The Preserved Duck eggs were served as an hors d’oeuvre in a fashionable Hong Kong restaurant, sliced in half, with a ginger-and-vinegar dip. It was my first trip to Asia, and I had rarely seen anything so revolting on a dinner table. They leered up at me like the eyeballs of some nightmarish monster, dark and threatening. Their albumens were a filthy, translucent brown, their yolks an oozy black, ringed with a layer of greenish, mouldy grey. About them hung a faintly sulphurous haze. I tried one, just to be polite, but its noxious aroma made me feel nauseous and I found it hard to swallow. Afterwards, a slick of toxic black slime from the yolk clung to my chopsticks, threatening to pollute everything else I ate.
–Fuchsia Dunlop, Shark’s Fin and Sichuan Pepper
Fuchsia Dunlop, British writer, cook, and expert on Chinese cuisine, does a magnificent job of describing the discomfort, and even horror, of being presented with foods that violate our cultural preconceptions of what is good to eat in her 2008 memoir, Shark’s Fin and Sichuan Pepper. However, and not for the last time, Dunlop pushed past her initial revulsion at the food on her plate and acquired an appreciation for this strange delicacy. As she explains in her 2013 cookbook Every Grain of Rice: Simple Chinese Home Cooking, “the first time I encountered these dark, mysterious-looking eggs, […] I was terrified. But their appearance is misleading, because they actually taste quite like eggs, but more intense and delicious”. In fact, her cookbook includes quite a few recipes with century eggs and suggestions on how to serve them.
What exactly are they? Pidan (皮蛋 – “skin egg”), as they are called in Chinese, their strange appearance has given rise to a host of more esoteric names in the West, such as century eggs or thousand-year-old eggs. As Fuchsia Dunlop explains, they “are made by treating duck eggs with strong alkalis that “cook” them chemically. It’s a technology that dates back at least to the sixteenth century. In the past, the eggs were caked in a paste made from mud, tea, rice husks and salt, with alkaline ingredients such as wood ash, soda, lye and lime; these days they may be simply immersed in an alkaline liquid. The eggs are sold, in boxes, in most Chinese supermarkets. They just need to be peeled before eating.”
Would you eat it? I decided one day that I not only wanted to eat then, I wanted to make them! I found not one Chinese cookbook with instructions on how this could be accomplished (although, granted, I was limited to English language cookbooks). Perusing the scientific literature on the subject, though, yielded a treasure trove of information on traditional and modern processing methods, complete with charts, diagrams, and exact measurements. These papers also validated for me the safety of the product that, at the end of the endeavour, I would be putting into my mouth. The method I outline below, using sodium hydroxide (NaOH) as the allkali, is based especially on one such paper, “Alkaline-Fermented Foods: A Review with Emphasis on Pidan Fermentation,” published in 1996 in the journal Critical Reviews in Microbiology.
Pidan cross-section, which looks uncannily like a piece of rainbow obsidian
What is Sodium Hydroxide?
Sodium Hydroxide (NaOH), also known as lye or caustic soda, is a solid, white, odourless compound, often sold as a powder. It is a strong alkali, with a pH of 12.88 in a 0.1 molar solution (compared to baking soda, which has a pH of 8.3 at the same concentration). It is highly caustic, and will decompose proteins at room temperature. Sodium hydroxide has many industrial uses. In the paper industry, it is an important component of the solution used to separate different plant fibres during the pulping process. It is utilized as a cleaning agent and to neutralize acids. It is also used in soap making, food processing and water purification.
Is it safe?
As I mentioned in the previous section, sodium hydroxide is not an uncommon tool in food manufacturing. In fact, you have probably already eaten food treated with sodium hydroxide a number of times in your life. From Wikipedia:
Food uses of sodium hydroxide include washing or chemical peeling of fruits and vegetables, chocolate and cocoa processing, caramel coloring production, poultry scalding, soft drink processing, and thickening ice cream. Olives are often soaked in sodium hydroxide for softening; Pretzels and German lye rolls are glazed with a sodium hydroxide solution before baking to make them crisp.
A few other examples from Wikipedia:
- Most yellow coloured Chinese noodles are made with lye-water but are commonly mistaken for containing egg.
- The Norwegian dish known as lutefisk (from lutfisk, “lye fish”).
