Course:FNH200/Projects/2023/Century Egg

Introduction

Picture 1.0 Century Egg in T&T Supermarket

Century egg, also known as preserved egg or thousand-year egg, is a unique and traditional Chinese delicacy with a history dating back centuries.

History

Picture 1.1 Century Egg in Amazon

Century eggs, with origins dating back to the Ming Dynasty around 600 years ago, were possibly invented in Hunan, China. A homeowner accidentally buried duck eggs in slaked lime used for construction. After two months, the eggs turned black and developed an unforgettable flavor. These eggs had an extended shelf life of several months, vital for ancient food preservation. The unique production process contributed to their distinct taste, making century eggs a popular delicacy across China.

Methods

The traditional approach, stemming from buried slaked lime, involves a mixture of wood ash, calcium oxide, and salt as the alkaline agent. Tea leaves, weighing 1.4 kg, are placed in boiling water, followed by individually covering each egg with a mixture of 1.4 kg calcium oxide, 4.1 kg sea salt, and 3.2 kg burned oak ash (increased to 3.2 kg in winter). Eggs are then wrapped in rice chaff to prevent adhesion, stored in cloth-covered jars or woven baskets, and allowed to react for several months.

In the modern method, the egg transformation process is streamlined. Eggs are soaked for 10 days in a mixture of table salt, calcium hydroxide, and sodium carbonate, followed by aging in an airtight container. This method takes only a few weeks to yield century eggs with traditional characteristics.

While lead(II) oxide was once used to expedite the reaction, its extreme toxicity led to its prohibition. Zinc oxide has since emerged as a safer alternative, although excessive absorption can result in copper deficiency. As such, the use of zinc oxide is carefully controlled in modern times.

Chemical Properties

Chemical fermentation

Sodium carbonate + calcium oxide (quicklime) + water  → sodium hydroxide (a strong base)

1. Sodium hydroxide diffuses through the eggshell and promotes protein denaturation of the egg white and degrades the protein into smaller pieces
2. The protein fragments ovalbumin, which interacts with one another and water in the egg white, forming thick protein filaments which allow the egg white to coagulate into a gel.
3. Lead (II) oxide may be added to:
   • Close the eggshell pores to prevent oxidation, which turns the egg white yellow instead of dark brown
   • Prevent sodium hydroxide from hydrolyzing the coagulated protein, which may lead to over-fermentation

During the curing process, salt is added to kill spoilage-causing microorganisms in the century eggs, which contribute to the extended shelf-life as opposed to regular eggs.

Nutritional Value

Each century egg contains:

• 80 calories
• 4 grams of fat
• 8 grams of protein
• 3 grams of carbohydrate
• 250 milligrams of cholesterol
• 440 milligrams of sodium (they are high in sodium due to the curing process, thus preserving the eggs as well as killing spoilage-causing microorganisms)
• Vitamin A, vitamin D, and iron

Compared to regular eggs, the preserved eggs are lower in calories, fat, and carbohydrate, and higher in protein, iron, and selenium among others.

Health benefits of Century eggs:

Macronutrients:

• Protein: for muscle repair and maintenance, help with the production of hormones, and promote healthy skin, anils, and hair
• Reduced fat and carbohydrate: will not contribute to a lot of weight gain

Micronutrients:

• Vitamin D: enhance calcium absorption for improved bone health
• Vitamin A: improve vision
• Iron: prevent anemia as iron helps with the production of red blood cells  
• Selenium: an antioxidant that regulates blood pressure and promotes healthy thyroid

Appearance, Aroma, Taste, and Texture

The distinct aroma of century eggs comes from ammonia and hydrogen sulfide released during protein degradation. The egg white turns into a translucent dark brown color, which is a result of the staining of tea leaf colors and the Maillard reaction in a highly alkaline environment.

The greenish-yellow egg yolk is proportionally larger, which is partially runny and surrounded by concentric rings of brown, green, and navy blue. These colors are derived from iron (III) sulfide, which is formed from iron and sulfur already present in the yolk. The yolk has a velvety and creamy texture that is in contrast with the gelatinous texture of the egg white. The egg has a succulent taste which is often compared to the taste of mature blue cheese.

Health Implications

Poisonous Century Eggs

A report by China Central Television done in June 2013 showed that three plants producing preserved duck eggs in Jiangxi, China, were using industrial copper sulfate (ICS) to speed up the food processing time. Century eggs are usually preserved for about two months, where the egg turns into a stiff, dark jelly structure; ICS’s presence can halve this preservation process to a month only. This news was shocking since ICS contains high levels of toxic ingredients such as arsenic, lead, and cadmium, which may cause kidney damage. Out of all the toxic ingredients ICS includes, consumers are generally most concerned about the presence of lead in century eggs. Due to this concern, many products print “lead-free” in bolded, large font on the food labels, as shown in picture 1.1.

