Properties of Food: Protein (AQA GCSE Food Preparation & Nutrition)

Properties of food: protein

• Protein denaturation can be defined as follows:

A change in the structure of a protein that occurs when the amino acid chain unfolds

Protein denaturation diagram

• Proteins can denature during food preparation under certain conditions:

  • Contact with an acid, e.g.

    • marinating: This is soaking food in a lemon juice or vinegar marinade can break down the protein fibres in meat

  • Mechanical action, e.g.

    • whisking eggs

    • kneading dough

  • Application of heat, e.g.

    • cooking meat

    • baking bread

• Protein denaturation can cause the proteins in food to change, for example during:

  • protein coagulation

  • gluten formation

  • foam formation

Protein coagulation

• During coagulation denatured proteins clump together, trapping water molecules between the proteins and changing the appearance and texture of a food

  • When food is overcooked the water can be forced out of the coagulated proteins, resulting in food that is dry and dense in texture

Coagulation diagram

• Examples of coagulation include:

  • cooked egg

    • When heat is applied the proteins in an egg mixture denature and coagulate, turning the liquid egg into a solid

    • This can be seen when an egg turns white as it is fried, or when a quiche mixture sets during cooking

  • cooked meat

    • The proteins in meat coagulate and shrink when they denature due to heating

  • curdled milk

    • The proteins in milk can denature with exposure to an acid or when heated

    • The denatured milk proteins clump together, producing a lumpy appearance

    • Curdling is an important part of the cheese-making process, during which:

      1. milk separates into solid clumps of protein (curds) and liquid (whey)

      2. the whey is removed

      3. the curds are pressed together and processed, e.g. by the addition of salt, to form cheese

Gluten formation

• Gluten forms when water is added to flour

  • The addition of water allows two proteins in flour (glutenin and gliadin) to move around and come into contact with each other

  • The two proteins join together to produce gluten

  • Kneading causes a network of gluten to form throughout the dough

Gluten diagram

• Gluten is stretchy and strong; this means that the dough can expand and support itself when gas bubbles form, e.g. carbon dioxide from yeast fermentation when bread is proved

• Gluten denatures during heating, causing it to coagulate; this maintains the shape of any gas pockets within the dough, resulting in well-risen bread and cakes after baking

• Examples of foods that rely on gluten formation include:

  • bread

  • pasta

  • cake

Foam formation

• Foams form when air is introduced into liquids that contain proteins, e.g. by whisking or beating

• The mechanical action causes proteins to denature and rearrange themselves into longer strands

• The rearranged proteins coagulate to form a mesh-like structure that retains bubbles of trapped air

  • Heating at this stage can cause the proteins to form a solid structure, e.g. in a cooked meringue

Foam diagram

• The length of time for which a liquid is whisked can affect the quality of the resulting foam, e.g.

  • when egg white is whisked into soft peaks the proteins are only slightly stretched and the egg white can return to its original state if the foam is left for the air to escape

  • when egg white is whisked into hard peaks the proteins are fully denatured and the process is not reversible

  • in over-whisked egg whites, the bonds holding the mesh of proteins together can break and the foam may collapse

• Examples of foods that involve foam formation include:

  • whipped cream

  • chocolate mousse

  • whisked cake batter

  • meringue