1. Egg White Turns White Even Though Nothing Is Added
Whether you make a fried egg or a boiled egg, the white part turns white once it is cooked. You did not mix anything in during the process, so the change must have happened inside the egg white itself. There is a similar phenomenon in nature: when transparent water vapor gathers into tiny water droplets, clouds appear white. Even transparent things can look white when small particles gather together. However, this is just an example of a similar mechanism; it does not mean egg whites turn white for exactly the same reason. To understand what is happening inside the egg white, we first look at its contents.
2. Raw Egg White Is Mostly Water and Protein
According to the Standard Tables of Food Composition in Japan, raw egg white consists of 88.3 g of water and 10.1 g of protein per 100 g [1]. This means egg white is about 90% water and about 10% protein. Before heating, the protein is dissolved in the water [3]. Just as salt or sugar becomes invisible when dissolved in water, the protein is not visible while it is dissolved. However, protein particles are much larger molecules than salt or sugar. Egg white contains several types of protein, such as ovalbumin and ovotransferrin. We will look at ovalbumin as an example later.
3. Egg White Proteins Are Much Smaller Than the Wavelength of Light
Ovalbumin is a round protein with a diameter of about 3 to 7 nm (nanometers) [5]. One nm is one millionth of a millimeter. Visible light has a wavelength of approximately 390 to 700 nm [5]. A single protein particle is only about one-hundredth the size of that wavelength. Because the particles are so small, light is thought to pass through them without much interference. This is one explanation for why raw egg white usually looks transparent [5].
4. Heat Unfolds Egg White Proteins and Makes Them Stick Together
Proteins are made of long chains folded into complex shapes. When heat is applied, this three-dimensional shape breaks down. The parts that were folded inside come to the surface, and the proteins stick to each other through these exposed parts [4]. This is called heat denaturation. According to a report by the Japan Science and Technology Agency, denatured proteins gather randomly without a fixed direction. They form a non-uniform network with many gaps, trapping water and solidifying [3]. This is how ordinary white egg white forms.
5. The Protein Network Scatters Light, So Egg White Looks White
When proteins gather into a network, that network scatters light in all directions [6]. This scattered light reaches your eyes, making the egg white look white and opaque. Note that this explanation is found mostly in general audience materials; the detailed physics has not been confirmed in primary Japanese sources. The temperature at which proteins solidify varies by type. Ovotransferrin is said to solidify around 60°C, and ovalbumin around 80°C [7]. Another source states ovalbumin solidifies at 75 to 78°C or higher, so there is a range in the numbers [4]. Egg white begins to turn white when the temperature exceeds about 60°C and solidifies firmly around 80°C. The softness of the white in a soft-boiled egg is thought to be due to this intermediate state.
6. When Egg White Stays Clear or Is White Before Cooking
Depending on the conditions, egg white can solidify while remaining transparent. In one experiment, purified egg albumin heated at 80°C formed a cloudy gel, but heating at 160°C for 5 minutes resulted in a uniform transparent gel [2]. This was a laboratory experiment with purified protein, not something that happens in home cooking. Patent documents also show that adjusting the pH (acidity) or salt amount before heating can create transparent gels [9]. Whether it turns white seems to depend on the heating conditions. Additionally, the white of a freshly laid egg contains a lot of carbon dioxide gas, which makes it look cloudy. The gas slowly escapes through tiny holes in the shell, and the white becomes clearer [8]. So, "raw is transparent" is not always true.
7. How to Observe Egg White Turning White Yourself
For today's activity: Watch an adult make a fried egg. Observe where and in what order the white turns white. Do not get close to the fire or the frying pan. Always watch from a safe distance with an adult. You might see a difference in the speed of whitening between the edge and the center of the pan. If you want to research more, you can look up egg white ingredients in the "Food Composition Database" by the Ministry of Education, Culture, Sports, Science and Technology, or basic egg facts in the "Egg Encyclopedia" by JA Zen-Noh Tamago, both available in Japanese.