BiologyChemistryOther

Century eggs: What are they?

Abstract

Century eggs are one of the traditional delicacies originating from China. This research investigated the difference between traditional and industrial methods of making century eggs, through various factors such as the number of steps required and time required for century eggs to complete their formation. Secondary to this comparison, the chemical process and scientific theory behind the formation of these eggs was explored. The traditional method involved wrapping a batch of eggs in a chemical clay and leaving them for 10 days. The industrial method, however, involved soaking the eggs in a chemical solution for a while. Both methods observed century egg formation in which the albumen proteins denature under an alkaline condition. However, the stated 10 day period of the traditional method was not enough to completely preserve the eggs. Future experiments could include chemically dissolving the eggshells in advance to reduce the diffusion barrier and distance for the alkaline conditions to reach the albumin of the egg. In turn, this would reduce the time required for century eggs to form, meaning that production costs could be lowered.

Introduction

Century eggs are a traditional Chinese delicacy not only widely consumed in China but are also famous on an international level, due to their distinctive colour and look [1]. Century eggs have other well-known names such as pidan, the pine-patterned egg and millennium egg. Although the discovery was believed to be a coincidence, methods were later developed to preserve chicken, quail eggs, and most commonly duck egg [1].

The century egg was discovered more than 600 years ago during the Ming Dynasty. Although the exact origin of these eggs was not made clear, a popular legend, that began in the teahouse of a man in Wujiang, was said to be the most probable origin [1].

Despite the number of studies conducted over the past decades, the collective understanding into the formation of century eggs is still generally unknown to the general Chinese public. This is likely due to lack of education and publicity of these studies and the studies are generally difficult for common people to understand. As a result, this research will also consider the scientific principle behind how these eggs have formed in sufficient detail, while at the same time keeping it at a level of which the general public can understand.

Depending on the method of processing and proportions of each ingredient, century eggs may take any time between several weeks to several months to fully develop. When the right proportion of each ingredient is added to water, a series of chemical reactions occur. First, the calcium oxide would react with water: CaO + H2O → Ca(OH)2. The product calcium hydroxide then further reacts to produce a strongly alkaline environment: Ca(OH)2 + Na2CO3 → CaCO3 + 2NaOH and Ca(OH)2 + K2CO3 → CaCO3 + 2KOH. The sodium hydroxide, potassium hydroxide and calcium carbonate dissociate into hydroxyl ions, potassium, sodium or calcium ions, respectively. These metal ions diffuse through the eggshell. The hydroxyl ions dissociate into the water, giving the alkaline conditions for the egg to be placed in [2].

The high pH causes the protein to denature breaking down the interactions between the amino acids in the secondary and tertiary structures (the secondary structure is the initial folding of the amino acid sequence into an alpha helix or a beta-pleated sheet and tertiary structure is the further folding of the protein to make a 3D arrangement)[3]. This leads to the proteins in the albumen (egg white) to solidify and, since water is present, gelatinise. The strong alkaline conditions also break down fats through a process known as saponification, this leads to the development of a unique flavour for the century egg [4]. At the same time, the metal ions, as well as tannins (a class of chemicals that bind to and precipitate proteins) found in the black tea, prompt the protein to solidify [5]. Consequently, the proteins such as those in the albumen are broken down into different amino acids, such as threonine, isoleucine and leucine. These amino acids further produce hydrogen, ammonia and a very small quantity of hydrogen sulphide. Ammonia and hydrogen sulphide, which are well-known for their pungent smell, are essential to provide flavours for the century eggs [1]. Furthermore, the amino acids would take salt form under alkaline conditions, resulting in the formation of crystals, with the shapes of pines, in the albumen of a century egg. Another main source of protein in the egg is the yolk, which is high in sulphur. As the yolk proteins are broken down through the action of hydroxyl ions, they produce hydrogen disulphide and hydrogen. The pigments in the egg yolk are combined with all kinds of metal ions, which leads to the dark green colour of the yolk [1].

The complete gelatinization of the albumen is a sign that the century egg has fully formed. Therefore, other chemicals, such as lead oxide, are often added to the solution to avoid the gelatinized protein from reliquefying [1].