- Bagels are often boiled in a lye solution before baking, contributing to their shiny crust.
- Hominy is dried maize (corn) kernels reconstituted by soaking in lye-water. Hominy is used to create Masa, a popular flour used in Mexican cuisine to make Corn tortillas and tamales.
- And of course: Sodium hydroxide is also the chemical that causes gelling of egg whites in the production of Century eggs.
Here’s another way to think about it: we use dangerous levels of heat to prepare our food all the time. The effects of high heat are very similar to those of sodium hydroxide. High heat will burn your skin and flesh. It can break glass, melt through plastics, and cause water to spatter. But we don’t think twice about it because we are familiar with the effects of heat and know how to protect ourselves and use it safely.
Verdict: Sodium hydroxide seems to be safe to use in the manufacturing of food, as long as it is used properly.
But, please read over the safety measures carefully before starting!
Safety Measures
- Sodium hydroxide can burn your skin and flesh. Wear protective gloves.
- Sodium hydroxide reacts violently when it comes in contact with water. It can spatter and froth. For this reason, you want to add it to a pot filled with liquid, not the other way around.
- Sodium hydroxide is highly corrosive. Do not use metal vessels or utensils for making century eggs (except high-grade stainless steel, see the section below on containers). It will also corrode glass and destroy certain types of plastic.
- Sodium hydroxide is safe to pour down the drain once your eggs are done.
- Sodium hydroxide is not flammable or combustible.
Concerning Containers
Since sodium hydroxide is highly corrosive, you do not want it to come in contact with metals. It will also corrode glass over long periods of time, so mason jars are also out of the question.
High-quality (“food grade” or “surgical grade”) stainless steel is safe for storing sodium hydroxide. Look for the numbers 18 / 8 or 18 /10, which refer to the percentage of chromium and nickel in the steel, respectively. These are also referred to as Type 304, part of the 300 series. Lower grades of stainless steel (in the 200 series) are safe for most food storage, but they contain manganese instead of nickel and are therefore less resistant to corrosion. Avoid these for this recipe.
Certain plastics are reportedly safe for storing lye, while others are not. I have heard that polypropylene (PP, SPI code 5) and high-density polyethylene (HDPE, SPI code 2) are the safest options. Definitely avoid the ubiquitous polyethylene terephthalate (PET/PETE, SPI code 1). The lye will eat through it in no time.
I did not feel comfortable with using plastic at all for such a long exposure, so I went for a large stainless steel saucepan marked 18/8 instead.
Materials and Equipment
- Scale
- Food-grade sodium hydroxide (I bought this one)
- Large high-grade stainless steel pot with lid (see section above on containers)
- Eggcups, or something to hold the egg upright as the wax dries
- Beeswax (I like to buy beeswax pellets, because they are easier to measure out and melt)
This recipe has two parts. In the first part, we soak the duck eggs in a solution of sodium hydroxide, salt, and tea until the proteins in the egg white have denatured, forming a dark-coloured gel. In the second part, we seal the pores of the eggshell using beeswax, to protect the egg from pathogens and from oxidation. The whole process takes about 6 weeks, also only three of those days involve any actual work.
Joëlle
Yields 12 Century eggs
Serves 12
Step-by-step instructions on how to make the delicious Chinese delicacy pidan (皮蛋) at home.
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Ingredients
- 12 fresh duck eggs
- 1L Water
- 25g Loose leaf black tea (or about 10 teabags)
- 40g food grade sodium hydroxide (NaOH)
- 50g Salt
- 150g Beeswax
Instructions
- Bring water to a full boil in a large stainless steel pot. Remove from heat, add teabags and salt and mix until the salt has fully dissolved.
- Let the tea cool down to room temperature, and then remove the teabags.
- In a well-ventilated area, and wearing protective gloves, slowly pour in the sodium hydroxide. Mix with a stainless steel utensil until dissolved.
- Carefully add the eggs to the mixture, one by one. I used a stainless steel slotted spoon to lower them down into the liquid.
- Cover the pot with a lid and label it with the date and content. Store the pot in a cool, dark place, somewhere safe where nobody will accidently open it or cause the liquid to spill out, for 24-30 days (lower end of the range in the summer, higher end of the range in the winter).
