Your favorite book might turn yellow due to various reasons. One of them could be that you are generously eating near your book and dropping food! But there is also another dimension to it. It is because of oxidation.

Lignin is an active component of wood that makes trees stand upright. It could be said that it is a glue that binds all the other components, including cellulose fibers together. Lignin is equivalent to the concrete used in buildings and cellulose the steel frame. But Lignin has more to it. It helps protect the wood from pests and other damages.

Paper is essentially composed of white cellulose which comes from wood. But wood also contains a dark substance called lignin, which ends up in the paper. When the paper is exposed to air and sunlight, the lignin turns yellow, thus turning the paper yellow. In other words, oxidation is the reason why paper turns yellow.

As books are now mass-produced, the paper quality is compromised. To produce as economically as possible, these papers have more lignin in them than finer papers. Brown papers used for packaging and cardboards are stiff and strong because they have more lignin in them, and they are not treated with any kind of bleaching chemicals. It does not matter how dark they are because there is hardly any print on them.