Food acidity and micro-organisms

Some foods, such as citrus fruits, are acidic; others, such as sodium bicarbonate, are alkaline [alkaline: opposite to acidic. Alkaline substances react with acids, neutralising them, and forming salts. ]. 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.

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.

Microorganisms in food

Three types of microorganisms affect food: bacteria [bacteria: Single-celled micro-organisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium. ], moulds [moulds: multi-cellular microorganisms. They are made up of many tiny fungi (spores), which travel by air to land on food surfaces. ] and yeasts [yeasts: Yeasts are single-celled organisms found on the skin of fruit which aid its deterioration. ]. By a natural chain of events, these microorganisms will cause food to change - often, but not always, making food "bad" or unpalatable [unpalatable: unpleasant to taste ].

• Bacteria are single-celled microorganisms that reproduce by binary fission (splitting in half).
• Moulds are multi-cellular microorganisms. They are made up of many tiny fungi (spores), which travel by air to land on food surfaces.
• Yeasts are single-celled organisms found on the skin of fruit that aid its deterioration.

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 [enzymes: proteins which catalyse or speed up chemical reactions inside our bodies ] 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.

The diagram below shows three key ways to reduce the action of air content and reduce discolouration.

Food temperature

Both heating and cooling food can change its susceptibility to microorganisms [microorganisms: microscopic (too small to see) organisms such as bacteria and viruses ], and therefore its ability to stay fresh.

Cooling

Keeping 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.

Freezing

Storing food in a freezer makes long-term storage possible by making bacteria dormant.

Many 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 dormantdormant: A volcano is classed as dormant when it is temporarily inactive but not fully extinct..

Cook-chilled foods

Cook-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.

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