How do you identify a double-yolk egg during grading?

What is a double-yolk egg?

A double-yolk egg has two yolks inside the same shell, both surrounded by the same white. It forms when two yolks are released almost at the same time during the production of the egg in the hen. In practice, these eggs are often larger than average and sometimes elongated.

This is a feature of the inside of the egg, so it cannot be seen from the outside. A normal yolk is seen on candling as a shadow. To understand what is being looked for, see the yolk on candling.

What does the rule say about the yolk?

RIISPOA describes the yolk of a Class A egg as visible on candling only as a shadow, with an apparent outline, that moves slightly when the egg is turned and returns to the central position (art. 225, III). The description speaks of one yolk. An egg with two yolks does not fit it.

The European regulation uses the same idea: the yolk is visible on candling only as a shadow, is slightly mobile when the egg is turned, and returns to a central position (Delegated Regulation (EU) 2023/2465, art. 3(1)(c)).

The cited texts do not name double-yolk eggs. Reading the description as excluding them is a practical interpretation, and the establishment should record it in its grading procedure.

How does candling reveal two yolks?

On candling, the egg is viewed against a strong light. A normal yolk appears as a single shadow that follows the turn of the egg and returns to the centre. With two yolks, the shadow departs from that pattern: the grader sees more than one shadow or a shape out of the ordinary.

When an egg is suspicious, the operator can follow a simple sequence:

1. Turn the egg slowly under the light and watch the yolk shadow.
2. Look for more than one shadow, or a shadow with a shape outside the usual pattern.
3. Set the egg aside for checking and record the case, instead of deciding on the spot.

These steps are grading practice, not a procedure set by a specific article. The final destination of the egg should follow the establishment's record.

What does Vigiovo do with eggs of abnormal shape?

Vigiovo does not confirm two yolks, because the camera analyses the shell and not the contents of the egg. What it can notice is the outer shape. Eggs with an abnormal shape, cut by the edge of the image or overlapping another egg are marked as unverified and sent for manual checking.

A double-yolk egg with a normal shape passes the camera like any other egg. That is why candling remains the step that confirms the inside, and the camera does not replace that check. For what Vigiovo classifies, see how it works.

Frequently asked questions

Can a double-yolk egg be sold as Class A?

The cited texts do not name double-yolk eggs. Class A describes the yolk as a single shadow, with an apparent outline (RIISPOA, art. 225, III). So the decision rests with the establishment, based on candling, and it should be recorded.

Does the Vigiovo camera detect double-yolk eggs?

It does not confirm them. The camera analyses the shell and can mark eggs with an abnormal shape as unverified, for manual checking. Two yolks inside the egg are seen only on candling, which is mandatory in registered establishments (RIISPOA, art. 223).

Are double-yolk eggs always larger?

No. In practice they are often larger than average and sometimes elongated. That is a sign to pay attention to, not proof: size alone does not confirm two yolks, and the check should be made on candling.

Sources

1. Decreto nº 9.013/2017 (RIISPOA), Presidência da República (Brasil)
2. Delegated Regulation (EU) 2023/2465, marketing standards for eggs, EUR-Lex