Century eggs, also known as pidan, preserved eggs, or millennium eggs, are a traditional Asian delicacy. This food is created through a unique curing process that transforms the raw egg into a deep, savory treat with a gelatinous texture. Understanding the science behind its preservation and its altered nutritional profile is key to determining its place in a modern diet.
How Century Eggs Are Prepared
The creation of a century egg relies on an alkaline preservation method that chemically transforms the fresh egg. Traditionally, duck, chicken, or quail eggs are coated in a thick, wet paste made from clay, wood ash, salt, quicklime (calcium oxide), and rice hulls. The rice hulls prevent sticking while the paste slowly cures the egg over several weeks to a few months.
The alkaline agents in the curing mixture raise the egg’s pH level, often reaching an alkalinity between 9 and 12. This highly alkaline environment causes the protein within the egg white (albumen) to denature and coagulate, turning it into a firm, dark brown or black translucent jelly. The yolk shifts to a dark green or gray color, resulting from the reaction between sulfur compounds and iron, forming iron(II) sulfide.
Modern production methods often simplify this process by soaking the eggs in an alkaline solution containing table salt, sodium carbonate, and calcium hydroxide. This immersion technique achieves the same chemical transformation in a shorter timeframe, sometimes just a few weeks. In both methods, the chemical reaction preserves the egg and develops its unique flavor profile.
Nutritional Breakdown and Health Advantages
The century egg remains a concentrated source of high-quality protein, similar to a fresh egg. A typical preserved egg provides between 6 to 8 grams of protein, essential for muscle maintenance and tissue repair. The alkaline curing process breaks down some complex proteins and fats into smaller, more easily digestible components, such as amino acids and fatty acids.
The preservation process maintains or concentrates beneficial micronutrients. Century eggs are a source of B vitamins, including Niacin and Vitamin B12, which support energy metabolism and nerve function. They also contain minerals like iron, necessary for transporting oxygen in the blood, and selenium, which acts as an antioxidant.
Safety Concerns and Consumption Guidelines
High Sodium Content
The most significant nutritional drawback stems directly from the preservation method, resulting in a high sodium content. The salt used in the curing mixture permeates the egg; a single century egg can contain up to 400 to 500 milligrams of sodium. This amount represents a substantial portion of the recommended daily intake and requires moderation, particularly for individuals managing hypertension.
Heavy Metal Contamination
Historically, lead oxide (litharge) was sometimes added to the curing mixture to accelerate the process. While modern, regulated manufacturers have replaced this practice with safer alternatives like zinc oxide, caution is required. Consumers should look for products explicitly labeled “lead-free” to ensure compliance with food safety standards.
Unregulated production remains a risk, as some producers use industrial-grade copper sulfate to cut curing time. This can introduce toxic levels of heavy metals such as lead, cadmium, and arsenic. These heavy metals pose a serious threat to health and can lead to organ damage with frequent consumption. To consume century eggs safely, purchase products from reputable sources. Due to the high sodium load, consuming a half or a quarter of an egg occasionally as part of a meal is the recommended guideline.