Have you ever noticed that some brown eggs appear to have a hazy, or chalky white coating on their shell? What you’re seeing is the start of an extra layer of calcium that is sometimes formed on the egg, and it looks distinctly different than eggs that just happen to have a lighter shell colour.
The egg in the middle (above) has a very noticeable white chalky coating on top of its shell. If you look closely, you’ll notice the egg on its left also has the beginning of a chalky white coating. It looks as though you can scratch it off, but you can’t.
This is not uncommon – and not to worry, it won’t affect you or the egg’s quality – but what it does mean is the hen was under stress before she laid the egg.
How an egg is made by the hen
A hen’s ovaries produce many yolks that gradually grow as they’re attached to the ovary. When a yolk gets to the right size, the follicle (casing) that it is growing inside breaks open and the yolk is released. It begins to travel down a tube called the oviduct.
As it travels down the oviduct, the other layers of the egg form around the yolk. In the first six hours, the clear part of the egg (the egg white) forms around the yolk. Then the egg passes into the shell gland where, in the next 18 hours, the skin-like shell membranes are added. Lastly, the hard shell is formed around the egg. It takes 24 hours for the whole egg development process to occur.
When a hen is ready to lay her egg, she’s highly motivated to seek out a dimly lit, secluded spot. This is because it’s built into in her genetics to lay her eggs somewhere protected. So about an hour to an hour and a half before the hen is ready to lay her egg, she starts to look for a nest.
Nests are important to hens
On many cage-free egg farms, farmers are providing their hens with private nest areas. When there are enough nests for the flock – about one nest per five hens is usually enough – and they are spread out in a way that there are no “preferred” nests to compete for, then the nests meet the hens’ needs just fine.
However, if the nest is occupied by another bird when a hen is ready to lay an egg, or if there is competition to get into the nest, the hen will become frustrated and a bit stressed. She won’t lay her egg at the usual time. Instead she will hold onto her egg a little longer until she can get into a nest.
When she withholds her egg, an extra coating of calcium (a shell layer) begins to form on the outside of the existing shell while the egg sits in her shell gland. This doesn’t happen very often in cage-free systems but it happens a lot in cage systems where hens either don’t have enough nests to share, or have no nests at all.
Know how to pick the ‘good egg’
Next time you’re in the grocery store picking out your eggs, take a peek inside the carton and see if you can spot any brown eggs that look a bit lighter than the rest. Take a closer look at one of the lighter brown eggs to see if you can spot an extra layer of calcium.
Remember, some brown eggs are naturally lighter in colour, but those with extra calcium on them look chalky, like you can scratch that layer off, even though you can’t.
Check for brown egg brands that have very few of these chalky coloured eggs in their cartons. That way you can be assured there are hardly any stressed hens in the brand’s egg supplier flocks.
Unfortunately, it’s practically impossible to see any extra calcium on white eggs, which tend to be the least expensive eggs since they mostly come from farms using cages. If you were able to see the chalky layer on a white egg, you would see it quite often.
The chalky white layer on brown eggs may fade when you first take them out of the fridge because moisture forms on the shell, but when the eggs dry off again, the chalkiness will reappear.
More resources and tips on picking the good egg:
- Read up on how egg-laying hens are raised in Canada.
- Learn how the industry is working to achieve higher welfare for laying hens.
- Print and distribute our egg labelling brochure (PDF) and our egg label poster (jpg).
- Learn more about food labels and what to look for when buying eggs.
Email our farm team for more information.