- Now, it is time to remove the eggs from the sodium hydroxide tea and to cure them in a sealed environment. Melt beeswax in a small double-boiler (I used a heat-proof glass dish placed inside a saucepan, submerged in water about halfway).
- While the wax is melting, bring out your pot of eggs from their safe storage space. Wearing gloves, scoop the eggs out of the sodium hydroxide tea one by one with a slotted spoon. Rinse them well until cool water, and place them on a clean towel to dry.
- When the beeswax has melted, pick up an egg (with gloves on!) and dip it halfway into the wax. Place the egg wax side-up in an egg cup for the wax to dry. Dip the remaining 11 eggs in the wax and set them in their egg cups, then go back to the first egg and dip it again in the wax (on the same side as before). You want to dip each egg in the wax three or four times per side so that the egg is completely encased in a solid layer of wax.
- Once all the eggs are encased and dry, place them in a container, and them in a cool, dry place (out of the refrigerator) for another few weeks, or for up to three months. They don't keep well once opened, so open only what you need at a time, and leave the others encased in beeswax until you are ready to serve them.
Notes
Make sure to follow the safety measures and evaluation process outlined on my blog post. Good luck!
Calories
Fat
Carbs
Protein
Safety and Quality Evaluation
(or How can I be Sure this won’t Kill Me?)
Once your eggs have sat enough time in their beeswax shells, it is time to break them open and enjoy them. But how can you tell if your eggs are good to eat? Here is a checklist you can go through as you open your century eggs to assess their quality before you put them in your mouth (or serve them to your guests!):
Before Removing the Beeswax
- No eggshell is exposed
- There is no mold
- When shaken, there is no watery sound
Eggs encased in besswax
While Removing the Beeswax and Eggshell
- The eggshell has no cracks
- The egg separates easily from the shell
After Removing the Eggshell
- The egg is semitransparent in deep brown, dark-green, or tea-brown
- The egg is intact and elastic, with no turbidity
- The egg may have little “pine-floral crystals” (see picture below)
During the curing process, the salts in the egg might start forming beautiful dendritic crystals, called “pine flowers” (松花) in Chinese
After Cutting the Egg into Pieces
- The yolk is light green, tea-brown, and orange-brown in color,
- The yolk is hard or semi-hard, and sticky but not flowing
- The smell of ammonia is present, but not overpowering
Ok. Are you still with me? If your egg has passed the evaluation so far, it is beyond safe for human consumption. Time for the final ordeal: the taste test!
- The egg has a cooling mouth feel
- The egg has a clean, slightly salty, slightly peppery taste
Or in the words of Fuchsia Dunlop, it should “taste quite like eggs, but more intense and delicious”!
Congratulations! You have made century eggs!
If the egg is exposed to oxygen during the curing process, it will revert of a paler colour. It is still edible.
Cleaning Up
I love beeswax because it is natural and safe. I don’t mind having it come in contact with my food for long periods of time (as opposed to, say, plastic or paraffin wax). Wax has been used in the past to coat eggs in order to preserve them. It effectively seals the pores of the eggshell and protects the egg from the outside environment (especially from the two main culprits of spoilage: oxygen and microorganisms). The big disadvantage, I realised I was doing this project, is that it is extremely difficult to clean from your pots and counter afterwards! I recommend waiting for it to dry and then scraping it off, and also using oil dabbed onto a paper towel to help you wipe the wax off (and then washing the oil off with dish soap and hot water).
Thoughts for Next Time
Next time I make century eggs, I will try a different method for sealing and curing the eggs, because beeswax is a little hard to work with. I might try a high-quality food-grade clay (such as food-grade bentonite clay) instead. I will post an update with the results when I do!
References
Dunlop, F. (2008). Shark’s Fin and Sichuan Pepper : a Sweet-Sour Memoir of Eating in China. London: Ebury Press.
Dunlop, F., & Terry, C. (2013). Every grain of rice: simple Chinese home cooking. New York: W.W. Norton.
Wang, J., & Fung, D. Y. C. (1996). Alkaline-Fermented Foods: A Review with Emphasis on Pidan Fermentation. Critical Reviews in Microbiology, 22(2), 101–138. doi: 10.3109/10408419609106457