There are two main methods of producing century eggs. The traditional method involves wrapping the egg with a mixture of clay and other ingredients. The newly devised industrial method produces century eggs by placing them into a chemical solution consisting mainly of calcium oxide. The lead oxide used in the traditional method originally is replaced by other metal compounds in the industrial method and most traditional methods by now because large and frequent consumption of lead could result in lead poisoning, a very serious condition which could cause health problems such as insomnia, inability to concentrate, anaemia, joint pain and brain damage and also death [1].

Although there are issues with lead poisoning, most century eggs currently found on the market are lead-free. However, this does not mean that there is no trace of lead in these eggs, rather that they contain a tiny amount of lead which is below regulation level. In fact, century eggs can be beneficial to one’s health. In terms of nutrition, century duck eggs are often rich in iron, amino acid and vitamin E. However, the proteins denatured by the alkaline conditions may be difficult to absorb, which likely occurs within the gut. Lysinoalanine is an amino acid commonly found in processed foods and is formed during alkali treatment of protein which has led to kidney failure in rats, but this has not been proven[1][6]. According to traditional Chinese medicine, century eggs help to reduce eye problems, toothaches, high blood pressure and tinnitus (a medical condition where there is a ringing noise in the ears)[7]. However, there is not yet any physical evidence to support these claims.

The process to form pidan eggs is often time-consuming and requires some amount of human labour. Although the century eggs are not very expensive, any reduction in the time taken, the amount of ingredients used, or the labour put in will result in a considerable increase in economic profit. By conducting both the traditional and industrial methods, this research aims to make a comparison in time and work required in each.

Methods

Industrial Method

A feed solution was made using a combination of chemicals in which the duck eggs were placed into (see Table 4 in the Appendix). This involved mixing sodium carbonate and black tea powder to the bottom of each container, before adding boiled water and calcium oxide. Each container with the feed solution was mixed thoroughly until all the contents dissolved. 25ml of the surface layer from each container was extracted and placed into a mortar. Zinc chloride was added and grounded into the mortar, before this mixture was added back into the containers. The solutions were then allowed to rest for three hours at the end of which sodium chloride was added. After a further 24 hours of sitting, the solutions were mixed thoroughly again and the duck eggs were split between the containers (see Fig. 13 in the Appendix) for 45 days. The three containers were covered with a layer of gloves filled with water to fully submerge the eggs. Chicken eggs were also placed into the solution for comparison to the duck eggs. Due to a lack of funding and storage issues, the size of the experiment was scaled down. Ideally, all duck eggs should be placed in one container filled with the feed solution. However, there was great difficulty in finding a large enough container to fit all the eggs in so splitting up of the solution and eggs between three containers allowed the eggs to be easily transported if there was any need to change the place of storage[8].

Traditional Method

This experiment involved making a feed clay that was used to wrap the eggs in (see Table 5 in Appendix for ingredient list). Calcium oxide, sodium carbonate, plant ash (a by-product of burning charcoal), sodium chloride and tea leaves were added together. Water was then added and all chemicals were mixed thoroughly to form the clay-like mixture. The clay was left for 24 hours before it was used to wrap each egg and rolled in rice hulls so a layer covered the outside of the clay mixture (see Fig. 14 and Fig. 15 in the Appendix) All 12 containers were sealed (see Fig. 16 in the Appendix) for a 10-day duration. One egg was cracked open every day to note any observations. One of the 12 containers contained only chicken eggs which were only opened at the end of the 10 days. Another contained only duck eggs for the same purpose. The other 10 containers had a few duck eggs and one chicken egg indicated by a blue ink mark in each[9].

Results

Industrial Method

At the end of the industrial method, a few eggs were randomly picked out of the batch to be opened and examined. After 45 days, five eggs were opened and four of the eggs completely formed century eggs, as shown in Fig. 1. As figure 1 has shown, the albumen of the century eggs has turned into a black-green colour and has gelatinized. The yolk which Fig. 1 does not show has also turned dark green and solidified. Even though one of the eggs had not fully solidified, all five produced the distinctive smell and flavour of a century egg. Likely, this egg was not a long way from completion.

Figure 1. Photograph of Successful Century Egg Formation from the Industrial Method

The century egg has formed due to its dark green/black colour.

