A micro-organism is an unicellular plant or animal of such a small size that it cannot be seen without a microscope. Protozoa are microscopic one-celled animals, while bacteria, yeasts, moulds and fungi are forms of plant life. All living micro-organisms have the ability to utilise food substances for growth and energy purposes. They have the capacity to multiply and react to environmental changes. Microbiology is the study of the structure, function and the uses of micro-organisms. Microbiology is of special importance in modern food technology.

Bacteria are single-celled plants found in soil, water, dust and air. Certain bacteria that cause disease are called pathogens. Many bacteria are useful to man such as those which ferment apple juice to produce cider. Some bacteria are capable of turning dead plants into manure. Certain bacteria can synthesise Vitamin B. Some have the capacity of fixing atmospheric nitrogen in the soil, thus enriching the latter. Bacteria may be destroyed by sunlight, ultra-violet rays, extreme heat and the use of certain chemical substances. Freezing and drying inhibits bacterial growth. An average bacterium is one micron, i.e., 1mm in size. Aerobic bacteria thrive and multiply in the presence of air while the 1000 anaerobic bacteria can do without air.

All micro-organisms grow well at body temperature i.e., at 98.6° F or 37° C. Bacteria grow and multiply rapidly between 20°C-43°C (68°F-109°F). When the temperature goes beyond 40° C (104°F) there is a definite decline in bacterial growth. Bacterial growth nearly ceases at 45°C (113°F). Non-sporing bacteria are killed at tempera­tures of about 60°C (140°C) but the time (duration) needed to kill the bacteria varies considerably. Milk will be safe to drink when it is heated to 62°C (l43°F) for 30 minutes. Extreme cold temperature does not kill bacteria; it only inhibits their growth and activity. When foods are kept below 10°C and above 65°C the danger of bacterial or microbial contamination is almost negligible.