Protein: acid denaturation

Protein molecules deform and unravel when exposed to acid.

Proteins are large molecules found in our bodies and food, consisting of many smaller components called amino acids.  Proteins have the properties they do because of the shape and arrangement of their amino acids.  A weak bond, known as a hydrogen bond, forms between a hydrogen atom and an oxygen atom in the amino acids.  This gives the protein its shape.

What is denaturing and how does it happen?
A protein becomes denatured when its normal shape gets deformed because some of the hydrogen bonds are broken.  Weak hydrogen bonds break when too much heat is applied or when they are exposed to an acid (like citric acid from lemon juice).   As proteins deform or unravel parts of structure that were hidden away get exposed and form bonds with other protein molecules, so they coagulate (stick together) and become insoluble in water.  Curing salmon using lemon and lime juice (eg. to make a gravadlax or ceviche) is an example of protein acid denaturation.

Explore

1. Place an egg white into a clean bowl
2. Observe the colour and texture of the egg white
3. Now add 3ml of lemon juice to the egg white and stir
4. Record what happens to the colour and texture of the egg white

Egg white turns solid and goes white instead of clear when it denatures

More information

Meat and Education.com worksheet: What makes meat tender

Meat and Education.com teachers' notes:  What happens when meat is cooked

OCR topic exploration pack: Preparation techniques