Proteins & Carbohydrates

Food Preparation & Nutrition: AQA GCSE, topic 3.2.1

Changing Properties of Proteins

Here's an overview of the chemical and functional properties of proteins:

Chemical bonds hold proteins' complex structures together. These break down during cooking, causing proteins to denature.
Proteins fall apart and change shape. This is often an irreversible transformation.
Things that cause proteins to be denatured include:
Temperature changes.
Acidic substances (e.g. fruits like kiwifruit, mango, pineapple and fig are good at breaking down meat proteins).
Agitation (beating or whisking).

When denatured, protein molecules bang into other protein molecules and join together (coagulate). When this happens, water gets stuck between the protein molecules.
Coagulation alters food texture and how it looks - e.g. egg white transforms from a clear liquid into a white solid.
Coagulation can help things stick together - e.g. when eggs are mixed with breadcrumbs.
Too much coagulation can be problematic. The food becomes dry and chewy as the protein tightens and water is pushed out of the molecules.

Chocolate mousse and meringues are examples of foams. We make them by trapping gas inside liquids.
When we agitate liquids storing protein (e.g. by whisking or beating), we cause the liquids' proteins to denature. The denatured proteins expand, trapping air in the liquid.
Then, when coagulation happens the air gets trapped and a foam is formed.
Over-beating or over-whisking will break coagulation bonds, causing the air to escape and the foam to fall apart.
When some foams are cooked, they become solid (e.g. egg white foams → meringues).

Gluten is a form of protein. Wheat flours contain gluten.
We get gluten by mixing water and flour to form dough.
Gluten molecules can bend and stretch - we say they are coiled. This is why dough is stretchy.
We knead dough to lengthen, strengthen and stretch the gluten strands - this is called 'working' the gluten.
Gluten coagulates at high temperatures, while dough remains stretched. As a result, foods, such as well-risen bread, gain an airy texture.

Carbohydrates change in three main ways during cooking. Here's an overview of these changes:

Starch gelatinisation thickens starchy foods, like pasta sauces and gravy.
Starch granules hang in liquid until they're heated. The intermolecular bonds begin to break and water is absorbed. This causes the starch granules to soften and expand (swell).
The starch granules start to burst once temperatures reach 62°C-80°C. Starch is released into the liquid.
The liquid is thickened by the addition of starch. The more starch that is released, the thicker the liquid becomes.

Cooling the liquid turns it into a solid gel, which can be used for desserts which require set fillings (like key lime pie).
Gelatinisation occurs when cooking spaghetti and other forms of pasta - the strands expand, become soft and starch is released into the liquid.

Dextrinisation describes the process of starch molecules breaking down into dextrins (smaller molecules) when starchy foods are exposed to dry heat.
The process causes food to brown, develop a crispier texture and taste different.
Cooking time affects how dark and crispy the food turns. The longer the cooking time, the browner and crispier the food becomes.

Caramelisation occurs when sugar molecules are exposed to high heat - they break down, the sugar browns and its flavour changes.
There are different stages of caramelisation:
Initially, the liquid is sweet-tasting and runny.
The liquid thickens with time and develops a caramel-like texture.
The liquid eventually hardens and develops a candy-like texture as it cools.

It's often necessary to add water at the start of the heating process to avoid burning the caramelised sugar. Once caramelised sugar starts to burn, it turns brittle and black.
Caramelisation adds sweetness.
Some desserts are therefore caramelised (e.g. creme brulee) as well as some savoury foods, like shallots.