According to Canada Food Inspection Agency, lead is not permitted to be added to foods sold in Canada, so lead should not be a health concern in Canada. It is interesting to note that lead is present in all foods at deficient levels. Trace amounts of lead can enter the food chain through scenarios like plants uptake from soil and fish absorb lead from the water.

Nutritional Concerns

According to Health Canada, Canadians are recommended to consume 1500 milligrams (mg) daily and not exceed 2300 mg daily. One preserved duck egg contains 440mg, as shown in picture 1.0, approximately one-third of the recommended amount. Sodium is an essential nutrient for many vital functions like regulating bodily fluids and contracting our muscles. Too much sodium will cause the body to retain more water, which will increase the liquid in the body and therefore increase the blood pressure; This, in turn, puts strain on blood vessels and the heart leading to a risk of stroke and heart disease. Hypertension Canada stated an average sodium consumption of 3500mg per day, far exceeding the limit of 2300 mg.  In conclusion, those who adopt a high-sodium diet or are at risk of hypertension should consume century eggs with precaution.

Regulations

There is no specific policy or requirement targeting century eggs. However, based on Canadian food importing requirements under Canadian Food Inspection Agency, outlines requirements specific to importing processed egg products including grades, standards, labeling requirements, and so on.

Specific import requirements for processed egg products

Processed egg products not only need to fulfill the general import requirements but also the following requirements:

1. The responsible government official in the exporting country must sign an official document attesting to the fact that processed egg products meet the requirements under the SFCA and SFCR before the import is allowed.

2. It is mandatory to include the country of origin on the label of processed egg products.

3. For shelled eggs:

• Before pasteurization, eggs that were pasteurized in a shell must have received a grade of A.
• It is required that grade C eggs, nest run eggs, and ungraded eggs be delivered to a licensed establishment where they are processed and treated.
• Ungraded eggs imported from abroad must be packaged in containers labeled as "Ungraded Eggs" or "Oeufs not classified".

Standards and grades for imported processed egg products

Based on the definition of state in Safe Food for Canadians Regulations (SFCR), the following standards of Dried Whole Egg and Frozen Whole Egg should be cautious within importing century eggs:

1. When importing dried whole egg products, products can not contain more than 5% of water.
2. In addition, dried whole egg products should not contain more than 50,000 viable bacteria per gram and 10 coliform organisms per gram.

Labeling requirements

1. Common name

2. Net quantity

3. Name and principal place of business

4. Country of origin

5. Date markings and storage instructions

6. Inspection legend

7. Lot code

Century egg salad

Three-color steamed eggs

Century egg congee

Preserved egg puffs

Common Ways to Consume Century eggs

Century eggs are generally safe to be consumed without cooking. As mentioned above, century eggs have an intense and special aroma, so they are ideally served with other spices and ingredients to enhance their flavour. Here are some popular traditional dishes made with century eggs:

1. Century egg salad: century eggs are served as a cold appetizer with Sichuan chili oil, pepper, and silken tofu.
2. Three-color steamed eggs: the three colors stand for century eggs, salted eggs (another kind of preserved duck eggs), and fresh eggs. Century and salted eggs are arranged in a plate and fresh eggs are beaten up with water and poured over. The dish is then steamed before serving.
3. Century egg congee: century eggs are boiled with rice and lean pork to make congee. It is a common breakfast option in Guangzhou, China.
4. Preserved egg puffs: the eggs are wrapped with pickled ginger, white sesame, lotus seed paste, and puff pastry dough. It originated as a kind of “bridal pastry” in the traditional Chinese culture which symbolizes prosperity.

Exam Question and Answers

What is the major health concern relating to consuming century eggs in Canada?

a. Contain tannins, which prevent absorption of vitamins and minerals leading to deficiency

b. Presence of lead oxide; this substance is detrimental to human body and may cause brain damage and various cancers

c. High in sodium, which pose a risk for hypertension and cardiovascular disease

d. Preserved for over 100 years and may pose a risk for diarrhea and colon cancer

The correct answer is C. According to Health Canada, Canadians are recommended to consume 1500 milligrams (mg) daily and not exceed 2300 mg daily. One preserved duck egg contains 440mg, which is already approximately one-third of the recommended amount. As mentioned in our project, “too much sodium will cause the body to retain more water, which will increase the liquid in the body and therefore increase the blood pressure”.