Acidity and temperature

Acidity or alkalinity of food is measured on the pH scale, which runs from 1 to 14. A strong acid will have a pH of 1, and a strong alkali will have a high pH value, like 14.

Some foods, such as citrus fruits, are acidic. Others, like sodium bicarbonate, are alkaline. Foods that are neither acid nor alkaline, like pure water, are called neutral and have a pH value of 7.

Acidity or alkalinity affects:

• the taste of final product
• the rate at which microorganisms grow within and upon food

Examples

Acidity or alkalinity can affect food in the following ways:

• Acidic fruit mixed with milk will cause the mixture to curdle.
• Vinegar (acid) is added to meringue to give it a soft marshmallow texture.
• Bicarbonate of soda (alkaline) acts as a raising agent during baking.
• Lemon juice (acid) helps prevent fruits like apples from discolouring.

Microorganisms in food

 Bacteria, mould and yeast are all types of microorganism. They cause food to change, sometimes making food unpalatable.

Heating and cooling can make food less resistant to microorganisms. This affects its ability to stay fresh.

Chilling

Dairy produce, cooked foods and raw ingredients should be kept at a temperature between 0°C and 5°C. This will slow the growth of microorganisms, but won't stop it.

Freezing

Meats, vegetables and ready meals can be frozen. Freezing maintains high standards of freshness and safety. Freezers store products at -18°C or below. Freezing does not kill microorganisms, but will keep them dormant until the food is unfrozen.

Reheating foods

Food must be reheated to at least 72°C to avoid the risk of food poisoning. Leftovers should be disposed of quickly