Why Does an Egg Becomes Solid Upon Heating?
Generally, when heat is applied to any matter, its atoms/molecules gain energy to overcome the attractive interatomic/intermolecular forces and transform to a state that has higher degree of freedom. For example, solid converts to liquid (melting), liquid to gas (vaporization), or in some cases solid directly transforming into gas (sublimation). Contrary to this, however, when an egg is cooked or boiled, it transforms from a transparent liquid to an opaque solid mass. Have you ever wondered why this anomaly arises?
As you might know, proteins are the major constituent of eggs following water. When these proteins are subjected to heat while cooking, they undergo a process called “Protein Denaturation” which is responsible for this unprecedented occurrence. Evident from its name, protein denaturation is a phenomenon in which the native state of proteins gets disrupted when an external stress is applied to it. This external stress can be in the form of change in pH, application of heat, mixing to a non–polar solvent or addition of any salt. To answer the question at hand & appreciate the concept of protein denaturation on a deeper scientific level, we need to understand how proteins are formed & the conformations they attain in their native state.
The Four Levels of Protein Structure
Proteins are a complex entity with complicated three–dimensional structure that evolves at four levels i.e. Primary, Secondary, Tertiary and Quaternary structures. The building blocks of proteins (& essentially for all living organisms) are amino acids which are organic molecules having a basic “amine” (–NH2) & an acidic “carboxyl” (–COOH) functional groups (that’s why the name “amino–acid”) bonded to a carbon atom — called ‘α–carbon’– that further has a “side chain” attached to it (Fig 1). It is this “side chain” that imparts distinctive identity to any amino acid since apart from it they all possess similar molecular arrangement & composition.
It requires twenty different forms of amino acids to formulate all the proteins found in a human body. Look closely, and you will find all these amino acids having distinct side chains attached to a common backbone comprising of α–carbon, amine & carboxyl functional groups.
When amino acids undergo condensation polymerization — a chain reaction where amine group of an amino acid reacts with carboxyl group of another to form peptide linkages (–CONH) (Fig 2)– they form a macromolecule called polypeptide. These polypeptides when achieve a distinct three-dimensional structure, by virtue of underlying intermolecular interactions, they are termed as proteins. The conformation in a 3D space is the very essence of a protein as it determines the function it can & will perform inside the body.
The structure of any polypeptide can broadly be segmented into its (i) “backbone” — consisting of all the α–carbon atoms and the peptide linkages in the polymer chain — and (ii) “side–chains” — collection of all individual side chains attached to the amino acids constructing that polypeptide. The sequence in which different amino acids fall within the chain of polypeptide forms “Primary Structure” of the protein. Even a single alteration in this sequence would result in formation of a totally different protein with a unique 3D conformation and ability to function in a particular way.
The polarity of peptide linkages in the “backbone” of polypeptides is responsible for the secondary structure of proteins — Due to the high electronegativity of Oxygen & Nitrogen, a partial negative & positive charge is respectively developed on ‘O’ & ‘H’ atoms of the amide bond (Fig 3). This promotes electrostatic attraction between the ‘O’ & ‘H’ atoms of two peptide linkages (also termed as hydrogen bonding). When amide bonds of the same polypeptide take part in hydrogen bonding, they attain α–helix structure which resembles a folded ribbon (Fig 3(a)). On the other hand, involvement of separate (two or more) polypeptides gives rise to a Ᏸ–sheet structure (Fig 3(b)).
Tertiary structure of proteins originates from intermolecular interactions within the “side–chains” of polypeptides. In contrast with secondary structure, there are various kinds of intermolecular forces between the side chains, besides hydrogen bonding, that partake in the formation of tertiary structure including electrostatic attraction between charged functional groups, dipole–dipole interactions, hydrophobic associations & disulfide linkages. Among them, the major contribution comes from the undergoing hydrophobic interactions between the non–polar side chains in the protein. These hydrophobic interactions arise due to the “like dissolves like” rule where a polar solvent has strong propensity to dissolve polar solutes (e.g. — a combination of water with any salt) and likewise, a non–polar solvent tends to dissolve a non–polar solute. Typically, when a non–polar solute is dissolved in a polar solvent then due to their incompatibility, the solute molecules aggregate together to form clusters (micelles) to minimize the level of their interaction with surrounding polar molecules of the solvent. This is the very reason why phase separation of oil happens when it is mixed with water. Because of hydrophobic interactions, the proteins form globular (spherical) structure in water where the hydrophobic side–chains locate in the core of globule — far away from water molecules — and the hydrophilic groups find themselves on the surface of globule to interact with water (Fig 4). An excellent illustration to visualize globular structure in proteins is a yarn ball where wool threads depicts the polymer chains of the protein which are wrapped around on itself to give it a globular shape.
Some proteins consist of a single polypeptide chain (e.g. ribonuclease), but many times multiple polypeptides come together to form a protein (Fig 5). Hemoglobin, the oxygen carrier in our blood, is one such example of protein which is made of four polypeptides with two α–helix and two Ᏸ–sheet structures. These kinds of proteins, due to the presence of multiple polypeptides, have the ability to perform multiple functions at the same time. Each polypeptide in these proteins, is called a subunit and their relative arrangement makes up the quaternary structure of a protein.
Protein Denaturation of Eggs While Cooking
Both components of an egg — egg white & egg yolk — are colloidal solutions of proteins in water. Majority of intermolecular interactions, particularly the hydrogen bonding & hydrophobic interactions, that gives proteins their complex globular structure are weak in nature & thus can easily be broken down when they are set to high temperatures while cooking. As a result, proteins loose their secondary, tertiary and quaternary structures with the application of heat. However, covalent bonds in peptide linkages of proteins are quite strong for them to be affected at these temperatures and hence protein retains its primary structure. When these forces are overcome, the polymer chains of the globular proteins unwind as they weave & entangle together to give rise to an opaque solid mass. This process is quite analogous to the hand knitting of yarn where yarn threads are unraveled while preparing a knitted sweater (Fig 6). The egg white solidifies faster due to its higher protein content as compare to egg yolk which has more fat and thereby less protein content.
The phenomenon of polymer chains unwinding in protein denaturation is essentially a reversible process i.e. when subjected to right conditions, proteins will regain their 3D conformations. The reason, however, you cannot convert back your omelette or boiled egg to a raw egg is because various new covalent bonds are also formed during this heating process which cannot be reversed.
As mentioned at the start, there are various forms of external stresses that can cause protein denaturation and will essentially show similar effects. For example, when raw egg is put in acetone — a hydrophobic solvent— polypeptide chains again unfold and as a result egg solidifies (watch this video). Likewise, adding an egg to salty water will stimulate the same response. This is the reason why it is widely suggested to add some table salt to the water while boiling the eggs, because in case the shell cracks and egg white starts to seep out, it will solidify moment after it comes in contact with the salt water and would prevent any further leakage.
So the next time you cook eggs for your breakfast, remember that you are just breaking down the complex protein structures inside it.
