Backyard chicken keepers take great pride in their hens’ eggs and for good reason- they taste better, perform better in baking and are more nutritious than commercially produced eggs. There are times, however, when the freshness of an egg from home-raised hens may be in question; occasionally a hen may lay her eggs outside the coop, (often referred to as ‘yard eggs’ or ‘found eggs’) sometimes an egg from the day’s collection becomes mixed in with older eggs that are intended to be eaten first, or perhaps an egg is discovered in a coat pocket (it happens more often than I like to admit). Marilyn Monroe, my White Orpington, hid her first eggs in my lemon balm & lavender.Marilyn had some help amassing this clutch in my lavender. I have no idea how long they had been there before I discovered them & threw them away.

Whatever the reason an egg’s age may be a mystery, one method used to gauge its approximate age is the “float test” wherein an egg is placed into a glass of water and it’s position in the water observed. In order to understand how the float test indicates an egg’s approximate age, we need to know a little bit about egg structure.

The average eggshell contains thousands of tiny pores through which air, moisture and bacteria can pass. A hen coats the eggshell with an outer layer of protective coating referred to as the bloom or cuticle, which is designed to cover the shell’s pores and defend the egg’s contents from microbes that can spoil it before she can hatch it.  In between the eggshell and egg white are two membranes that provide additional layers of defense against microbes.

Just before an egg is laid, the outer and inner membranes adhere closely to the shell and each other, but when the egg is laid, the membranes begin to pull away from the shell and an air space develops between then.  (Incidentally, this air cell serves as a small oxygen reservoir for a chick just prior to hatching.)

The essence of the float test is that newly laid eggs will lay flat on the bottom of a glass of water and very old eggs will float to the top. I would like to emphasize that the float test does NOT reveal whether an egg is spoiled, rotten, contaminated or bad. A freshly collected egg from a dirty nest box can harbor salmonella yet sink to the bottom of a glass, conversely, “[a] floating egg is quite old but not necessarily unsafe to eat.”1Proper egg handling is the only way to ensure egg consumption safety. Safe handling entails maintaining clean nest boxes, promptly collecting, carefully storing, and thoroughly cooking eggs. Of course, the older an egg is, the longer it has been exposed to temperature and humidity variations, therefore the likelihood of an old egg having been exposed to contaminants is higher than a freshly laid egg, all other factors being equal.

When in doubt, throw it out.

The Inside Story About Salmonella Transmission 
There is a mistaken belief that Salmonella is transmitted to people primarily through dirty eggshells when, in fact, most cases of the illness are the result of an egg that was infected with Salmonella inside the hen’s ovary.2

In the commercial poultry industry, stringent procedures for cleaning and inspecting eggs were implemented in the 1970s (which) made illness from Salmonella caused by chicken feces on the outside of egg shells extremely rare. However, unlike Salmonella infections from eggs in past decades, the epidemic that started in the 1980s and continues to cause illnesses today is due to SE (Salmonella enteritidis) being inside intact eggs with clean shells. The reason is that SE can silently infect the ovaries of healthy appearing hens and contaminate the inside eggs before the shells are formed. (A)n infected hen can lay many normal eggs while only occasionally laying eggs contaminated with SE.”

Here’s the take-home message: with ‘found eggs,’ I feel that it is better not to eat them than to play Salmonella Russian roulette with my health. I have had Salmonella poisoning and prefer not to revisit that beast. While the float test is a fun way to gauge the size of an air cell and guess when the egg may have been laid, it is not an indicator that the egg is safe to eat. I know that whenever I throw out an egg of unknown safety, my hens are already hard at work making more.

Sources & further reading:

The Chicken Encyclopedia, Gail Damerow  see: “egg floating”
2 How to reduce the risk of Salmonella transmitted through a hen’s ovary
What is egg shell quality and how to preserve it
Salmonella Enteritidis
Egg Safety Center