Why do newspapers turn yellow over time?

Newspapers turn yellow over time because they contain lignin, a substance in wood that oxidates when exposed to air and light. Learn more about lignin and paper processing in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:00):
Welcome to brain Stuff from house stuff Works dot com
where smart happens. Hiem Marcia Brain with today's question why
do newspapers turn yellow over time? Paper is made from wood,
which is made up mainly of white cellulos. In the

(00:23):
ideal case, paper would contain nothing but white cellulos, But
wood also has a lot of a dark substance in
it called lignant, which ends up in the paper despite
the best efforts to keep it out. The exposure of
this lignant to air and sunlight is what turns paper yellow.
Lignin makes wood stiff and helps trees to stand upright.

(00:47):
You could say that it acts like a glue to
bind the cellulose fibers together. Lignant is a polymer, a
substance that's formed by joining together simple molecules into giant
molecules that act differently than the simple molecules would. According
to Dr Who Men Chang, a professor of wood and
paper science at NC State University, a tree could only

(01:10):
grow about six ft tall if it didn't have lignant.
Lignant also helps to protect the wood from pests and
other damage. Newsprint, which must be produced as economically as possible,
has more lignant in it than finer papers. At the mill,
the wood that will be turned into newsprint is ground
up lignant and all. That's not uncommon because paper manufacturers

(01:34):
use the benefits of lignant in some types of paper.
For example, brown craft paper, which is the dark brown
paper used in grocery store bags, and cardboard are stiff
and sturdy because they have more lignant in them, and
because those kinds of paper aren't treated with bleaching chemicals.
It doesn't matter how dark they are because the printing

(01:56):
on them is limited. To make a fine white paper,
the mill puts the wood through a chemical solvent process
which separates and discards the lignant. Pure cellulose is white,
and the paper made from it will be white and
it will resist yellowing. Lignant eventually turns paper yellow because
of oxidation. That is, the lignant molecules, when exposed to

(02:19):
oxygen in the air, begin to change and become less stable.
The lignant will absorb more light, giving off a darker color.
Chang says that if newsprint were kept completely out of
sunlight and air, it would remain white. After only a
few hours of sunlight and ocygen, however, it will start
to change color. Be sure to check out our new

(02:43):
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