Why are some eggs hard to peel?

If done correctly the boiled egg is one of the simplest ways to cook an egg. Since it may be difficult to peel extremely fresh eggs, boiled eggs are best with eggs that have already spent a few days in the refrigerator. See below for a comparison of boiled egg to 'sous vide egg'.

Why are peeling fresh eggs shells so difficult?

With eggs that are just a day or two old, the membrane beneath the shell sticks tightly to the shell making peeling the egg almost impossible. After a few days in the refrigerator the egg becomes easier to peel. Why is this? In fresh eggs the albumen sticks to the inner shell membrane more strongly than it sticks to itself because of the more acidic environment of the egg. The white of a freshly laid egg has a pH between 7.6 and 7.9 and an opalescent (cloudy) appearance due to the presence of carbon dioxide. After the protective coat is washed off the egg shell the egg becomes porous and begins to absorb air and loose some carbon dioxide contained in the albumen. This reduces the acidity of the egg which causes (after several days in the refrigerator) the pH to increase to around 9.2. At higher pH the inner membrane does not stick as much to the albumen so the shell peels off easier. In addition, as the egg gets older it will shrink and the air space between the egg shell and the membrane will get larger.

Research shows that the reduced acidity helps with peeling. The tradeoff, however, is that in older eggs the yolk tends to move further from being centered. This happens because the white gets thinner and is less able to hold the yolk in place. The best compromise is to use eggs that have been stored on their sides in the refrigerator for 7 to 10 days.

Tips on Peeling Boiled Eggs :

What is the best way to peel a boiled egg so the egg white does not stick to the shell? Here are several responses, although all bets are off for eggs only a couple days old.

>>> After boiling, pour off the hot water, shake pan back and forth to crack the shells. Cover eggs in cold water and let set for a couple of minutes. Leave water and eggs in pan and peel, using water to rinse away excess shells.

>>>Or, take the egg from boiling water and immediately run cold water on it. Peel the egg under cold running water starting with rounded top where the air pouch is.

Why do eggs crack? How can I prevent eggs from cracking while boiling?

The bottom, rounded end of an egg contains a small air bubble. As the egg heats up, the air inside the bubble expands. As the hot air pushes outwards, it puts pressure onto the shell, making it crack. You can prevent this by making a pin-pick in the bottom rounded end of the egg. This will let the expanding air escape.

You can also prevent cracking by letting eggs come to room temperature before cooking them. Add the eggs to tap water and then apply the heat. Do not add eggs directly into boiling water.

Adding either salt or vinegar in the water will not prevent the egg from cracking but will congeal the white if it starts to leak out of the shell from a small crack, making the cracked egg, still pleasing to look at.

Tips on Cooking Boiled Eggs

Place eggs in a pot of cold water. To avoid cracking the eggs place them in the pot with a spoon. Turn on the heat and wait until the water has come to a boil. Cover the pot, then turn off the flame. Let the pot sit with the cover on for the desired amount of time (see below).

Size	Degree of Doneness	Time Required
Medium	Soft-cooked yolk	4 minutes
	Medium-cooked yolk	6 minutes
	Hard-cooked yolk	11 minutes
Large	Soft-cooked yolk	5 minutes
	Medium-cooked yolk	7 minutes
	Hard-cooked yolk	12 minutes
Extra Large	Soft-cooked yolk	6 minutes
	Medium-cooked yolk	8 minutes
	Hard-cooked yolk	13 minutes

The temperature of the egg at the start of the cooking process will affect the cooking time. An egg that is at room temperature at the start of the cooking process will require about 1 minute less cooking time for each time listed above. The times listed above are based on eggs taken directly from the refrigerator. Source

At what temperature do eggs set?

It is not really necessary to keep the water boiling when cooking eggs since the proteins in eggs coagulate well below 212°F. Egg whites begin to thicken at 145°F/63°C and become a tender solid when the temperature reaches 150°F/ 65°C (although ovomucin yolk cords coagulate at much higher temperatures) . The yolk protein will start to thicken at 150°F and set at 158°F/70°C. The whole egg will set at around 165°F/73°C. If eggs are cooked at 212F for too long they will just get rubbery as proteins continue to coagulate and water gets pushed out from between protein molecules.

Keep in mind salmonella is killed instantly when subjected to a temperature of 160° F . An egg (white and yolk) requires a temperature of up to 158°F before it sets properly. The white alone requires a somewhat lower temperature before it coagulates, usually in the 140° to 150°F range. These temperatures are only slightly less than what is required to destroy all of the harmful bacteria that may be present, so heating eggs to 160° F should not cause eggs to be overcooked, unless they are held at that temperature (or higher) for an extended period.

Following are the general temperatures at which various egg parts and egg products will coagulate:

egg white 60-65°C
egg yolk 65-70°C
custard 82°C

Generally, whole egg begins to become opaque at around 60° C and increases in viscosity to 72° C. At 75° C it is a soft curd and increases in firmness up to 87° C.

Boiled Eggs vs. Sous Vide Eggs

Several chefs have claimed the perfect egg to be the 65 °C (145°F) egg where both whites and yolk have similar consistencies. We have found in our own tests that eggs still exhibit a runny white while the yolk is more solid at these temperatures (see photos above and below). As shown below the boiled egg exhibits a reverse effect with the whites coagulating first compared to the sous vide egg.

Sous Vide egg at 147F (1hr)
Runny whites and sem-solid yolk

Classic 8 minute boiled egg
Solid whites and runny yolk

For more details on Sous vide eggs click here.

What causes the green film on boiled eggs?

A greenish gray film will form on the surface of the yolk when the temperature of the yolk exceeds 158° F. This discoloration is not unhealthful. It's just a visual indicator of a natural chemical reaction. Sulfur from amino acids in the white of your egg (or albumen) is reacting with iron from the yolk which causes a film of ferrous sulphide to form on the yolk's surface. If the film is thin, your egg will still taste fine. Heat speeds up this reaction, so the longer your eggs cook, the greater the chance of discoloration. That's why it is best when cooking hard boiled eggs to turn off the flame when the water boils and let the egg sit for 17 minutes in the hot water bath with the lid on. Then plunge the egg into cold water.

Are these eggs too old -- How to Test for Egg Freshness

As an egg ages it gets lighter by evaporation of water through the porous shell, causing the growth of the air space. A fresh egg is heavy and should feel well filled. Hence the old water test for freshness, a fresh egg sinks and an old egg floats. When broken, the white of a fresh egg should be compact round the yellow. It spreads out, as it gets older. Holding it up to the light can also test the freshness of an egg. A very small air chamber indicates a very fresh egg. Typically eggs reach the shops within 2-3 days of being laid. Eggs should be used within 1 month and kept under the conditions described.

SEE ALSO: HOW FRESH IS THAT EGG?

Note : In normal, "non" salted water, an egg that lays on the bottom is fresh. An egg that stands on end is still useable. And egg that floats is rotten.

When eggs age there is an increase in yolk size, thinning of the egg white, weakening of vitellenin membrane and deterioration of odor and flavor. The deterioration depends upon the storage conditions.

SEE ALSO: EGGS SOUS VIDE