. A neutral product like milk has a pH of 7; meat has a pH of
about 6, carrots have a pH of 5 and oranges about 4. The more acidic a
product is, the lower the pH value will be.

Just like humans, micro-organisms also need nutrients: sugars, proteins,
fats, minerals and vitamins. These are rarely in short supply,
because they can be found in all food products.

To thrive, micro-organisms need a temperature of between 5 and
65°C. At temperatures above 65°C it becomes very difficult for them
to survive; and they definitely die if boiled, as long as they are boiled
for a certain length of time, such as 10 minutes. When heated, the
micro-organisms slowly die off, but not all at the same time. Heating
at temperatures lower than 100°C thus has to be sustained for a longer
period. The growth of micro-organisms is also slowed down significantly
at temperatures between 0 and 5°C (as in a refrigerator), which
makes it possible to store the food products for a few additional days.
At temperatures below 0°C microbial growth is stopped completely,
but the micro-organisms themselves remain alive. They will become
active again as soon as the temperature rises above 0°C.

To preserve food, it is sometimes necessary to make drastic changes to
the micro-organisms’ living conditions. We can remove water (drying),
increase the acidity, or first heat the products (to kill the bacteria)
and then store them in air-tight containers to prevent oxygen from
entering (preserving/canning). These and other methods will be discussed
later in this booklet.

Do micro-organisms grow differently on vegetables and fruit?
Vegetables and fruit have a lot in common. But there are also important
differences, which determine the type of spoilage they are most
susceptible to. Damaged fruits, which are usually somewhat acidic,
are very susceptible to the growth of yeasts and moulds. Vegetables
are generally less acidic, and their spoilage is usually caused by bacteria.
Though not visible to the naked eye, bacteria can still be present
in large numbers.