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Hunger and technology

Egg preservation in China

H.C. Hou (Hou Xiangchuan)
Institute of Nutrition and Hygiene, Academy of Military Medical Sciences, Shanghai, People's Republic of China

According to Tibbles, eggs are preserved in western countries to restrain the growth of bacteria by storing them in a "pure" atmosphere at 30°F ( - 1°C). Other methods used to prolong shelf-life are wrapping tightly in paper, packing in dry lime, or varnishing the shells with glyceride, salicylic acid, boroglyceride, colloidin, shellac, or petroleum jelly. The best results are achieved when the eggs are put into a solution of water-glass (silicate of soda) or lime water.

In China, methods have been developed to preserve eggs in such a way as to cause chemical and physical changes in both egg white and yolk, imparting a new flavour that many people enjoy and conferring the added benefit of prolonged shelf-life.

PRESERVED EGGS (PIDAN)

Processing Methods

Perhaps the most important method for preserving eggs is the one that produces pidan, or eggs coated on the outside. Depending on preference, methods may be modified to produce songhuadan (pine floral eggs) or caidan (colourful eggs). The earliest known description of an egg preservation method is that of Wang Zizhen during the Ming Dynasty about 500 years ago. Blunt and Wang described an essentially similar method in 1918:

Into an infusion of 11/2, lbs of strong black tea is stirred successively 9 lbs of lime, 41/2 lbs of common salt and about one bushel of freshly burned wood ashes. This pasty mixture is put away to cool over night. Next day 1,000 duck's eggs of the best quality are cleaned and one by one carefully and evenly covered with the mixture and stored away for 5 months. Then they are covered further with rice hulls, and so, with a coating fully 1/4 inch thick, are ready for the market. They improve on further keeping, however, for at first they have a strong taste of lime which gradually disappears. The eggs are eaten without cooking.

Since then, a number of other methods differing only slightly from the above have appeared in the literature. For example, Chen Chunjen, in his book on Chinese foods published in 1963, classified methods into four types, although they are not radically different from one another or from that of Blunt and Wang. One of his methods is as follows:

For 100 duck eggs, take 21/2 catties [one catty equals about 600 g] of lime, dissolve in 1.6 catties of boiling water, then mix with charcoal powder (1.5 catties), caustic soda (0.5 catty), and table salt (0.4 catty). If a floral design in the egg white is desired, pine twigs with needles should be added during the boiling of water and discarded after boiling. The above mixture should be stirred and ground into a paste, which is then ready for coating over each of the 100 eggs. Finally, cover the coated eggs with a thin layer of rice hulls to prevent them from sticking together, and store in an earthenware jar, which is then sealed with mud. The jar is then put in a dry place. After two weeks' storage, the preserved eggs are ready to be eaten.

Another method uses similar ingredients except that tea is used instead of water. The tea is first mixed with yellow mud into a thin paste, into which the eggs are dipped one by one so as to be covered completely with a layer of the paste before they are removed. The lime, table salt, and caustic soda are mixed and heated in a cooking pot until bubbling. The mud-coated eggs are then put in, turned over to get fully coated, and then removed to store in an earthenware pot which is then sealed. After one month the eggs are ready to be eaten.

More recent literature describes many methods that vary somewhat in the amounts of ingredients used in the paste or immersion fluid as well as in procedures. However, the main ingredients-lime, salt, tea, and ash-are invariably the same. Duck eggs are generally preferred, as they are thought to produce a better result, although hens' eggs can be used. The time required for processing or aging varies from one week to five months, depending on the alkali concentration in the processing medium and temperature. Two preservation methods, one for coating and the other for immersing eggs, are as follows:

Coating method. To preserve 100 fresh eggs, use 0.7 catty of table salt, 0.4 catty of anhydrous sodium carbonate (Na₂CO₃ ), 0.04 catty of red tea leaves, 0.7 catty of lime (CaO), 8 catties of grass ash, and 0.4 catty of rice hulls. First, boil the tea leaves in water, filter out the leaves, and add sufficient boiled water to make 2.6 catties of liquid. Mix the salt, lime, and sodium carbonate in a wooden tub, add the liquid tea, and pound the mixture with a wooden pestle until quite smooth. Pass the grass ash through a sieve, add it to the mixture, and stir into a paste. Set aside overnight to allow time for the chemical reaction to take place. Clean the eggs and let them dry in the open air before coating them with the paste. When the eggs are completely coated, roll them over rice hulls, and place them one by one in a porcelain jar, which should then be tightly sealed with a mixture of yellow mud and a little table salt. Eggs preserved in this manner can be eaten after 15 days in hot summer weather, after 20 days in the autumn, and after 30 days in winter.

