LAB 11: Egg Structure & Grading

Purpose: A brief introduction to commercial aspects of the avian egg.


PLAY VIDEO
Here is a vintage OAC hand-out which has been around since the earliest opportunities for plagiarism made possible by the Mark I Xerox photocopier.  Hopefully the copyright has expired?

Key words:

Shell - cuticle
  - spongy calcareous layer (palisade)
  - mammillary layer
  - pores (for gas exchange)
Shell membranes - outer shell membrane
  - inner shell membrane
  - air cell
Albumen - outer thin
  - firm or middle thick
  - inner thin
  - chalaziferous layer
  - chalazae (ch = k sound when spoken)
Yolk - germinal disc (blastoderm)
  - vitelline (yolk) membrane
  - light yolk layer
  - dark yolk layer

FORMATION

- core of yolk present in the hen's ovary when the female chick hatches
- the rest of the egg takes about 2 weeks to develop
- growth of the yolk accelerates 10 days before ovulation (when the yolk is released into the oviduct)
- yolk is 30-33% of total egg weight
- the albumen is formed in several hours and is about 60% of total egg weight
- inner and outer shell membranes are thin (0.01 to 0.02 mm thick) and are main defence against bacterial invasion
- the shell forms last (9 to 12% of total egg weight) and is composed of
     - calcium carbonate (94%),
     - magnesium carbonate (1%),
     - calcium phosphate (1%) and
     -  protein (4%).


GRADING

- required for interprovincial and international transport
- occurs at registered grading station, by an inspector at any location, or by producer with adequate facilities approved by authorities
- registration number identifies where the egg is graded
- lots of regulations relating to refrigeration, packaging and identification of the product

Canada Grade A.
Shell has not more than three small stain spots. The shell appearance is normal or nearly normal but may have rough areas and ridges other than heavy ridges. The shell cannot have ANY cracks.
 
There are several sizes:-
Extra large at least 64 g
Large around 56 g
Medium around 49 g
Small around 42 g
Peewee less than 42 g 
The yolk has an indistinct yolk outline and in the centre of the egg.
The albumen is firm.
The air cell is fixed in position and shallow.

Canada Grade B
The shell may have spots of dirt or stain but not a whole lot.
Eggs are not subdivided by size but must be at least 49 g.
The shell cannot have any cracks.
The yolk may have a distinct outline and may be oblong and float freely when twirled.
The air cell is not too deep.

Canada Grade C
The shell may be stained or cracked, but the egg must not be leaking. The egg must be clean. The yolk may have a distinct outline and oblong shape.  Blood spots present but not too large.