It is often found at the top of grocery lists, it is an ingredient that is used daily in the kitchen and it can be eaten at any time of the day; what is it? It’s the egg of course, an ingredient that can stand alone as a “star” or play an important role behind the scenes. The egg can be eaten plain after boiling it, poaching it, scrambling it, frying it and more, but it also plays an important role in other foods such as meringues, custards, flans, creams and icings. There are endless possibilities to this very versatile ingredient that is hidden in a fragile shell concealing its incredible properties.
Anatomy of the egg
The chicken’s egg, our favorite and most common in the egg category, is the gamete produced by female chickens. If the egg were to be fertilized it would eventually lead to chicks, but we often eat unfertilized eggs or eggs that did not go through the incubation period and thus the embryo did not have time to form.
The egg is mainly made up of the albumen, also known as egg white, and the yolk. The difference between the white and yellow part of the egg also denotes the chemical differences in the properties of the egg. The yolk is suspended in the center of the egg by two elastic spiraling cords that are attached to each end of the egg. This is surrounded by thick and thin albumen, which is then coated with a thin membrane. Its outermost layer is the shell, which is made primarily of calcium carbonate and serves to protect the egg.
The egg is one of the most significant sources of proteins and other vitamins. The difference between the egg yolk and egg white is that the yolk also contains fat, which is one of the reasons why the egg yolk is essential for creams and custards. The egg white makes up nearly 2/3 of the egg, however 90% of it is water. On the other hand the yolk, which is the only source of fat of the egg, is also the only source of cholesterol, and has three times as many calories as the white. The table shows some of the differences in the nutritional values of the egg white and yolk. But let’s focus on the most important component of the egg, the protein.
Egg White vs. Yolk
Eggs whites contain three major proteins that affect the appearance, cooking time and flavor of the egg. Ovalbumin, one of the main sources of nourishment, makes up approximately 50% of the egg white, and it is the only protein that has reactive sulfur groups, that can affect color and flavor. An example of this is H2S, hydrogen sulfide, which gives their ‘eggy’ aroma to cooked eggs, but in high amounts emanates the smell of “rotten eggs” which is very unpleasant. Have you ever found the yolk in your hard-boiled egg looking green-grey? Well, the color is in fact formed due to the H2S found in the egg white, which reacts with the iron found in the yolk forming ferrous sulfide. It often appears in older eggs, but it is safe to eat. Next in percent amount is, ovotransferrin, the first protein that coagulates when an egg is being cooked. This protein binds very tightly to iron, which prevents bacteria but also causes color changes, to yellow or pink. Then, there is ovomucin, which is only 1.5% of the egg white, but plays a very important role in the consistency of the egg white. This protein creates a compact and attractive appearance of the egg white when it is cooked, instead of the gooey texture of the raw egg.
What about the egg yolk? What makes it so cool? First of all the yolk is one of the single cells that can be seen with the naked eye, and its main purpose is to serve as the food for the developing embryo if the egg is fertilized. Therefore, the egg yolk is primarily nutritive as it contains most of the iron, vitamin A and thiamin of the whole egg. As seen in the table above, it is also a source of cholesterol and fat, as well as ¾ of the calories in an egg. The yellow color comes from pigments known as xanthophylls, which the chicken obtains from eating alfalfa sprouts and corn. One of the yolk’s talents is the property of emulsification. The yolk is a sack of water with a lot of floating proteins and lecithin aggregates. Lecithin, is made up of a triglyceride backbone, with a phosphate group and a choline. Its emulsifying properties come from the phospholipids, which are partly hydrophilic (bonded with water) and partly hydrophobic (repelled from water), and thus can bond to both water and a fat/oil .
“Are these ribbons? Nope, it’s a protein!”
We have talked about the anatomy of the egg, the properties of the egg white and yolk, and now it is important to talk about protein structure. The basic components of a protein are amino acids that form long chains called polypeptides, the primary structure of a protein. As an avid knitter, I sometimes think of proteins as complicated knitting patterns. Amino acid chemistry is very important in eggs as it affects form, texture and flavor. These polypeptide chains twist to form helices and sheets, which are known as the secondary structures. When the helices and sheets fold 3-dimentionally they form the tertiary structure of the protein, and when multiple polypeptide chains bond together, they form the quaternary structure (seen in the ribbon-looking picture on the right). The ‘glue’ that holds a protein together and gives each one its unique structure are the types of bonds present in proteins. Here is a list of these bonds:
Heat it, Beat it, Mix it
When eggs are cooked some of these bonds break, and thus the proteins denature meaning that their 3-D structures change. As the proteins unfold, they begin to tangle with each other, forming new and tighter bonds that cause large protein networks. Because they are more densely held together the egg structure begins to change, as both the egg white and yolk coagulate to form a thickened and firmer mass. The egg white also begins turning white/opaque, because as the protein networks tighten and less light can be transferred through, so the egg white loses its translucence. Relating this to my knitting analogy, these proteins are like yarn that is untangled and then knitted into a much tighter new pattern. There is of course a limit to how much proteins should coagulate. If eggs for instance are heated for too long, the networks become too tight after squeezing out water. This gives eggs a very ‘rubbery’ texture that is ok for hard-boiled eggs, not particularly appetizing for scrambled eggs (in my opinion).
