Thousand Year Eggs???
Century
egg, also known as pidan, is a type of preserved egg, millennium egg, skin egg or
black egg. Moreover, it is one type of most traditional and popular preserved
product which are made from duck eggs, chicken eggs or quail eggs in Thailand, China and South East Asian countries. Besides that, it is
made by using a mixture of alkaline solutions, salt, black tea, and metal ions
for 4–6 weeks at room temperature. Moreover, century eggs have many
characteristics such unique flavour, dark green yolks and dark brown or
transparent egg whites.
Century eggs
According
to Food Regulation 1985 regulation 177, preserved egg shall be fresh poultry
egg that has been preserved by the application of salt or any substance.
Besides that, preserved egg may contain permitted food conditioner. In fact,
century eggs can be made by using fresh chicken eggs, duck eggs or quail eggs.
Century
eggs can be classified into two categories in which it depends on the yolk is
semi-solid or hard. In fact, century eggs with semi-solid yolk have a more
pleasant, fragrant taste without any undesirable pungent lime flavour. Besides
that, century eggs with semi-solid yolk also do not have any aftertaste. To
obtain or produce eggs with semi-solid yolk, the paste or coating fluid used
contains less table salt, small amount of lead oxide and lower alkalinity. On
the other hand, century eggs with hard yolks have a slightly pungent, salty
taste and a longer-lasting aftertaste. Moreover, the amount of ingredients used
to produce century eggs with hard yolks are different and lead oxide is
omitted.
Generally,
century eggs are made by pickling the eggs in 4.2% sodium hydroxide (NaOH)
solution and 5% sodium chloride (NaCl) solution at ambient temperature for 20
days. The production of century eggs is due to the penetration of alkali
through the egg shell and membrane. This leads to chemical changes in the egg
components. In fact, albumen and yolk will become solidified and hardened
gradually. Under strong alkali treatment, the proteins and fats of the egg will
break into variety of peptides, amino acids, and volatile compounds. The
prolonged treatment of alkaline may alter the nutrients found in egg white. In
this context, for making century eggs, chemical reactions that take place and
contribute to the special sensory characteristics of century eggs are as stated
below:
During
the fermentation process, the sodium hydroxide formed will penetrate through
the eggshell pores and causes physiochemical changes inside the eggs. The
degradation of egg proteins is caused by the sodium hydroxide which results in
the gelation and colour change of the albumen. Besides that, the tea which is
used in the coating method can help to enhance the brown colour of the albumen.
Furthermore, crystallization of the degraded protein products can also lead to
the formation of fine, snow-flake patterns in the albumen. For the egg yolk,
diffusion of sodium hydroxide into it can cause the proteins to coagulate into
semi-solid or solid state. Moreover, the formation of ferrous sulphide will
also cause the yolk to be in dark-green colour. Last but not least, the action
of sodium hydroxide during fermentation will result in the release of hydrogen
sulphide and ammonia. The presence of ammonia can increase the pH value of the final
products.
In
fact, manufacturers usually use three different methods to produce century
eggs. These methods are coating, immersion and the combination of both methods.
For coating methods, fresh eggs are coated with a paste mixture of tea, salt,
ashes and lime. Then, the eggs are rolled in mud to prevent the eggs from
sticking. After that, a large crock is lined with garden soil and the coated
eggs are laid in top. Next, covered it with more soil and the crock is placed
in a cool dark place and cured for 100 days.
Differences between coating method and immersion method.
Flow
chart for the processing of century eggs by using coating method.
Processing of century eggs by using coating method and some chemical reactions involved
Source of video: https://www.youtube.com/watch?v=NL8kMj-GlLs
On the other hand, for
immersion method, 72 g of sodium chloride and 42 g of sodium hydroxide are
mixed completely in 1 L of water. Then, the eggs are submerged into the saline
solution and stored at 15-20 oC for 10 days.
After that, the pickled eggs are taken out and rinsed. Next, they are allowed
to dry naturally. Furthermore, the eggs can be coated with polyvinyl acetate(PVA) or red soil can
be added to the saline solution after the pickled eggs are removed. Then, the
eggs are coated with mud and rolled them in rice husk for 2 weeks. This method
is more convenient and has a short fermentation time. In either method, lead
oxide or zinc oxide can be added to produce soft-yolked century eggs.
Flow
chart for the processing of century eggs by using immersion method.
Source:
https://www.thefooddictator.com/cory-lees-thousand-year-old-quail-egg-pickled-ginger/, http://www.instructables.com/id/century-eggs/
Differences between coating method and immersion method.
Furthermore,
century eggs contain high value of nutrition especially its egg white is rich
in proteins and essential mineral such as calcium and iron although some of the
amino acids and vitamins in the preserved egg white may be destroyed by the
alkali treatment. When compared with the fresh eggs, century eggs have a higher
protein and fat content. Besides that, the century eggs also have a higher ash
content. Moreover, century eggs have lower mineral contents than the fresh eggs.
This may due to the strong alkali treatment which destroys the mineral
contents.
Nutrient contents of century eggs and fresh eggs per 100 g.
Century eggs packaged in cardboard
(left), plastic container (centre) and plastic (right).
In conclusion, century egg is one
type of preserved egg that can be found commonly in the market. It can be eaten
without further preparation, on their own or as a side dish. Besides that, alkali treatment induces the
formation of cross-linking proteins. Thus, more attention should be paid to the
formation of such undesirable products during alkaline processing of
protein-rich foods, such as century eggs. Furthermore, the nutritive value of
proteins in century eggs may be reduced. This is due to the destruction of
essential amino acids such as threonine as well as the racemization of some
amino acids. In the future, the forming mechanism of specific characteristics
of preserved egg should be studied, and some new technology should be built to
control nutrient loss and safety of preserved egg in the pickling process.
1. Billow,
Alexa (2018) What Are Century Eggs? [Online], [Accessed 5th April 2018]. Available from
World Wide Web: https://www.youtube.com/watch?v=NL8kMj-GlLs&t=55
3. Century Eggs (2012) [Online], [Accessed 10th April 2018]. Available from World Wide Web: http://www.instructables.com/id/century-eggs/
4. Cory Lee’s Thousand Year Old Quail Egg With Pickled Ginger (2016) [Online], [Accessed 25th April 2018]. Available from World Wide Web: https://www.thefooddictator.com/cory-lees-thousand-year-old-quail-egg-pickled-ginger/
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