Food Bacteria Groups

Food temperature Both heating and cooling food can

change its susceptibility to microorganisms, and therefore its ability to stay fresh.

CoolingKeeping foods such as dairy produce, cooked foods and raw food ingredients in a refrigerator at a temperature from 0°C to 3°C will slow the growth of microorganisms such as bacteria, but won't stop it entirely.

Storing Food Separately

FreezingMany different types of food are quickly frozen in factories to maintain high standards of freshness and safety. Products are stored in commercial freezers at between -18°C and -29°C. Home freezers store products at -18°C. Freezing does not kill bacteria, but will keep them dormant.

Cook-chilled foodsCook-chilled foods can be cooked from frozen without defrosting. The downside is that cook-chilled foods have a short lifespan and have to be thoroughly reheated before eating - to at least 72°C - to avoid the risk of food poisoning. Leftovers should be disposed of quickly. In the factory, the foods are prepared by cooking them to 70°C for two minutes to destroy microorganisms. Food is packaged into portions and blast-chilled. Here the temperature is instantly lowered and stored at 0°C to 4°C.

Ultra Heat Treatment (UHT)UHT is used for products such as "long-life" milk. The product is heated to a temperature of 132°C to 140°C for one second to destroy all bacteria. Then the food is rapidly cooled.

Microorganisms in food

Microorganisms in food Three types of microorganisms affect

food: bacteria, moulds and yeasts. By a natural chain of events, these microorganisms will cause food to change - often, but not always, making food "bad" or unpalatable. The graphic should help you to remember the difference between bacteria, moulds and yeasts.

Microorganisms in food

Food acidity and micro-organisms Food acidity and micro-organisms Some foods, such as citrus fruits, are

acidic; others, such as sodium bicarbonate, are alkaline. The more acid the food is, the lower it will be on the pH scale. The more alkaline the food is, the higher it will be on the pH scale. Foods that are neither acid nor alkaline (for example pure water) are called neutral and have a pH value of 7.

Food acidity and micro-organisms

Acidity or alkalinity Acidity or alkalinity of food is important

for two reasons: it affects the taste of a product and it affects the rate at which microorganisms grow within and upon food. For example, lemon juice can be used to reduce the pH level of yoghurt, to a pH below 4.5. This inhibits the growth of microorganisms, making the yoghurt last longer, but it will also give it a sharp taste.

Oxidation of foods Food can also be affected by the oxygen

in air. When some fresh fruit, such as an apple or pear, is cut or peeled, the outside surfaces darken and turn brown, making the food seem unattractive. The browning is caused by enzymes in the fruit reacting to the oxygen in the air. By adding an acid (eg lemon juice) or sugary solution (eg syrup), the enzymic browning or discolouring action is slowed down.

Oxidation of foods