Traditional method

Overall, the 10 days allowed for the eggs to be wrapped in the feed clay did not show much progression in century egg formation. Throughout the 10 days, five eggs were opened every day to check for progress and see how they changed. The layer of clay wrapped around each egg was removed and each egg was washed with water.

Table 1. Observations Recorded Everyday During the Traditional Method

Observations were made for the full duration of the recommended 10 days.

Day

Figure

Observations

Day 1

Figure 2. Photograph Showing One of the Cracked Eggs Opened on Day 1.

The egg yolk and albumen can be seen and are unchanged.

Five eggs were cracked open but there were no changes observed to the albumen and the yolk (see Fig. 2), which was the expected outcome. Both were still liquid-like and the yolk still yellow. Also, there was no distinctive smell produced.

Day 2

Figure 3. Photograph Showing One of the Cracked Eggs Opened on Day 2.

On the left hand side of the contents of the egg, both the yolk and albumen can be visualised.

Again, there was no change to the five opened eggs. The yolk remained yellow and liquid-like as shown in Fig. 3.

Day 3

Figure 4. Photograph Showing One of the Cracked Eggs Opened on Day 3.

The egg yolk and albumen are visible, although the egg yolk is visibly broken.

Five eggs were opened with no change to both the albumen and yolk of the eggs as shown in Fig. 4.

Day 4

Figure 5. Photograph Showing One of the Cracked Eggs Opened on Day 4.

The egg yolk and albumen are visible in this picture contained in a plastic bowl.

The albumen and the yolk both remained as a liquid and the yolk, although split, is still clearly yellow as Fig. 5 shows. Furthermore, there was no unique smell.

Day 5

Figure 6. Photograph Showing One of the Cracked Eggs Opened on Day 5.

On the right-hand side of the egg contents, both the yolk and albumen can be visualised. The shell itself appears to have a marbled effect, which is staining caused by the clay.

As the eggs were being washed, the eggshells felt like they were covered by a layer of membrane. However, the inside of the duck eggs had no obvious difference to the previous days. Both the yolk and the albumen were still liquid-like and the yolk specifically still yellow, as seen in Fig. 6.

Day 6

Figure 7. Photograph Showing One of the Cracked Eggs Opened on Day 6.

The egg yolk and albumen can be seen contained in the shell. There is a visible change of colour, to green, in the yolk on the top right corner.

When the eggshells were cracked open, the top layer of the yolk had started to solidify, with all of them beginning to turn green. The yolk was still liquid-like below the layer, but it had turned into a golden colour, as shown in Fig. 7. A weak smell unique to a century egg was produced by all five eggs. Some sedimentation of the gelatinised egg yolk was observed on the eggshell (see Fig. 17 in the Appendix).

Day 7

Figure 8. Photograph Showing One of the Cracked Eggs Opened on Day 7.

The egg yolk and albumen can be seen in the cracked eggshell, with a clear change in yolk colour to green.

Three of the five duck eggs opened on day 7 had yolks with a dark green colour, as shown in Fig. 8. The yolks of these three eggs have also gelatinized more than the eggs opened on day 6 (see Fig. 18 in the appendix for a different perspective) Four of the five duck eggs, including the three with the most progress, produced the unique smell. The one that did not produce this flavour was an egg that had rotted (see Fig. 19 in the Appendix). The rotten duck eggshell had a crack in it which could have been the reason for it rotting.

Day 8

Figure 9. Photograph Showing One of the Cracked Eggs Opened on Day 8.

The egg yolk and albumen can be seen contained in the cracked eggshell, with changes in colour for both egg yolk and albumen seen.

Only the first egg opened had further progress in turning into a century egg. The yolk of this egg already looked very similar to that of a complete century egg, and the albumen had turned golden and gelatinized, as seen in Fig. 8. The other four duck eggs opened still had yellow yolk with a pale yellow, the gel-like outer layer covering it (see Fig. 20 in the Appendix).

Day 9

Figure 10. Photograph Showing One of the Cracked Eggs Opened on Day 9.

The cracked eggshell contains the visible egg yolk and albumen with unexpected observations of no change from a normal duck egg.

