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.