Immersion method. Immersion fluid is made by adding about 100 catties of water to a vessel containing a mixture of 6.5 to 8 catties of sodium carbonate, 15 to 35 catties of lime, 3 to 7 catties of table salt, and 0.2 to 0.5 catty of lead oxide, stirring continuously. When the mixture is cool, immerse the eggs one by one and maintain the mixture at a temperature two to three degrees below room temperature. After 30 days, the eggs, having acquired a dark coffee colour and a pleasant taste, are ready for eating.

A further simplified procedure has recently come into use: Mix table salt, tea leaf dust, and lead oxide with water; then gradually add in sodium hydroxide, stirring all the while, until the solution contains 5 per cent sodium hydroxide, 4 per cent table salt, 2 per cent tea leaf dust, and 0.2 per cent lead oxide. The eggs are first immersed in the fluid mixture for 25 days, after which the fluid is drained off and the vessel is sealed with a plastic membrane. After another 25 days the eggs are ready for eating.

Prolonging the Shelf-Life of Pidan

In order to prolong the shelf-life of the ripened, preserved eggs(pidan) without paste coating, a method of coating with a thin membrane may be used. The coating fluid consists of white wax, oleic acid, ammonia water, and boiled cold water. The eggs are thoroughly dipped in the mixture, and removed to let excess fluid drain off. They are then placed in ordinary, unsealed packing cases. Eggs so treated kept well for over 200 days in Nanjing, (Nanking) during the hot season, and tasted even better than eggs coated with mud by the old method. Membrane-coated eggs stored in earthenware containers or sealed plastic bags were inferior in quality to those stored in open cases, and developed an unpleasant taste.

Classification of Pidan

Pidan eggs are now classified into two categories, according to whether the yolk is semi-solid or hard. The semi-solid yolk has a pleasant, fragrant taste without any pungent lime flavour and with no after-taste. In processing eggs to obtain a semisolid yolk, the paste or coating fluid has less table salt and lower alkalinity and contains a small amount of lead oxide. Pidan with hard yolks have a slightly pungent, somewhat salty taste and a longer-lasting aftertaste. The amounts of the ingredients in the coating mixture used for hard-yolked pidan are somewhat different, and lead oxide is omitted.

Chemical Changes during Processing

After the ingredients to make pidan are mixed, the following chemical reactions take place:

CaO + H₂O -> Ca(OH)₂
Ca(OH)₂ + Na₂CO₃ -> 2NaOH + CaCO₃
Na₂O₃ + H₂O -> 2NaOH + O₂
K₂O + H₂O -> 2KOH
(The Na₂O₃ and K₂O are from the plant ash.)

Because of the porosity of the egg shell, NaOH is first adsorbed to the surface, and, owing to a change in the osmotic pressure, NaOH enters the egg through the pores and subsequently penetrates the semi-permeable membrane, coming into contact with the egg protein, causing it to become denaturized and hydrolysed into polypeptides and finally into amino acids.

It has been found in recent processing methods that the best pidan product is achieved when the NaOH concentration is maintained at 3.6 to 4.6 per cent. A concentration of 3.6 per cent coagulates the egg protein in four to eight days, while a 4.6 per cent concentration coagulates it in two to four days. With the former concentration, the pidan ripens in 55 to 65 days; with the latter, in 35 to 40 days. When the NaOH concentration is lower than 1.6 per cent, the egg protein does not coagulate, and thus the end-product is not pidan. The most suitable processing temperature is between 20 and 30 C. The higher temperature accelerates processing. The presence of tea leaf tannin, lead oxide, and calcium oxide also influences the coagulation process as well as osmosis, and thus it influences the ripening too.

TABLE 1. Nutrient Contents of Fresh Duck Eggs, Pidan, and Salted Eggs (per 100 9)

TABLE 2. Amino-Acid Contents of Fresh Duck Eggs and Pidan (mg) per 100 grams)

TABLE 3. Fatty Acids in the Yolk of Fresh Duck Eggs and Pidan (Percentage of Total Fat)

*The figure on the left of the colon indicates the number of carbon atoms in the acid and that on the right the number of double bonds. Thus "14:0" designates the saturated (having no double bonds) acid with 14 carbon atoms.

