Proteins
Proteins are one of those molecules that are essential for life. In order for us to make proteins in our bodies (in the form of muscles, haemoglobin and other cells) we need to eat proteins. That’s why no matter what sort of a diet you are on, it will contain proteins. They are found in meat, eggs, pulses, fish, nuts, beans, cheese, soya products and vegetarian foods.
Proteins are a special type of polymer made up of chains of small molecules called amino acids. Each protein is made up from about 20 different types of amino acid. Folds in the amino acid chain produce the shape of the protein, and this determines that particular protein’s chemical and biological properties. There are several types of internal bonds that form links between the amino acids, one of which is a hydrogen bond. These bonds can be made to break down, making the protein change shape in a process called denaturation. Many proteins are tightly coiled up (globular proteins) so when the internal bonds are broken down, they expand outwards, like a ball of string being unravelled.
So what causes denaturation? The most usual cause is heat - you guessed it - cooking! Most proteins can be denatured at temperatures of around 40OC. When more heat is applied, the proteins start to undergo chemical reactions that can cause them to break up or to join together into even larger molecules. And it is these chemical changes that are at the heart of cooking.
Eggs
Get cooking!
Eggs are mostly proteins dissolved in water, and some of these are globular proteins. These are the type of proteins that are folded into compact balls and they have a fixed sequence of amino acids. High temperatures make these proteins stretch out, and the long chains of amino acids tangle with each other and become cross-linked with hydrogen bonds. This leads to the formation of a solid, three-dimensional network of protein molecules, turning the liquid egg into a solid.
Eggs consist of two main parts – the white and the yolk, both of which contain proteins. These parts solidify when cooked, but at different rates. The white hardens at a lower temperature than the yolk - a soft-boiled egg will have a hard white but a runny yolk. Heating eggs with milk will lead to an even more solid product. Overheating eggs creates too many hydrogen bonds to form within the egg, and that’s what makes your custard go lumpy and your scrambled eggs rubbery!
Let’s see those whites!
Egg whites can also be denatured by beating them. And the secret is in the bubbles of air that are introduced into the egg whites. Some of the amino acids that make up the proteins are attracted to water, they’re hydrophilic, or water-loving. Other amino acids are water-fearing, or hydrophobic. Egg whites contain both types. In its natural state, the hydrophobic amino acids in the protein are packed near the centre, away from the water in the egg white, and the hydrophilic ones are on the outside of the protein, near the water (because they love it!).
When you whisk an egg white, you introduce air bubbles, and part of the protein is exposed to air, and part is still in water. But not all parts of the protein like air (or water!), so the protein starts to uncurl as the amino acids re-arrange themselves. The water-loving ones moving towards the water, and the water-hating ones moving towards the air bubble. Once the proteins uncurl, they start to form hydrogen bonds with each other, just like they do when they’re heated. This creates a 3D network that hold the air bubbles in place. If you then heat these captured air bubbles, they expand as the gas inside them heats up. If done correctly, the network solidifies and the structure hardens, just like you get when making a pavlova!
Learning to love the yellow
Because some of the proteins in egg yolks are water-loving, and others are water-loathing, egg yolk is an excellent emulsifier. Emulsifiers allow oil and water to mix, without separating, which is why they’re so important in making things like mayonnaise or margarine.
Mix egg yolks with oil and water, and one part of the protein will stick to the oil, and another part will stick to the water.
Another substance that’s found in egg yolks that acts as an emulsifier is lecithin. This is a phospholipid, which is a fat-like molecule that has a water-loving “head” and a water-loathing “tail”. This long molecule buries its tail in the oil and pokes its head out into the water. By doing so, it forms a barrier that prevents the surfaces of the oil droplets from touching, keeping them apart and stopping them from joining together and hence separating out from the water.
How bread works…
There’s nothing more delicious than a slice of freshly baked bread. The two most important things about bread are its tiny bubbles, and the gluten that binds them together.
Flour is made up of small starch molecules, and gluten is formed when water is added and the dough is kneaded. Adding water makes the proteins on the outside of the starch molecules become “sticky” in a process known as hydration. These sticky molecules bind together, and if they’re moved apart, the proteins between them become stretched - forming gluten. This happens when two different protein molecules (gliadin and glutenin) interact with each other to form a “protein complex”.