Not one of the yolks was dark green, as would have expected. All five duck eggs had a paler yellow, gel-like outer layer covering the yolk, as seen in Fig. 10. The albumen was not golden or gelatinized. The five duck eggs did not produce a unique smell. The eggs would have been expected to have almost turned into a complete century egg.

Day 10

Figure 11. Photograph Showing One of the 10 Cracked Duck Eggs Opened on Day 10.

The cracked eggshell contains the visible egg yolk and albumen, again with an unexpected observation of no alteration in colour. This shows no progression in the formation of the century egg.

Figure 12. Photograph Showing One of the Chicken Eggs Opened on Day 10 From the Traditional Method.

The green colour and slight gelatinisation can be seen, which shows that the century egg formation is taking place but is not yet complete.

The eggs had similar observations to that of day 9. The last 10 duck eggs and all 14 chicken eggs were opened. All the duck eggs had no dark green colour, as would have expected (Fig. 11). The albumen did not gelatinize. As for the chicken eggs, all had a dark green and gel-like outer layer covering the yolk, this can be seen in Fig. 12. The observed changes within these eggs were similar to some of the duck eggs opened on day 7 and 8.

On day 10, a total of 20 duck eggs and 15 chicken eggs were opened. One egg from both the duck and chicken batches remained unopened and were sent to Jilin University laboratory along with an original duck egg and a century egg bought on the market. These four eggs were analysed for their compositions, as shown in Table 2.

Table 2. Translated Food Nutrition Composition Detection Report.

As the original report was in Chinese, it has been translated to English. For the original report see Fig. 21 in the Appendix.

(Taken for every 100 grams of edible parts)

Components

   

Res

ults

Normal duck egg

Duck Century egg produced by

traditional method experiment

Chicken Century egg produced by traditional method experiment

Century egg bought from the market

 

Edible parts (%)

 

87

88

83

87

Water content

(g)

 

70.3

61.3

66.4

69.3

Energy (KJ)

 

753

795

745

802

Protein (g)

 

12.6

12.7

14.8

11.1

Fat (g)

 

13

12.7

10.6

14.6

Carbohydrates

(g)

 

3.1

6.3

5.8

2.8

Cholesterol (mg)

 

565

647

595

685

Total Vitamin A (μ gRE)

 

261

134

310

192

Total Vitamin E (mg)

 

6.98

6.25

1.06

4.50

Retinol (μg)

 

261

134

310

192

Thiamine (mg)

 

0.17

0.16

0.02

0.08

Riboflavin (mg)

 

0.35

0.33

0.13

0.30

Calcium (mg)

 

62

118

26

34

Phosphorus (mg)

 

226

231

263

130

Potassium (mg)

 

135

184

148

74

Sodium (mg)

 

106

120

0

90.6

From the composition report, it is clear that the water content was the highest in normal duck eggs and the lowest in the century egg produced by traditional method. The century egg bought from the market had only a 1.4% difference in water content to that of the bought duck egg. However, the century duck egg produced by the traditional method had a difference of 12.8% to the normal duck egg. In terms of energy, the chicken century egg produced by the traditional method had the lowest while the century egg bought on the market had the highest. The duck century egg produced by the traditional method had a similarly high energy composition to the market bought century egg. For the protein composition, the chicken century egg produced by the traditional method had the highest protein content while the century egg bought from the market had the lowest protein content. The duck century egg produced by the traditional method had a 0.79% difference to that of a normal duck egg and a 14.4% difference to the market bought one.

Discussion

Since the production of century eggs was thought to be by accidental discovery rather than developed specifically, this meant that the method was likely to be passed down by word of mouth and therefore slightly altered over the generations. Although there is not an ideal nutritional composition for century eggs, the idea of having nutritional benefits that will contribute to our daily nutritional intake is important, but not specifically ideal to century eggs alone. Other ideal century egg formation processes may not be in terms of their nutritional composition but rather reducing cost and time, for example, having the most efficient amount of ingredients and reducing the time it takes to form these century eggs.

