We boiled 30 eggs in one go so you can peel the perfect egg

Eggs are a very cost-effective food, and many people are used to eating one egg every day. However, if the eggshell is not peeled smoothly, the egg will be bumpy after peeling, and it will always lack a bit of "satisfaction" when eaten. Just like this...

Look at the half-peeled eggshell in the picture below. Whose dream egg is this?

How can we boil an egg that is easy to peel? With a scientific attitude, today we will design an experiment to explore it.

Before the experiment, we need to understand what factors determine the difficulty of peeling the eggshell.

Whether a boiled egg is easy to peel depends on the adhesion between the inner shell membrane (the almost transparent film on the surface of the egg white when peeling the egg) and the egg white. If the inner shell membrane has strong adhesion to the egg white, it is not easy to peel the shell; otherwise, it is easy to peel the shell. Have you ever tried to lift the inner shell membrane and peel off a piece of eggshell as shown in the picture below? This is a manifestation of poor adhesion.

Photo: Photo taken by the author

So, what factors affect the adhesion between the inner shell membrane and the protein?

Experimental Materials and Methods

1

Experimental design

Controlled experiments are a common experimental method in scientific research, which explores which variables will affect the results by controlling variables and setting controls.

This experiment set up seven control groups, corresponding to seven egg-boiling techniques, to see which of these seven techniques would affect the difficulty of peeling boiled eggs. Some of these seven techniques are supported by research theories, while others are popular methods spread online.

The variables corresponding to the seven control groups are the freshness of the eggs, the pH value of the water used to cook the eggs, the storage temperature of the eggs, the water temperature when the eggs are put into the pot, the cooking time, whether salt or vinegar is added to the water used to cook the eggs, and whether the eggs are soaked in cold water after cooking .

However, the experiments in this article are too simple and the conclusions still need to be rigorously verified. I just want to use this article to let everyone understand scientific ideas and experimental methods, and perhaps everyone can become a "home kitchen scientist" at home.

2

Experimental Materials

Ingredients: 30 eggs (20 fresh eggs, 10 stale eggs), 3 grams of salt, 5-10 grams of vinegar, 3-5 grams of baking soda.

Tools: 1 small cooking pot, 1 thermometer, 2 medium bowls, 1 timer.

Auxiliary tools: refrigerator and ice tray (for making ice water to cool eggs).

3

Experimental procedures

According to the seven control groups set up, the fish were boiled and shelled separately, and other variables were kept consistent as much as possible during the process.

Experimental results and analysis

1

The freshness of the eggs

The fresher the egg, the greater the adhesion between the inner shell membrane and the egg white, making it more difficult to peel the shell after cooking.

The difference in the freshness of eggs is due to the different pH values ​​of the egg whites . The pH value of fresh egg whites is lower than that of stale eggs. Changes in pH value will lead to changes in functional properties such as protein solubility, emulsification stability and gel strength, which have a comprehensive impact on the adhesion between the protein and the inner shell membrane. In general, the higher the pH value, the weaker the adhesion and the easier it is to peel the shell . From this perspective, it seems easier for us to understand why boiled eggs from roadside stalls are easier to peel - perhaps it is because the eggs are not so fresh.

Our experiments found that less fresh eggs are not as smooth to peel than fresher eggs, but they can still be peeled intact (see Figures 1-1, 1-2).

Image: Left: not very fresh room temperature eggs; right: relatively fresh room temperature eggs. Both were put into cold water and boiled for 8 minutes after the water boiled.

2

pH of egg boiling water

Friends who want to eat fresh eggs don't have to worry. It is not impossible to have both "fresh eggs" and "easy to peel eggshells". We can put a little baking soda in the water when boiling eggs to increase the pH value of the egg white near the eggshell of fresh eggs , making them easier to peel after boiling. Experiments have found that fresh eggs boiled with baking soda are very smooth to peel, as shown in Figure 2-2.

Picture: Add a teaspoon (3-4 grams) of baking soda to a fresh room temperature egg, put it in cold water, and cook for 8 minutes after the water boils.

3

Storage temperature of eggs

Studies have found that compared with eggs stored at room temperature (around 25 degrees Celsius), eggs stored in the refrigerator (4-6 degrees Celsius) have higher protein viscosity, stronger adhesion to the inner shell membrane, and are more difficult to peel after cooking.

Our experiment found that the shells of eggs taken out of the refrigerator and boiled directly were cracked, which affected the smoothness of peeling, but they could still be peeled completely, as shown in Figure 3-2; and after the eggs were warmed to 48 degrees Celsius in a warm water bath, they were more difficult to peel after being cooked, as shown in Figure 3-3. This may be caused by the use of boiling water to make the egg white and the inner shell membrane adhere quickly and tightly.

(Please note: The purpose of this step is to verify the impact of the difference between the egg temperature and the water temperature. If the eggs are boiled in room temperature water, the refrigerated eggs will gradually heat up, affecting the judging results, so boiling water is used to boil the eggs.)

Picture: Fresh and refrigerated eggs are put into boiling water and boiled for 8 minutes

Picture: Fresh, warm water bath at 48 degrees Celsius. Put the eggs into boiling water and cook for 8 minutes after the water boils.

4

Water temperature when eggs are put into the pot

Considering that the egg white coagulation speed is different under different water temperatures, we boiled eggs in room temperature water of about 25 degrees Celsius, warm water of 40 degrees Celsius, and boiling water. The results showed that the egg white cracked after the eggs were cooked in boiling water, which affected the smoothness of peeling, but the shells were still completely peeled, as shown in Figure 4-2; the eggs in room temperature water and warm water were easier to peel after cooking , as shown in Figures 1-1 and 4-3. This may be because the eggs are gradually heated in room temperature water and warm water, and the heat is relatively uniform, which prevents the outer layer of egg white from coagulating quickly and adhering tightly to the inner shell membrane.

