Buzz
Lignin provides rigidity to wood and helps trees maintain their structure. Over time, it reacts with oxygen, causing paper to turn yellow due to oxidation.
Hitoshi Nishimura/Getty ImagesMain Points
- Newspapers yellow over time because of lignin in the paper, which undergoes oxidation when exposed to air and sunlight, leading to discoloration.
- Newsprint contains higher amounts of lignin compared to premium paper, making it more prone to yellowing when exposed to sunlight and air.
- Premium white paper is produced by removing lignin through a chemical process, which helps it resist yellowing as time passes.
Paper comes from wood, primarily composed of white cellulose. Wood also contains a dark substance called lignin, which is transferred to paper along with the cellulose. The exposure of lignin to oxygen and sunlight leads to the yellowing of paper.
Lignin is responsible for making wood rigid and allowing trees to stand tall. It functions similarly to glue by bonding cellulose fibers together. As a polymer, lignin is formed through the joining of simpler molecules into large structures that behave differently from the smaller molecules. Dr. Hou-Min Chang, a professor of wood and paper science at N.C. State University in Raleigh, N.C., compares lignin to concrete in buildings, with cellulose acting as the steel framework. According to Chang, without lignin, a tree would only reach about 6 feet in height. Lignin also plays a vital role in protecting wood from pests and other potential damage.
Newsprint, which is produced as affordably as possible, contains more lignin than finer papers. At the mill, the wood destined for newsprint is ground up along with its lignin content.
Certain paper types, such as brown kraft paper (used for grocery bags) and cardboard, take advantage of lignin's benefits. These papers are rigid and durable due to their higher lignin content, and they aren't treated with bleaching chemicals. Their darker appearance is inconsequential because printing on these materials is minimal.
To produce fine white paper, the wood undergoes a chemical solvent process that separates and removes the lignin. Since pure cellulose is white, the resulting paper remains white and resistant to yellowing.
Lignin is responsible for turning paper yellow over time due to oxidation. When lignin molecules interact with oxygen in the air, they undergo changes that make them less stable, absorbing more light and giving off a darker hue. Chang notes that if newsprint were kept away from sunlight and air, it would remain white. However, exposure to just a few hours of sunlight and oxygen will start the color change.
