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

> A dirty egg has material stuck to its shell, such as faeces, blood, soil or yolk. A stained egg has a colour change in the shell itself, with no material on top. The US regulation defines dirt as adhering or foreign material (7 CFR 57.1), and Vigiovo measures both by area and colour, against each egg's own shading. So the difference is one of origin and measurement, not only of appearance.

Updated: 2026-10-09 · URL: https://vigiovo.com/en/resources/dirty-vs-stained-eggs · Português: https://vigiovo.com/recursos/ovo-sujo-ou-manchado.md

## Key takeaways

- Dirt is adhering or foreign material on the shell (7 CFR 57.1); a stain is a colour change in the shell itself.
- Class A requires a clean shell (RIISPOA, art. 225, I), and the Brazilian text does not define a colour stain.
- A camera measures area and colour, not the material: dirt and stains go through the same method.
- A stain whose colour is close to the shell has little contrast, so the limit must be calibrated with eggs from your own line.

## 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](https://vigiovo.com/en/resources/glossary#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](https://vigiovo.com/en/resources/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.

**Dirt and stain: practical assessment, based on the regulation and on Vigiovo's method**

| Criterion | Adhering dirt | Shell stain |
| --- | --- | --- |
| What it is | Adhering or foreign material: faeces, blood, soil, yolk | A colour change in the shell itself, with no material on top |
| Basis in the rules | 7 CFR 57.1 (dirty egg); clean shell, RIISPOA, art. 225, I | Not defined in the rules; falls under the clean-shell requirement |
| Effect on Class A | Takes the egg out of Class A | Needs a case-by-case decision based on a clean shell |
| How Vigiovo measures it | Area above the egg's shading, and colour deviation in Lab | Same measurement; the limit comes from the line's calibration |
| Typical difficulty | Dark or coloured material usually contrasts with the shell | A colour close to the shell has little contrast and may fall below the limit |

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](https://vigiovo.com/en/resources/glossary#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](https://vigiovo.com/en/resources/lab-colour-space-for-shell-stains). For the whole grading picture, see [dirty eggs](https://vigiovo.com/en/resources/dirty-egg-grading).

## 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](https://vigiovo.com/en/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

- [7 CFR § 57.1 Definitions (Inspection of Eggs)](https://www.ecfr.gov/current/title-7/subtitle-B/chapter-I/subchapter-C/part-57/subpart-A/section-57.1), U.S. Electronic Code of Federal Regulations
- [Decreto nº 9.013/2017 (RIISPOA)](https://www.planalto.gov.br/ccivil_03/_ato2015-2018/2017/decreto/d9013.htm), Presidência da República (Brasil)
- [Como o Vigiovo classifica ovos sem IA / How Vigiovo grades eggs without AI](https://vigiovo.com/como-funciona), Vigiovo

## Read next

- [Cracked vs body-checked (fissured) eggs](https://vigiovo.com/en/resources/cracked-vs-body-checked-eggs.md) (Comparison, 2026-10-09): The difference between a cracked egg and a body-checked (fissured) egg, why only the fissured one can be Class A, who sees each defect and where each goes.
- [Check, broken or leaker: what's the difference?](https://vigiovo.com/en/resources/check-broken-or-leaker.md) (Comparison, 2026-10-09): Check, broken and leaker eggs compared: shell, membranes and what Brazil's RIISPOA and 7 CFR 57.1 say about each one, and where each one goes.
- [How to identify cracked eggs: types, methods and rules](https://vigiovo.com/en/resources/how-to-identify-cracked-eggs.md) (Guide, 2026-10-09): What a cracked egg (check) is, how it differs from a leaker, which methods detect each kind of crack and where cracked eggs must go under the regulations.

## How Vigiovo helps

Vigiovo grades eggs on the conveyor as clean, dirty, cracked or unverified, in real time, with deterministic computer vision: every decision comes with the measurements behind it, on an ordinary CPU. https://vigiovo.com/en/how-it-works.md

4 min read.