I Didn’t Know That

Q: Why do newspapers turn yellow with time?

A: Because of the lignin they contain. Wood, from which paper is made, is composed of two primary materials: cellulose fibers and lignin, which binds the fibers together. Both are included in newsprint, which must be made as cheaply as possible. Cellulose is normally white and lignin can be bleached white. But when lignin is exposed to sunlight and air, it becomes oxidized and slowly turns yellow. To make good writing paper, paper mills put the cellulose/lignin mixture through a process that removes the lignin, leaving only white cellulose. Brown kraft paper--such as that in grocery bags and cardboard--is stronger than ordinary paper because it has a higher lignin content and is not bleached. For further information about how paper is made, check out https://www.bowater.com/paper.html. To answer a related question, about 80,000 sheets of paper can be made from the average tree, according to https://www.howstuffworks.com.