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SHELL
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Bumpy
and grainy in texture, an eggshell is covered with as many
as 17,000 tiny pores. Eggshell is made almost entirely of
calcium carbonate (CaCO
3
) crystals. It is a semipermeable
membrane, which means that air and moisture can pass through
its pores. The shell also has a thin outermost coating called
the
bloom
or
cuticle
that helps keep out bacteria
and dust.
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INNER
AND OUTER MEMBRANES
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Lying
between the eggshell and egg white, these two transparent
protein membranes provide efficient defense against bacterial
invasion. If you give these layers a tug, you’ll find
they’re surprisingly strong. They’re made partly
of keratin, a protein that’s also in human hair.
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AIR CELL
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An
air space forms when the contents of the egg cool and contract
after the egg is laid. The air cell usually rests between
the outer and inner membranes at the egg’s larger end, and
it accounts for the crater you often see at the end of a
hard-cooked egg. The air cell grows larger as an egg ages.
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ALBUMEN
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The
egg white is known as the albumen, which comes from
albus,
the Latin word for “white.” Four alternating
layers of thick and thin albumen contain approximately 40
different proteins, the main components of the egg white
in addition to water.
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CHALAZAE
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Opaque
ropes of egg white, the chalazae hold the yolk in the center
of the egg. Like little anchors, they attach the yolk’s casing
to the membrane lining the eggshell. The more prominent they
are, the fresher the egg.
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VITELLINE
MEMBRANE
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The
clear casing that encloses the yolk.
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YOLK
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The
yolk contains less water and more protein than the white,
some fat, and most of the vitamins and minerals of the egg.
These include iron, vitamin A, vitamin D, phosphorus, calcium,
thiamine, and riboflavin. The yolk is also a source of lecithin,
an effective emulsifier.
Yolk
color ranges from just a hint of yellow to a magnificent
deep orange, according to the feed and breed of the hen.
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