Picture: Fresh, room temperature eggs are put into boiling water and boiled for 8 minutes

Picture: Fresh, room temperature eggs are put into 40 degrees Celsius warm water and boiled for 8 minutes after the water boils.

5

Boil egg time

Studies have shown that the thermal gelation of egg white during egg boiling also affects the ease of peeling. Boiling eggs for too long will not only affect the taste, but also cause excessive adhesion between the egg white and the inner shell membrane, making it difficult to peel . By controlling the boiling time, the degree of egg white gelation can be optimized, thereby affecting the peeling effect.

However, our experiment found that the eggshell was difficult to peel when the egg was boiled for 12 minutes, as shown in Figure 5-2; the egg white broke when the egg was boiled for 16 minutes, but the shell could be peeled completely, as shown in Figure 5-3. The experimental conclusions are inconsistent with previous studies.

Picture: From left to right: fresh room temperature eggs put into cold water, boiled for 12 and 16 minutes after the water boils

However, the sample size of this experiment is too small, and it is normal for a single experiment to have deviations. The reliability of previous experiments is still higher. In addition, considering the taste of the egg yolk, it is recommended that you do not boil eggs for more than 10 minutes, about 8 minutes is enough. Of course, the specific conditions should depend on the fire size when boiling eggs.

6

Add salt or vinegar to the egg water

Some egg-boiling tips mention that adding salt or vinegar to the water can make the egg easier to peel. We haven't found any research to support this theory, but it's worth a try.

The experiment found that adding salt and vinegar did not make the eggshell easier to peel . On the contrary, the eggshell cracked after boiling with salt, as shown in Figure 6-1, and the eggshell was very difficult to peel after boiling with vinegar, as shown in Figure 6-2. The reason for this situation may be that adding salt increases the boiling point of water, but also shortens the time required for boiling water to cook eggs, making the normal 8-minute egg-boiling time seem too long; adding vinegar accelerates the coagulation of egg whites, causing the egg whites and the inner shell membrane to adhere quickly.

Picture: From top to bottom, fresh room temperature eggs are put into cold water, salt and vinegar are added respectively, and the water is boiled for 8 minutes

7

Soak in cold water after cooking

Most people have used the quick method of peeling eggs by quickly placing the boiled eggs in room temperature cold water or ice water to cool them down . This is because the expansion coefficients of the egg white and the inner shell membrane are different, and the temperature difference between boiling water and cold water (ice water) causes thermal expansion and contraction, which separates the egg white and the inner shell membrane.

Our experiment found that it was difficult to peel the boiled eggs after cooling them in ice water at about 0 degrees Celsius, as shown in Figure 7-1; it was easier to peel the eggs after cooling them in cold water at room temperature , as shown in Figure 7-2. The reason for this may be that the temperature change was too rapid and drastic when cooling them in ice water, resulting in the adhesion between the egg white and the inner shell membrane not being fully weakened, and the egg white shrinking unevenly, which increased the difficulty of peeling the eggs.

Picture: Fresh room temperature eggs are put into cold water, boiled for 8 minutes after the water boils, and then put into ice water and room temperature cold water to cool down

There is also an advanced version of this step: researchers have found that it is easier to peel a hard-boiled egg by cooling it in cold or ice water for 10 minutes and then boiling it in boiling water for 10 seconds . However, considering the complexity, time consumption and final effect of the operation, we don't need to do this in our daily life. It is enough to just do the previous step.

Experimental conclusion

Combining the above 7 simple comparative experiments, we finally came up with 4 useful tips for easy peeling of boiled eggs:

1. Place the eggs at room temperature to warm up in advance;

2. Add a little baking soda to the water used to boil eggs;

3. Put the eggs in cold water and cook for no more than 10 minutes;

4. After cooking, immediately put it into room temperature cold water to cool down.

However, it should be noted that the sample size of this experiment is small, the control experiment setting is relatively rough, and different eggs will also produce corresponding differences , so the experimental results are just for your entertainment.

From a nutritional point of view, it is recommended that you eat 300-350g of eggs per week, about 5.5-7 eggs, and the most recommended way to cook eggs is to boil them. You can also try to compare the above methods in your daily egg boiling to explore the best egg boiling skills, and ensure that you boil the perfect eggs that can be peeled off as soon as the eggshell is peeled.

References

[1] Su Yujie, Xu Zhenzhen, Yang Yanjun. Effect of pH value of whole egg liquid on its functional properties[J]. Food and Fermentation Industries, 2012, (038) 011: 63-67.

[2]Britton WM, Fletcher D L. Influence of storage environment on ease of shell removal from hard-cooked eggs[J]. Poultry Science, 1987, 66(3): 453-457.

[3] Ge Qinglian, Liu Yinyin, Ma Lina, et al. Effect of cooking time on egg texture characteristics and correlation analysis[J]. Journal of Jiangsu Agricultural Sciences, 2021, 49(7):4.

[4]Hale Jr KK, Britton W M. Peeling hard cooked eggs by rapid cooling and reheating[J]. Poultry Science, 1974, 53(3): 1069-1077.

Planning and production

Author: Wang Lu, registered dietitian

Review丨Zhong Kai, Director of Kexin Food and Health Information Exchange Center

Zhang Na, Associate Researcher, Peking University School of Public Health

Planning丨He Tong

Editor: He Tong

Proofread by Xu Lai and Lin Lin

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