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Published: Sunday July 13, 2014 MYT 12:00:00 AM
Updated: Sunday July 13, 2014 MYT 10:13:49 AM

Dark and jellified: Century eggs

The Curious Cook investigates what happens in the process of creating this pungent delicacy.

This story is about something curious, pretty ubiquitous in Asia, but seldom seen in the West. I was always inquisitive about that somewhat pungent delicacy misnamed Century Eggs or even Thousand Year Eggs, having discovered them as ingredients in several rather quirky (but very pleasant) Chinese dishes. Being a little familiar with the Maillard reaction, I was wondering what chemical processes cause the intense discolouration of the eggs and the history behind their manufacture.

First of all, I must apologise to the people who had asked me previously about the production of century eggs – I had been originally misinformed by some Thais who fed me a plausible origin, which turned out to be wholly untrue. And therefore, I was guilty of passing on their fabrications, which are actually quite generally accepted as genuine by lots of other people so perhaps they cannot be completely held at fault.

The main misconception, despite them actually being called “horse urine eggs” in Thailand and Laos, is that the manufacture of century eggs has something to do with equine piss. This is simply untrue because the alkalinity of horse urine never goes above a pH of 7.5 to 7.9. Even human urine exceeds this with a maximum pH of 9.4 – which though whiffy – is never alkaline enough to make century eggs. One requires an alkalinity of at least pH 12 to cause the reactions that cure normal duck (or chicken or quail) eggs into dark, distinctive and gelatinous century eggs.

No, the history of century eggs is rather more prosaic and it seems that a house builder in Hunan during the Ming Dynasty (some 600 years ago) left a batch of eggs accidentally for a couple of months in a pool of slaked lime used to prepare mortar. Incredibly, though fortuitously, instead of chucking the darkened eggs, he decided to crack open the denatured eggs and actually ate them. He liked them and decided to produce more – adding salt to the slaked lime to improve the flavour.

The recipe has changed little since then, with only the addition of strong black tea to improve the colour and flavour. So if you wish to produce your own century eggs, the traditional recipe is actually rather simple. All you need are the following (though it might be considered a good idea and polite to ask your family first if it is OK with them):

1 litre very strong Chinese black tea (keep the tea leaves)

750ml wood ash

750ml charcoal ash

425g quicklime

160g sea salt

18 fresh duck eggs

1.5kg rice chaff

Strong latex or plastic gloves

A large plastic or porcelain mixing bowl (don’t use a metallic mixing bowl)

Another plastic bowl to hold the rice chaff

A large clay container to be filled with soil and garden lime (or use your garden)

Face mask (optional)

You start by brewing the tea – by volume, use the ratio 1 cup of cheap Chinese tea to 10 cups of water. You need this tea to be pretty darn strong, so after brewing, leave the tea for a few hours to get really intense with the tea tannins.

Put on your safety gloves and place the ashes, quicklime and salt into the mixing bowl and mix with the tea until it becomes a dark mud – you might need only around 700ml of the tea. Don’t make the mud too watery – about the consistency of a wet cake baking mix will be fine. Add the tea leaves to the mud and mix in well. You might want to wear a face mask as well because of the smell.

Now it gets slightly messier, I’m afraid. Gently place the eggs into mud and carefully encase each egg completely with the mud. Leave for 15 to 20 minutes, then carefully roll each muddy egg in the rice chaff, making sure it is wholly coated with the chaff. Press the chaff onto the mud very gently as required – please avoid breaking the eggs. Place the chaff-coated eggs on a large plate to dry.

Leave the eggs in a cool place at least overnight or 24 hours for the chaff and mud to harden. Then bury them inside a large clay container with soil mixed with some gardening lime, covering the eggs well. Or bury them in a hole lined with a thick plastic sheet in a sheltered spot in your garden, again covered with garden soil and garden lime – please pick a spot which does not get flooded when it rains. Pour any remaining tea over the covered, buried eggs. The important thing is that the eggs are exposed to the weather as they cure better when they get rained on and then dry again naturally.

The eggs generally take two to three months to cure fully. After two months, exhume one of the eggs and check if it has been fully cured. If not, leave for another month.

century egg
Covered in mud and rice chaff, and wrapped in cellophane for sale.

Now comes the bit that interested me initially – the convoluted chemical processes within the egg that causes the black pigmentation and change of texture. I found a website which claims the reactions are as follows:

CaO + H2O -> Ca(OH)2

Ca(OH)2 + Na2CO3 -> 2NaOH + CaCO3

Na2O3 + H2O -> 2NaOH + O2

K2O + H2O -> 2KOH

But this is highly unlikely as it involves a chemical called disodium trioxide (Na2O3), which is an impossible compound. A further probe into the mystery and I find that the eggs are cured primarily due to the introduction of hydroxide ions and sodium into the albumen and yolk of the egg.

However, this glib explanation does not cover the true depth of the complexity of what happens inside the egg. For example, ovalbumin is the main protein in egg white – which in turn is made up of 385 amino acids.

The manner in which each individual amino acid is denatured by the addition of inorganic elements such as sodium and calcium is seriously complex, and probably very boring if probed further here. And we haven’t even gotten to the egg yolk, another complex assembly of numerous different proteins, fatty acids and carotenoids.

So I am afraid that the full mystery of how these eggs are cured cannot be explained in a simple newspaper article – but, like many quirky foods, I guess we don’t need to know all the fine chemistry details as it is simply fortunate that we have these foods, despite their unlikely origins.

Imagine the brave person who drank the first cloudy mug of fermented grain and founded the beer-brewing industry. Or the person who ingested the first mouthful of curdled rotten milk (which had been stepped on by bare human feet) and invented the cheese platter.

Regrettably, I have to now wade in with a little warning. As with all commercially-produced food items, producers are always looking for ways to speed up the production process – and investigations have found dangerous levels of lead oxide and other poisonous chemicals such as copper sulphate which are used to hasten the curing process of century eggs produced in China.

As lead is poisonous in any concentration, I would suggest avoiding century eggs from China for the moment. Or possibly for a long time, until that country gets to grips with basic human concepts such as consumer safety and quality standards.

Anyway, the reactions to tasting century eggs vary widely – from gentle appreciation to that look on people’s faces when they realise that they’ve just stood on something they shouldn’t have. And whatever they are, century eggs are definitely not binge-eating fare – unlike chocolate or good fried chicken, for example. In quantity, I suspect that they would be a little too weird for many digestive systems, so do enjoy your non-China exported century eggs sparingly.

Tags / Keywords: century egg

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