Nutrient Composition of the Eggs

Analysis of the ordinary nutrients in pidan and in fresh eggs, as compared in table l, shows that the two are quite similar except that pidan have a higher protein content and less carbohydrate. This may reflect the variety of eggs analysed here, since fresh duck eggs from Jiangsu Province are known to contain 14.2 per cent protein and 0.3 per cent carbohydrate.

There are probably changes in the quality and quantity of certain amino acids and fatty acids that influence the metabolism of processed eggs in the body. Recent reports from studies on patients with coronary disease indicate that these changes may be beneficial. Studies to confirm this are being carried out in our institute.

Tables 2 and 3 illustrate some differences in certain amino acid and fatty acid contents between fresh duck eggs and pidan. The significance of these differences remains to be elucidated.

Therapeutic Effect

It has been claimed that regular consumption of pidan in moderate amounts (two or three a day) will benefit persons with high blood pressure or coronary disease. In a preliminary experiment with rats, one month of feeding pidan at a 10 per cent level in the diet lowered blood cholesterol and ß-lipoprotein to a certain extent compared to results in the control group. It has also been said that consumption of pidan will improve appetite, clear vision, and protect the liver. This remains to be confirmed.

OTHER PROCESSES FOR PRESERVING EGGS

In addition to making pidan, there are two other methods of preserving eggs, which also change their nature, producing a form and flavour different from pidan, but nevertheless prolonging shelf-life.

1. Salted eggs (xiandan). Xiandan are processed as follows: To process 1,000 eggs, mix 12.5 catties of dry, yellow mud and 10 catties of table salt with 10 catties of water to make a paste. Cover each egg with a layer of paste and place in a vessel. Finally, pour the rest of the paste over the eggs and cover the vessel. Leave for about a month. The eggs can then be eaten after cooking.

A simpler method is to place eggs in a 35 per cent salt water solution that has been boiled and cooled. Seal vessel and store for about 25 days.

2. Pickled eggs (zaodan). Zaodan are processed as follows: Put about 8 catties of fermented grain mash in the bottom of a vessel and place eggs on top one by one to form a single layer. The shells of the eggs should be cracked before processing, but the membrane should remain intact. Add another layer of grain mash and another layer of eggs. Next, put 18 catties of grain mash over the top layer of eggs. Finally, sprinkle some table salt evenly over the mash. For 120 eggs, use 34 catties of grain mash and 3.75 catties of table salt. The eggs are processed for 41/2 to 5 months, with part of the time in the hot season.

Zaodan can be eaten cooked or uncooked and have an especially pleasant taste, probably because of some chemical changes in the ingredients mixed with the egg, producing acetic acid, alcohols, and glycerides. This results in eggs that drop readily out of their shells, with both white and yolk coagulated, yielding an aromatic, sweet odour.

Pidan and zaodan are now exported as well as being produced for home consumption- Exports are for the most part to Hongkong, Macao, Japan, and South-East Asian countries.

REFERENCES

Blunt, K., and C.C. Wang. "Chinese Preserved Eggs-Pidan," Nat. Mod. J. China. 4: 145 (1918).

Bureau of Commercial Inspection. The Manufacture of Pidan and Its Analysis. Chinese Commercial Finance Press, Beijing, 1979. (Chinese)

Chen Chunjen. Shi Jing Yang Sheng. 6th ed. Hong Kong Commercial Press, 1963. /Chinese)

Du Baosai. "Mechanism of Processing Songhuadan." Shipinkeji, no. 6,1980, p. 26. (Chinese)

Hu Xiongfei. "Soft-Shell Zaodan.", Shipinkeji, no. 11, 1979, p. 23. (Chinese)

Tibbles, William. foods: Their Origin, Composition and Manufacture. Balliere, Tindall and Cox, London, 1912.

Tso, E. Vitamin Content of Chinese Preserved Eggs (Duck)," Biochem. J., 20: 17 (1926).

Zhong Zhongxian et al. "The Coating of Pidan." Shipinkeji. no. 6 1979, p. 4. (Chinese)

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