The major difference between the traditional and industrial method was the time factor. The industrial method was completed over a duration of 45 days to allow the eggs to be left in an inaccessible place. Whereas, the traditional method was completed over a suggested duration of 10 days and is likely the reason for the large difference in stage of century egg formation observed [9]. However, only those eggs in the last 2 days showed unexpectedly slow progression towards completion, so this may be due to problems that only posed to the eggs and conditions in these two batches. One possible explanation could be that these containers were not fully airtight, but to conclusively show this would require further investigation.

Additionally, the ingredients used were slightly different between the two methods, which was expected because traditional methods use more old fashioned techniques and some of the chemicals used in the industrial method may not have been readily available at the time of its development. Although there were slight differences in ingredients, it is believed that this was not very significant in altering the timing of century egg completion observed at the end of the two experiments. Finally, the eggs involved in the industrial method were left outdoors in the UK and the eggs involved in the traditional method were left indoors in China, suggesting there may be slight differences due to the temperature because China generally has a higher temperature in the summer than the UK. Temperature affects the rate of reaction in general and the rate of diffusion, which means it could alter the speed of century egg formation since the alkaline conditions could penetrate the eggshell to alter the protein structure faster at its optimum temperature [10].

The eggs from the industrial method were not able to be sent to a laboratory for composition analysis so only the composition of the eggs of the traditional method could be discussed. The eggs did not completely form into century eggs using the suggested traditional method and therefore the composition was compared to a store-bought century egg to see if the stage of formation the eggs were at could be concluded [9]. This can be seen from Table 2 as there is a noticeable difference between the duck century egg produced through the traditional method and the century egg bought on the market. This could mean that the century eggs produced were still not close to completion. Since the eggs in the traditional method were not completed during the 10 days, it can not be said which method would have taken longer. Furthermore, the eggs in the industrial method were only opened after 45 days, meaning they could have been completed at an earlier date. Without a mathematical or scientific technique to extrapolate the results in the traditional method, it could not be determined which method would have taken longer for completion. Another plausible explanation would be due to the individual compositional differences between eggs. Since the differences shown in Table 2 are not very large and not all the composition differences fall between both the bought duck egg and century egg, this explanation may be more likely.

Furthermore, it can be concluded that the industrial method was developed as an improved method for manufacturing purposes. A lot of effort was required in terms of preparing the eggs during the traditional method since each had to be physically wrapped with a layer of the clay mixture and then rice hulls. This would certainly require manual work from employed workers and thus it is not only a time-consuming preparation process but it would also require more money to hire employees. On the other hand, the industrial method only required placing the eggs in the feed solution, which could be done by machines, and does not require as much time for the preparation process. This would cut down the cost of production and increase the profit margins for large factories dramatically.

Table 3 has many differences when compared to the results found for the traditional method. First, there is an 80 kJ (19 kcal) energy difference between the century egg from Table 3 and the duck century egg, from the traditional method, in Table 2. Also, the duck century egg produced from the traditional method was 10.6% lower in protein content, 18.7% higher in fat content and 40% higher in its carbohydrate level. It is most likely down to the unique nutritional composition of each egg.

Table 3. The Century and Duck Egg Nutrition Compositions

This was created by the Institute of Nutrition and Food Safety (Chinese Centre for Disease Control) published in 2002 [11].

(Taken for every 100 grams of food content)

 

Components

 

Century egg

Duck egg

Edible parts (%)

90

87

Water (g)

68.4

70.3

Energy (kJ)

715

753

Protein (g)

14.2

12.6

Fat (g)

10.7

13

Carbohydrates (g)

4.5

3.1

Cholesterol (mg)

608

565

Ash (g)

2.2

1.0

Total Vitamin A (μ gRE)

215

261

Total Vitamin E (mg)

3.05

4.98

Retinol (μg)

215

261

Thiamin (mg)

0.06

0.17

Riboflavin (mg)

0.18

0.35

Niacin (mg)

0.1

0.2

Calcium (mg)

63

62

Phosphorus (mg)

165

226

Potassium (mg)

152

135

Sodium (mg)

542.7

106.0

Magnesium (mg)

13

13

Iron (mg)

3.3

2.9

Zinc (mg)

1.48

1.67

Selenium (μg)

25.24

15.68

Copper (mg)

0.12

0.11

Manganese (mg)

0.06

0.04

Further research

As there was very limited time available, it would be optimal to repeat the two experiments, especially since the traditional method did not see the completion of the process. This would allow observations to be confirmed and for a more complete set of results to be given.