Let’s not forget however that there are other ways in which the proteins in eggs can denature. Apart from all the heating methods (boiling, frying, poaching etc.) there is beating and mixing. When you beat eggs the proteins also unfold, but with the motion of the whisk or mixer, air is introduced between the molecules. This is seen especially when beating raw egg whites. Egg whites are made primarily of protein and water, and no fat. As we beat them, the proteins of the albumen denature and the amino acids hold in air bubbles. The hydrophobic ones turn towards the air, while the hydrophilic ones hold in the moisture. The beating begins to form a foam like the ones formed from detergents, due to the interaction of the proteins with the air and water present in the egg white.
It is important to also talk about mixing. It was mentioned above the yolk’s emulsifying properties, but what happens when you mix eggs with sugar, and what about acids? It has been found that proteins can bond to polysaccharides, and this is an example seen in meringues. After beating the egg whites for a little, they form a white frothy mixture. By adding some sugar, the consistency becomes more stiff and dense, as the sugars dissolve in the water molecules that are already trapped in, and give more bulk and structure. They also provide sweet flavor (the egg white foam is tasteless), as well as a shiny white color.
Experiment 1 – Boiling eggs
After going over the main properties of the egg, I can now talk about the experiment we ran in class and what we found. The first experiment focused on investigating at what temperature the egg coagulates when being boiled. A circulator was used for this experiment, which can keep the temperature constant when set. The following table shows the results from the experiment. The eggs were placed in the circulator for about 2 hours. T = 0 was started after the first 2 hours of immersion.
Experiment 2 – Scrambled eggs
The second experiment which was conducted by a small group of people during class, involved cooking scrambled eggs at various temperatures. Overall, the conclusions of the experiment were: at low temperatures the eggs take a long time to cook, but they maintain their bright color and moisture. At medium temperatures the eggs cooked best, because they cooked relatively fast without drying out, and they maintained a great puffy texture. At the high temperature, the eggs cooked in 30s, however they were very dry, with a rubbery texture and they didn’t taste as good. Having a really high temperature caused the water in the eggs to boil off very quickly, but the proteins over-coagulated and so created overcooked eggs.
I will point out though that not everyone agreed on what the ‘best’ scrambled eggs were.
Experiment 3 – Meringues
I mentioned before what happens when you beat egg whites. The proteins unfold and the amino acids hold in air bubbles that cause the raw runny egg whites to form a foam. It is important to point out that beating the egg whites for too long will lead to the proteins over-denaturing and not being able to hold a form, thus the foam will start to look like bubbles popping, it will collapse and slowly go back to liquid form. As part of the experiment we let one batch of egg whites be beaten with the Kitchen Aid for approximately 20 mins. After holding a nice and firm texture, bubbles started to form and after it was set in a bowl it collapsed. For meringues, the secret is to add sugar after beating the eggs for a little. The water in the proteins dissolves the sugar which becomes part of the structure and adds elasticity, texture and sweetness to the foam. Half of the batch was baked plain, while to the other half were added chocolate chips. They were then baked in the oven at a moderate temperature that makes the water boil off, while cooking the meringue. The outside browns a little (hint: Maillard reaction), while the inside remains relatively moist. There was also a difference between the plain and chocolate chip meringues. The plain ones seemed to be more dry inside, while the ones with chocolate chips were more chewy. This can be attributed to the fat present in chocolate, but also the additional mass of the meringue that makes it harder to cook.
Experiment 4 – Lemon Curd
The last experiment conducted in class was making lemon curd. Yes, it was a delicious experiment, but let’s see what happened chemically. The recipe that was used for this edible ‘experiment’ was Alton Brown’s “Lemon Curd” recipe (http://www.foodnetwork.com/recipes/alton-brown/lemon-curd-recipe/index.html). Here are the ingredients:
- 5 egg yolks
- 1 cup sugar
- 4 lemons, (4 zested and 2 juiced)
- 1 stick butter, cut into pats and chilled
Initially the yolks were beaten with the sugar, until the mixture started turning pale a yellow/white color and its consistency was sticky. What happened in that case was that are the yolks were beaten with the whisk, they released a lot of the water that is present inside the yolk. That water dissolved some of the sugar, and the dark yellow color of the yolk mixed with the opaque white of dissolved sugar, forming that yellow/white color. The stickiness is also attributed to the supersaturation of the sugar, caused by the hydrogen bonding between water and sugar, much like when you are eating cotton candy (which is basically just spun sugar) and it gets sticky all over your fingers. Then the lemon juice and zest were added and the entire mixture was placed on a burner at a low heat. The mixture was whisked continuously. More juice was added after a few minutes, and since the mixture wasn’t thickening one more egg yolk was added. It was heated for approximately 15 mins and then pot was removed from the heat. The butter was added in little pieces and one at the time while the mixture was whisked. The curd finally thickened, and the entire class was ecstatic, because it meant we could finally eat our lemon curd filled meringues. Yum!
An EGGciting New Idea
Now, to wrap it all up I’d like to share with you something very interesting I found from an episode of Iron Chef (Food Network). What if instead of boiling, frying, poaching, beating, eggs, you turn them into a powder?
It turns out it is a method with which you can store eggs for a long time (for months and even more than a year), and reconstitute them using water to use them in any recipe, cakes, quiches and even scrambled eggs.
Here is a recipe/ procedure I found online:
That’s all folks! I hope this helped explain what an amazing ingredient egg can be. So let the EGGstravaganza begin!
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