Gluten is very elastic, and forms thin sheets that behave a bit like rubber balloons. In bread, these balloons become filled with carbon dioxide gas generated by yeast as the bread rises. To make great bread, these gluten sheets have to be strong enough not to break as the carbon dioxide forms. They also have to be plentiful enough to be able to capture the gas in small bubbles.
Yeasts are single celled fungal micro-organisms. They metabolise sugars and create carbon dioxide and alcohol as a by-product. Although there are over 160 different types of yeast, in bread-making yeasts that make little alcohol are favoured. The species of yeast used in almost all baking is Saccharomyces cerevisiae, which converts sugar into either alcohol and carbon dioxide or, in an oxygen rich environment, into carbon dioxide and water. Yeasts are able to ferment at temperatures as low as 5oC, but their rate of gas production increases exponentially up to about 38oC. At 40oC and above, the yeast is slowly killed.
Sugars
Glucose is the type of sugar that yeasts like to metabolise the most. But when you add sugar to dough, it’s usually sucrose and not glucose that you are adding. The yeast first converts it to glucose and fructose using an enzyme called sucrase. Yeasts also produce another enzyme called amylase, and this breaks down the starch molecules into another sugar called maltose, which is also metabolised by the yeast into carbon dioxide and alcohol.
Heat transfer
Heat is energy which flows from hot to cold. It will always flow from an object with a higher temperature to a cooler one. This causes the hotter object’s temperature to decrease and the cooler’s to increase, until both reach the same temperature. When this equilibrium is found, heat will stop flowing.
Heat is transferred in cooking by one of three methods – convection, conduction or radiation.
Convection
Convection is the process of heat transfer by movement in a fluid or in air. It’s the main process of heat transfer in cooking.
Boiling, deep frying and baking are all examples of convection.
Both boiling and deep frying use liquid to heat the food, although oil boils at a much higher temperature than water. When boiling water in a pan, the water at the bottom is heated. This hot water becomes less dense than the cold water above it. The less dense liquid rises, transferring its heat to the cooler water as it goes. It draws the cooler water in behind it, which in turn is then heated up. As the water rises to the surface it cools and sinks, to be heated up again in a continuous cycle.
When food is baked it is cooked by means of convection through air in an oven. This works in exactly the same way as water in a pan, but uses air instead of water. The air is heated at the bottom of the oven by an electric element or a gas flame, the hot air rises, transferring its heat and cooling down on the way. The cool air sinks to the bottom to be heated up again. And so it goes on. Fans are often used in ovens. They push the air around, giving a more even temperature throughout the oven - not just relying on the natural circulation of the hot air rising and the cool air sinking, which causes the top of the oven to be hotter than the bottom.
Conduction
Conduction is when heat is transferred inside a solid by the collision of atoms from a region of higher temperature to a region of lower temperature. As the atoms collide, they get hotter.
Unlike heating by convection it’s not the material itself that is moving, all movement is happening at an atomic level. When a metal pan is put onto a hob the heat is transferred from the hob through the metal in the bottom of the pan by conduction. Most metals are good conductors, but some metals are more conductive than others. Stainless steel isn’t a good conductor of heat, but copper is – that’s the reason why stainless steel pans often have copper bottoms.
Wood and plastic are not good conductors of heat, which is why spoons used to stir food during cooking tend to be made out of these materials. Heat is not conducted through the spoon to the handle, which makes it much easier to hold onto.
Radiation
When hot, all objects radiate heat. Unlike convection and conduction, radiation doesn’t need a medium through which to carry heat – it can even carry heat through a vacuum. Radiation is used in two forms of cookery - grilling and microwaving.
Microwave ovens use high frequency electromagnetic waves which penetrate the food and are absorbed by the water molecules inside. The water molecules vibrate when hit by a microwave at exactly the right frequency and this vibration generates heat. The food is further heated by the energy from this molecule being transferred by conduction to neighbouring molecules. Microwaves can penetrate up to one centimetre, so the inside of the food as well as the surface is heated.
Strictly speaking, grilling food uses all three forms of heat transfer. The heat radiates from the grill and is absorbed through the surface of the food nearest to the grill. Conventional grills use infra-red radiation, which has a shorter wavelength than microwaves. The heat is then conducted from the surface to the inside of the food, which is then cooked by convection.
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