Eggs are a neat little package of nutrition. They are one of the most versatile foods – eaten boiled, scrambled, poached, as frittata, quiche, omelette and so on. Additionally, they act as an important ingredient in many dishes due to their chemical properties.
- Shell membranes
- Air cell
The yolk makes up roughly 30% of the egg’s weight and is very nutrient dense. If you have noticed a tiny white spot on your egg yolk then you have spotted the germinal disc.
The yolks colour (ranging from pale yellow to red) will depend on the diet of the chicken and the pigments present in the feed e.g. beta carotene. Sometimes marigold petals are added to the feed to enhance the yellow/ orange pigment of the egg.
The albumen – more commonly known as the egg white – consists mostly of water and protein. The egg white ranges in viscosity from thin layers to thick layers. The viscosity is dependent on the protein content of different areas of the egg white. The thickest part of the white is that surrounding the yolk.
The chalazae is made up of twisted strands of albumen that act like an anchor to the egg yolk. This is the part many people try to pick out. The chalazae penetrates the egg white to keep the yolk centred within it.
There are two membranes between the eggs shell and the egg white. These membranes (an inner and outer one) protect the contents of the egg from bacterial infection.
At the wider end of the egg is an air cell. This is formed between the two membranes and is a pocket of air. The air cell forms as the egg cools and the contents contract – separating the inner membrane from the outer membrane. The purpose of the air cell is to provide a chick with a breath of air that it required to break out of the shell.
The shell is made of calcium carbonate and protects the contents of the egg. There are many tiny pores in the egg shell which allows the exchange of gases between the outside environment and the inside of the egg.
Why are some shells a different colour? This is due to the breed of the hen. The nutrient content and taste of the egg should not differ, however. Typically brown eggs come from larger hens which produce fewer eggs and require more feed. They may therefore be more expensive.
How to tell a fresh egg from an aged egg
A fresh egg will have a plump yolk that stands tall. A large portion of the albumen surrounding it will as well be thick and tall. As an egg ages, the yolk becomes flatter and the albumen becomes thinner, dispersing further. The air cell as well becomes larger as an egg gets older.
Nutrient content of eggs
Eggs contain a high quality, complete form of protein. It is absorbed as efficiently as the protein found in meat. A large egg contains approximately 4g of protein in the egg white and 3g of protein in the yolk. This makes it a great option for vegetarians.
The egg yolk consists of approximately 5g of fat. Roughly 2g of this is monounsaturated fat, 1g is polyunsaturated fat and just below 2g is saturated fat.
Egg yolk does have a high cholesterol content (approximately 186mg per large egg). However, it is saturated fat that has the most significant impact on LDL cholesterol levels. The heart foundation has suggested that six eggs per week can be included as part of a healthy, balanced diet. If you are someone who is extra sensitive to dietary cholesterol then you may need to discuss this with your GP or Accredited Practicing Dietitian.
Eggs contain every fat soluble vitamin – A, D, E and K – which is quite rare amongst individual foods. The egg white also contains good amounts of B vitamins. Eggs contain the minerals iodine, selenium, zinc and iron. Unfortunately the iron is not well absorbed due to it binding to phosvitin which inhibits absorption.
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This article was written by our dietitian Belinda Elwin who is a Dietitians Association of Australia member and Accredited Practising Dietitian and Nutritionist.
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