How to tell whether an egg is fresh

A fresh egg has a small air space at its wide end, a firm and cloudless white, and a round yolk that stands up in the middle. As an egg ages, the air space grows, the white thins and the yolk flattens. You can see most of this by cracking the egg onto a flat plate. The float test shows how old an egg is, but it cannot tell you whether the egg has spoiled.

What changes as an egg ages

When an egg is laid it is warm, and as it cools its contents shrink slightly. The two membranes inside the shell then separate at the wide end, and a small air space forms there. University of Florida extension explains that during storage the egg loses water through the shell, so the air cell enlarges.

The white changes too. University of Wisconsin extension notes that as an egg ages the white gets thinner and the yolk gets flatter, and that eggs stored warm age much faster than refrigerated ones. University of Florida extension adds that properly refrigerated eggs in their carton drop from AA quality to A quality in about one week, and from A to B in about five weeks.

A date on the carton gives a first estimate. The Thai standard asks for a packing date and/or a best-before date on packs sold directly to consumers. Iowa State University Extension notes that the dates printed on US cartons are used by processors to rotate stock, and that refrigerated eggs keep their quality for 3 to 5 weeks. A date is a guide to age. It does not replace the checks below.

The Thai standard gives numbers

The Thai egg standard, TAS 6702-2010, uses these signs to separate its grades:

Grade AA: air space not more than 0.3 cm deep, and it does not move when the egg is twirled. Broken out, the yolk is round and bulging in a firm white.
Grade A: air space not more than 0.5 cm, with a reasonably firm white.
Grade B: air space not more than 0.8 cm, a flattened yolk, and a weak, watery white that spreads.

The standard's general quality rules add that the egg should not smell abnormal, the white should not be cloudy, and there should be no visible mould. Graders check air space by candling, which means shining a bright light through the shell in a dark room.

Cracking an egg onto a plate

This is the most direct home check. A fresh egg shows a round yolk in a centred position, surrounded by thick white. An older egg shows a flatter yolk, often off to one side, with a thin white that has collapsed into a wide, runny pool. University of Florida extension describes the same difference.

Then use your nose. Extension services say a spoiled egg has an unpleasant smell when it is opened, whether it is raw or cooked. If an egg looks or smells wrong, the extension advice is to decide after cracking it into a bowl, not before.

You may sometimes see a small blood or meat spot. University of Wisconsin extension says these come from the hen's own tissue and not from an embryo, because most commercial eggs are unfertilised. The Thai standard allows such spots only in Grade B, and only up to 0.3 cm across.

The float test, and what it really shows

Put the egg in a bowl of water. Extension services in the US describe the result like this: a fresh egg sinks and lies flat, and an older egg stands upright or floats. The reason is the air cell. It grows as water leaves the egg, until it holds enough air to make the egg buoyant.

Two limits matter. First, the test measures the size of the air cell, which mainly reflects age and storage conditions. University of Nebraska-Lincoln extension says a floating egg is old, and that this does not on its own show that it is spoiled. Second, an egg that sinks has not been checked for smell or appearance inside. Treat the float test as a rough age check, and the plate and the nose as the final check.

In short

A fresh egg has a small air space, a firm clear white and a round, centred yolk.
Thai grades use air space limits of 0.3, 0.5 and 0.8 cm for AA, A and B.
Cracking an egg onto a plate shows more than any shell check.
The float test shows air cell size and age, not spoilage.
Dates are a guide, and sight and smell give the final answer.