Egg

Proteins

Eggs consist of a variety of proteins, which are polymers of amino acids whose properties are determined by the number and the arrangement of amino acids in the molecule. There are about 20 different common amino acids which can be classified into 2 main types based on their structure and function.

• Fibrous Proteins

Fibrous proteins have a linear shape, making them excellent for thickening and forming structures. Glutenin is a type of fibrous proteins which forms the backbone of gluten, it is crucial for the elasticity and the structure of the dough. Ovomucin is another type of fibrous proteins found in egg whites. It provides thickness and stability to foams, such as meringues. Ovomucin also breaks down and dissolves as the egg ages, causing egg whites to thin.

• Globular Proteins

Globular proteins have a spherical shape which can be altered by heat, acid or salt, changing their functionality. Enzymes are a common type of globular protein.

Protein coagulation

When eggs are heated, proteins denature and unfold, which then move through the mixture to bond with one another forming a rigid protein network. Tempering eggs into hot mixtures during the baking process must be done carefully to avoid pre-matured coagulation of eggs. This could be done by diluting the egg with a small amount of hot liquid before adding to the bulk of hot liquid. 

Factors affecting coagulation include:

• Proportion of eggs in a mixture: Diluted eggs makes coagulation more difficult as unfolded proteins in eggs are less likely to collide and to bond with one another.
• Rate of cooking: When eggs coagulate too quickly, egg proteins do not unfold properly, making it less likely to thicken or to gel. Hence, a water bath is often used to slow the process.
• Addition of sugar: Sugar slows coagulation by pulling water from yolks. If too much sugar is present in a mixture, coagulation could be stopped completely causing baked goods to appear undercooked even after extensive baking.

Composition of an Egg

• Egg white

Egg white consists of 10% of proteins and 90% of water, with small amount of mineral ash and glucose. It has little flavour and colour but develops a slight sulfur aroma when cooked. This is due to sulfur being released from the egg proteins when they are heated.

Egg white consists of thick and thin portions, with the thick portion thinning as the egg ages. As a result, the egg would lose the ability to form foams that are stable. Some of the key proteins found in egg white are ovalbumin, conalbumin, globulin, ovomucin and lysozyme.

• Egg yolk

Egg yolk contains 50% moisture and 50% yolk solids — proteins, fats, emulsifiers, small amount of mineral ash, yellow-orange carotenoids. It is rich in proteins like lecithin which is a type of phospholipids that bind to lipids acting as emulsifiers. Phospholipids is made up of 2 fatty acids and a phosphate group bonded to glycerol. The fatty acids are hydrophobic meaning that they would repel water molecules, whereas the phosphate group is hydrophilic meaning that it would be attracted to polar water molecules. Hence, this allows phospholipids to bond to both water and oil, stabilising emulsions in mixtures such as cake batter. When light bounces off the microscopically small droplets of oil and trapped air dispersed throughout the water in a fully emulsified mixture, this gives off a creamy look of emulsion.

Protective membrane of the egg yolk weakens as the egg ages, causing the yolk to pick up moisture from the egg white. As a result, it would be more difficult to separate the yolk from the white. Also, this increases the possibility of bacteria passing into the nutrient-rich yolk, making eggs more prone to bacteria contamination. Storing eggs in the fridge/in cool area could reduce this risk as bacteria would multiply at a much lower rate in cooler environments.

The hen’s feed has a significant impact on both the colour and the flavour of the egg yolk. If there are more carotenoids present — in alfalfa, yellow corn and marigold petals — the yolk would be more yellow-orange. If the hen’s feed consists of more omega-3 fatty acids, this could give rise to a different flavour.

• Eggshell

Egg shell account of approximately 11% of the total weight of an egg. It is porous allowing odours to penetrate and moisture and gases to escape. The eggshell is sometimes coated with mineral oil to delay moisture loss when distributing eggs.

Air pocket forms between the 2 protective membranes between the shell and the egg white at the egg’s larger end as it ages. This is because eggs lose moisture and shrink as they age, causing the air pocket to increase in size. Hence, older eggs with a lower density would float whereas fresh eggs would sink.

The colour of the eggshell depends on the hen’s breed. If hens have white feathers and white earlobes, the eggshell would be white; if hens have red feathers and red earlobes, the eggshell would be brown.

• Chalazae

Chalazae is the white cord that holds the yolk to the centre of the egg, disintegrating as the egg ages. It is similar in composition to ovomucin.

Functions of Eggs

• Provides structure

Eggs provide structures due to the thickening and gelling properties of their proteins. Egg whites contribute more to structure building than yolks.

• Aerating

Eggs produce a stable foam consisting of tiny bubbles of air surrounding by a liquid/solid film, assisting in the leavening process. 

In the process of egg foam formation, air bubbles are first beaten into the liquid whilst certain proteins denature or unfold. The unfolded proteins would quickly move through the liquid to the surface of the bubbles. As neighbouring proteins bond and aggregate around the bubbles, a filmy network is formed reducing the chance of the bubbles collapsing. This pocket of air can be expanded when more bubbles are beaten in, which causes the walls of the protein film to thin out. These protein films stretch when heated, making them more likely to break and to collapse in the oven.

Egg whites can be whipped up to 8 times their original volume, but there are several factors that affect meringue stability:

• Sugar: Sugar dissolves the meringues’ liquid protein film to prevent the trapped air bubbles from collapsing. However, if added too quickly protein molecules may not unfold properly. Sugar would also slow down the aggregation of protein molecules so it can protect against over-whipping. But for the same reason, sugar must be added only after egg whites have started to foam. As undissolved sugar crystals could weigh down the foam, make sure to use sugar with a fine grind when beating egg whites.
• Lipids: Lipids coat proteins preventing them from unfolding and aggregating, so they would slow down or even prevent aeration. They prevent protein film from forming by competing with proteins for a spot at the foam’s bubble surface, which then form a weaker network that causes bubbles to expand rapidly and collapse.
• Acids: Acids can lower the pH to stabilise the meringue. They should be added at the start to help the formation of a flexible and stable protein film around trapped air bubbles.
• Temperature: Ideal temperature at 21°C for whipping.
• Thickness of egg whites: Thinner older egg whites whip more easily to a high volume but forms less stable foam. This is because liquid film drains more easily from the bubbles.
• Whipping time: If egg whites are under-whipped, the proteins would not fully aggregate to form a strong film. But if whipped too quickly or for an extensive period of time, proteins would start to denature. This causes the proteins film that surrounds trapped air bubbles to be overstretched and too rigid. Hence, egg whites should be whipped at a maximum of medium-high speed.

• Emulsifying

Egg yolks help keep water and oil in emulsions from separating, crucial in batters and doughs. Eggs should be added slowly and at room temperature to maintain the emulsion, preventing the baked goods from not rising properly or having a coarser crumb.

• Adding shine

Egg wash (egg yolks diluted with milk) applied to the surface of baked goods create a glossy, brown finish. Adding salt to the egg wash thins it, making it easier to be applied evenly. This is because salt neutralises proteins in eggs, making the egg wash better hydrated by water.

• Adding moisture

Eggs contribute moisture to baked goods, enhancing texture and flavour.

Reference

• Figoni, P.L. (2010) How Baking Works: Exploring the Fundamentals of Baking Science. 3^rd ed. Providence, Rhode Island: Wiley.