Dirty or stained egg: how do you tell them apart during grading?

What is dirt and what is a stain on the shell?

Dirt is material that sits on the shell. A stain is the shell itself in a different colour. The definition of dirt comes from the US regulation: an egg with an unbroken shell and adhering or foreign material is dirty (7 CFR 57.1). The Brazilian rule requires a clean shell for Class A (RIISPOA, art. 225, I), without defining a stain.

In this guide, a stain is a colour change in the shell with no material on top. That is a practical definition, not the regulation's. For natural speckles and shell tones, see shell colour variation.

What is the difference in practice?

The difference lies in what sits on the shell. Dirt is something that has stuck to the egg, such as faeces, blood, soil or yolk. A stain is the shell in another colour, with nothing on top.

The table summarises the criteria. The Class A and measurement rows are practical assessment, based on the regulation and on Vigiovo's method, not a new rule.

The hardest case is a pigment stain close to the shell colour. It has little colour contrast and can pass without being counted as dirt. So the limit needs calibrating with real eggs from your line, not a default value.

How does a camera tell dirt from a stain?

A camera does not separate them by material, because it does not know what a mark is. It measures the area covered and the colour, relative to the egg itself. Vigiovo fits a surface to the brightness of each egg, measures how far each point falls below it and, in parallel, measures the colour deviation in Lab colour space. If the stains cover 3% or more of the visible shell, the egg is dirty.

Because the method is the same for both, the separation depends on the area and colour limits. For how colour measurement works, see Lab colour space for shell stains. For the whole grading picture, see dirty eggs.

How do you reduce doubt between dirty and stained?

Doubt appears with naturally varied shell colours and with uneven lighting. The team reduces it with calibration and stable light. The steps below are general calibration practice:

1. Calibrate the limits with labelled eggs from your own line, separated by shell colour.
2. Keep the lighting stable, because changes in light alter the measured colour.
3. Review eggs marked dirty with a small area, to separate a colour stain from real dirt.
4. Record dirty decisions by batch, to see whether the doubt repeats in one shed or shift.

If the egg is also cracked, the Vigiovo tie rule applies, which sends the egg to the more severe defect (see how it works).

Frequently asked questions

Can a stained egg be sold as Class A?

It depends on whether the shell is clean. Class A requires a clean shell (RIISPOA, art. 225, I), but the Brazilian text does not define a colour stain. So the decision should rest on the assessment of the shell and on the calibration of the line, not on a rule written for stains.

Can a camera tell dirt from a stain?

It measures area and colour, not the material. Adhering dirt and a colour stain go through the same method, and the limit comes from calibration. A stain whose colour is very close to the shell may fall below the limit, which is why calibration with real eggs matters.

Is blood on the shell dirt or a stain?

Blood adhering to the shell is dirt. It takes the egg out of Class A, because the shell must be clean (RIISPOA, art. 225, I). Blood inside the egg is an internal defect, and a surface camera does not see it.

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)
3. Como o Vigiovo classifica ovos sem IA / How Vigiovo grades eggs without AI, Vigiovo