In the future, an experiment that alters the ingredient composition could be done to see which one of the ingredients is the most important and which ingredients can be reduced. This would involve two sets of experiments that involves both of the methods while altering the amount of only one ingredient at a time. This should show which ingredient impacts each method the most, allowing for optimised mixtures to be identified and to reduce the cost of production, to bring a potential economic benefit.

There could also be an experiment done to see if it matters which alkaline is best in forming century eggs. This is because fundamentally it is the alkaline conditions in which these eggs are placed under that causes them to transform into century eggs. So, a possible question that could be answered is if the strength and type of alkali would affect the time it takes to form century eggs.

Further research should investigate the effect of temperature change has on century egg formation to find the optimum temperature. This could be done by doing both of the methods under different temperature conditions and within the same geographical location.

Lastly, a potential experiment could be set up to observe whether a change in the permeability of duck eggs would allow the compounds to penetrate faster and hence reduce the time it takes for the century eggs to form.

With these further research ideas, the processing of century eggs could be optimised. This may be by processing the eggs with the fewest ingredients required and leaving the eggs in a temperature that reduces the time it takes for century eggs to form. Also, there could be methods of chemically altering the eggshells so that the chemicals would diffuse into the egg faster. All these improvements could lead to even cheaper production price and greater production yield so the profits of these century eggs would be maximised.

Conclusion

Through the two experiments, the observational results for the industrial method saw the century eggs fully form, meanwhile the eggs both chicken and duck in the traditional method showed some progress towards the completed formation of century eggs. There are a few hypothesized explanations for this incompletion, including the possibility that the containers opened towards the end were not an airtight environment or there was not enough time given under those conditions.

Through this research, it remains unclear as to which method took the longest for century eggs to fully form from. This is due to an insufficient time period of 10 days for the traditional method and only opening the eggs made using the industrial method after 45 days [8][9]. Secondly, there was a difference in temperature environment, indicating that the rate of reaction and diffusion would have been different for the two experiments and thus a more reliable comparison is needed. Although the results may not be very reliable, a comparison can still be produced. There were slightly differences in the nutritional composition within Table 2, which is likely down to the uniqueness in the nutrition of individual eggs.

This research also may act to provide sufficient scientific knowledge for the educational purposes to the general public, since such knowledge is not commonly known even for those that consume century eggs regularly. The alkaline environment is essential for the formation of century eggs since it alters the protein structure, allowing the physical and chemical changes to be observed during the century egg development process.

One of the possible future research projects should include removing the eggshell and see if this would have any effect on both the rate of formation and the ingredients required.

Acknowledgements

I would like to thank my supervisor throughout this project for continued guidance and professional advice. I would also like to thank Mr. White-Foy for providing me with the opportunity to do a research project under the CREST Award scheme. I also appreciate the University of Jilin for accepting my experiment samples and analysing the nutritional composition for the traditional method. Finally, I would like to thank my secondary school, Dulwich College, for providing the chemicals in the industrial method, and Mr. Willet (a chemistry teacher at Dulwich College) for supervision during the industrial method experiment.

