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Design & Technology

Acidity and temperature


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Foods can be acidic, alkaline or neutral. The rate a food deteriorates is determined by acidity and temperature. Foods must be reheated properly to prevent food poisoning.

Food acidity

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.

numbered ph scale with examples of food

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


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.


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