Yolk-smeared eggs: how does it happen, and how do you prevent it?

What is a yolk-smeared egg?

A yolk-smeared egg is one with a whole shell covered in yolk from another egg. The US regulation defines dirt as adhering or foreign material on an unbroken shell (7 CFR 57.1), and yolk fits that definition. So the stain counts as dirt, not as an internal defect.

If the egg itself is also cracked, the case changes. A broken and dirty shell makes the egg unfit for human consumption (RIISPOA, art. 500, VI), and a dirty cracked egg may not be washed for egg products (RIISPOA, art. 228). See the difference between dirty and cracked eggs in cracked eggs.

How does yolk get onto a whole egg's shell?

Yolk reaches the shell of a whole egg when a broken egg leaks or spreads and touches others. In practice the cause is usually the breakage on the collection line itself, not dirt that was already in the shed.

Because yolk can reach more than one egg in the same stretch of conveyor, the stain often appears in groups. That pattern helps find the breakage point, which is where the team should act. The camera shows the stain, but not where it came from.

Why does yolk on the shell take an egg out of Class A?

Because Class A requires a clean shell. RIISPOA asks for a shell and cuticle that are "smooth, clean, intact" (art. 225, I), and yolk adhering to the shell counts as dirt. The egg therefore leaves Class A, even if the yolk inside is perfect.

Don't confuse it with the yolk adhering to the shell in RIISPOA art. 500, I. That is an interior defect, seen on candling, where the yolk sticks to the inside of the shell and makes the egg unfit for consumption. Yolk smeared on the outside by a broken neighbour is surface dirt and follows the dirty-egg rules.

How does Vigiovo detect yolk on the shell?

Vigiovo measures the stain by area and colour. Each stain is compared with the natural shading of that egg, and colour is measured in Lab colour space, in parallel with brightness. An egg is graded dirty when the stains cover 3% or more of the visible shell. Specular highlights are excluded from the colour measurement, so a shine is not counted as yolk. Details in how it works.

Yolk is yellow-orange, quite unlike a white or brown shell, so the contrast is usually good. The camera does not identify the material: yolk, blood and faeces all count as dirt. On shells whose colour is very close to yolk, the contrast drops and measurement gets harder, so the limit should be calibrated with eggs from your own line.

How do you prevent yolk-smeared eggs?

The best way is to keep broken eggs from reaching the conveyor and touching others. The practices below are routine farm and grading-station practice, with no fixed numbers:

1. Remove broken eggs from the line as soon as they appear, before the yolk spreads.
2. Inspect the points on the conveyor where eggs knock against each other most, such as transfers between sections.
3. Reduce shocks during egg transport, with speed and spacing suited to the line.
4. Count yolk stains by batch and by shed, to see whether the problem is concentrated at one point.

A camera placed before candling takes visibly dirty and cracked eggs off the line, which eases the next step. It does not replace prevention: whoever avoids breakage avoids yolk on the shell. For the rest of the flow, see the guide to dirty eggs.

Frequently asked questions

Can an egg with yolk on its shell be sold as Class A?

No. Class A requires a clean shell (RIISPOA, art. 225, I), and yolk smeared on the outside is dirt. Don't confuse it with the yolk adhering to the shell in art. 500, I, which is an interior defect seen on candling that makes the egg unfit for consumption.

Can a camera detect yolk on the shell?

Yes, by area and colour. Vigiovo compares the stain with the egg's shading, measures colour in Lab space and grades the egg as dirty when the stain covers 3% or more of the visible shell. The camera sees only the shell, so internal defects still need candling.

How can you tell whether yolk on the shell came from a broken egg?

A camera measures the stain but cannot identify where it came from. Spread yolk usually appears on more than one egg in the same stretch of conveyor, and that is where the team should look for the broken egg. Removing broken eggs as soon as they appear reduces the problem at its source.

Sources

1. 7 CFR § 57.1 Definitions (Inspection of Eggs), U.S. Electronic Code of Federal Regulations
2. Decreto nº 9.013/2017 (RIISPOA), Presidência da República (Brasil)