Reference

  1. “Century egg_360 Baike,” Baike.so.com, [Online], Published 23rd March 2020, Accessed 2nd July 2020, Available at: https://baike.so.com/doc/history/id/24969004
  2. “How is Pidan Formed?” Zhidao.baidu.com. [Online], Published 18th April 2017, Accessed 24th August 2020], Available at: https://zhidao.baidu.com/question/629200695881098884.html
  3. “Protein Structure | Learn Science At Scitable,” Nature.com, [Online], Published 2014, Accessed 21st August 2020, Available at: https://www.nature.com/scitable/topicpage/protein-structure-14122136/
  4. “Saponification – An Overview | Sciencedirect Topics,” Sciencedirect.com, [Online], Accessed 22nd August 2020, Available at: https://www.sciencedirect.com/topics/chemistry/saponification
  5. “Tannin – An Overview | Sciencedirect Topics,” Sciencedirect.com, [Online], Accessed 22nd August 2020, Available at: https://www.sciencedirect.com/topics/chemistry/tannin
  6. “Lysinoalanine – An Overview | Sciencedirect Topics,” Sciencedirect.com, [Online], Accessed 29th June 2020, Available at: https://www.sciencedirect.com/topics/nursing-and-health-professions/lysinoalanine
  7. “Century egg_BaiduBaike,” Baike.baidu.com, [Online], Published 20th August 2020, Accessed 2nd July 2020, Available at: https://baike.baidu.com/item/松花蛋
  8. “Method of making century eggs,” Wenku.baidu.com, [Online], Published 25th June 2013, Accessed 2nd July 2020, Available at: https://wenku.baidu.com/view/297ec6b40029bd64783e2cc7.html
  9. “Century egg making method_360Q&A,” Wenda.so.com, [Online], Published 29th Novemeber 2012, Accessed 2nd July 2020, Available at: https://wenda.so.com/q/1353323619122990?src=150
  10. Bicking. M, “EXTRACTION/Analytical Extraction”, Sciencedirect.com, [Online], Published 2000, Accessed 24th August 2020, Available at: https://www.sciencedirect.com/sdfe/pdf/download/eid/3-s2.0-B0122267702022717/first-page-pdf
  11. Yang. Y, Wang. G and Pan. X, “China Food Composition 2002,” Beijing: Peking University Medical Press, (2002), pp.136 – 137, Accessed 24th August 2020

Appendix

Table 4. Ingredients Used in the Industrial Method.

Industrial Method Ingredients

96 duck eggs

744 grams of sodium carbonate

2112 grams of calcium oxide

369.6 grams of sodium chloride

24 grams of black tea powder

13.6 grams of zinc chloride

10.56 kilograms of boiling water

Table 5. Ingredients Used in the Traditional Method.

Traditional Method Ingredients

65 duck eggs

20 chicken eggs

2250 grams of calcium oxide

135 grams of sodium carbonate

45 grams of plant ash

90 grams of sodium chloride

900 grams of water

10 tea bags of tea leaves

500 grams of rice hall

Figure 13. Duck eggs submerged in the feed solution in the Industrial Method

Figure 14. Eggs Wrapped in the Feed Clay without Rice Hulls in the Traditional Method

Figure 15. Eggs wrapped in the Feed Clay with an Outer Layer of Rice Hulls in the Traditional Method

Figure 16. Picture showing all the Containers with all the eggs that were used in the Traditional Method

 

Figure 17. Picture Showing the Sedimentation from the Gelatinisation of the Egg Yolk left on the Egg Shell on Day 6 in the Traditional Method

 

Figure 18. A picture showing another one of the Duck Eggs opened on Day 7 in the Traditional Method

 

Figure 19. A picture showing the inside of the Rotten Egg opened on Day 7 in the Traditional Method.

 

 

Figure 20. A picture of one of the other four Duck Eggs opened on Day 8 in the Traditional Method

Figure 21. Image Taken of the Chinese Composition Table of the Eggs Sent From the University Laboratory

The first column showed the composition of the normal duck eggs. The second column showed the composition of the duck eggs produced at the end of the traditional Method and the third column showed the chicken egg produced via the same method. The final column showed the composition of a century egg bought from the market.

Biography

Hao Gao

Hao is currently studying MBBS Medicine as an undergraduate at UCL, Hao is 21 years old and comes from Beijing, China. Hao likes to play the piano and badminton in his free time. Hao’s career ambition is to become a neurosurgeon and work at UCLH.

 

Jimin Zhang

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Jimin is currently studying biochemistry as an undergraduate at UCLA, Jimin is 22 years old and comes from Beijing, China. Jimin is thinking of studying medicine after he finishes his undergraduate degree.

 

 

Zihao He

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Zihao is currently studying a BSC in actuarial science as an undergraduate at LSE, Zihao is 22 years old and comes from China. Zihao likes to play the violin and football in his free time. Zihao’s career ambition is to go into actuary related careers.

 

Hongming Yu

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Hongming is currently studying a BEng in Mechanical Engineering as an undergraduate at Warwick University, Hongming is 22 years old and comes from Beijing, China. Hongming likes to work with drones in his spare time. Hongming’s career ambition is to go into UAV Engineering related careers.

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