are present almost everywhere: in the air, soil, on our hands, in
our bodies and in food. Not all of these tiny organisms are harmful
and some are essential for good health and the production of food
and drugs. Some can cause food spoilage and illness. Most forms of
food processing either destroy these micro-organisms or reduce their
numbers to safe levels.
can grow and multiply at temperatures between 15°C and 63°C, with
most rapid growth occurring around 37°C. It is important that food
is not held in this temperature range for long periods, as the food
may become contaminated with large numbers of micro-organisms and
cause illness. At higher temperatures most harmful microorganisms
are destroyed, and at lower temperatures, such as in the refrigerator
(1-4°C) or the deep freeze (-18°C), there is little or no growth.
When cold foods are warmed, the micro-organisms will start to grow
and multiply. Therefore it is important to heat food rapidly. The
shorter the time spent in the temperature range where rapid growth
of micro-organisms occurs, the lower the chance of food poisoning.
Some frozen foods
such as vegetables, precooked foods and smaller cuts of meat can be
cooked directly from the frozen state. Large cuts of meat should be
thawed prior to cooking or extra cooking time should be allowed to
ensure that the interior temperature reaches 71°C. Thawing is best
carried out by placing the food in a refrigerator, allow at least
16 hours per kilogram. If not used after thawing it can be kept in
the refrigerator chilling section for 1 to 2 days. Food that has been
allowed to thaw in the kitchen should be cooked soon after thawing
and not stored in a refrigerator. Packaged frozen foods have instructions
on how best to prepare the food for eating. It is not advisable to
re-freeze foods that have been thawed.
are very widespread, contamination of food can occur easily. Personal
hygiene and a few precautions can prevent this leading to food poisoning
and illness (see
Figure 39 and the picture above).