Why Do Book Pages Turn Yellow Over Time?
Just how old is this book?
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If you look at old newspaper clippings, aging paper documents and books that are past their prime, you'll notice that they likely have a yellow tinge. But why do old paper products turn this golden hue?

It's not that books would rather be blond, but rather that paper is made from components that yellow over time — at least when they're exposed to oxygen, Susan Richardson, a chemistry professor at the University of South Carolina, told Live Science.

Most paper is made from wood, which largely consists of cellulose and a natural wood component called lignin that gives land plant cell walls their rigidity and makes wood stiff and strong. Cellulose — a colorless substance — is remarkably good at reflecting light, which means we perceive it as being white. This is why paper — including the pages of everything from sheet music to dictionaries — is usually white. [Why Is Cow's Milk White?]

But when lignin is exposed to light and the surrounding air, its molecular structure changes. Lignin is a polymer, meaning it's built from batches of the same molecular unit bonded together. In the case of lignin, those repeating units are alcohols consisting of oxygen and hydrogen with a smattering of carbon atoms thrown in, Richardson said.

But lignin, and in part cellulose, is susceptible to oxidation — meaning it readily picks up extra oxygen molecules, and those molecules alter the polymer's structure. The added oxygen molecules break the bonds that hold those alcohol subunits together, creating molecular regions called chromophores. Chromophores (meaning "color bearers," or "color carriers" in Greek) reflect certain wavelengths of light that our eyes perceive as color. In the case of lignin oxidation, that color is yellow or brown.

Oxidation is also responsible for a sliced apple browning when it's abandoned on the kitchen counter. Oxygen in the air enters the fruit’s tissue, and enzymes called polyphenol oxidase (PPO) oxidize polyphenols (simple organic compounds) in the skin of the apple, Lynne McLandsborough, a professor of food science at the University of Massachusetts Amherst, told Scientific American. This process yields chemicals called o-quinones that then produce brown-colored melanin— the dark pigment present in our skin, eyes and hair.

Typically, paper manufacturers try to remove as much lignin as possible by using a bleaching process, according to Richardson. The more lignin that's removed, the longer the paper will remain white. But newspaper — which is made cheaply — has more lignin in it than a typical textbook page, so it turns a yellow-brown color faster than other types of paper, she said.

Interestingly, the producers of brown paper grocery bags and cardboard shipping boxes take advantage of lignin because it makes their products sturdier. These paper products aren't bleached, leaving them much browner than a typical newspaper, but also stiff enough to give a bag carrying a milk carton and other groceries its strength.

According to Richardson, theoretically, you could preserve your high school yearbookin pristine condition, provided you kept out both oxygen and light indefinitely.

"Oxygen is the enemy," she said. "Keep the book in a perfectly sealed box and replace the oxygen with nitrogen, argon or another inert [meaning it doesn't readily undergo chemical reactions] gas, and you're set."

But while oxygen-rich conditions are bad for paper, sunlight and high-moisture levels can also negatively impact paper preservation, Richardson noted. For instance, any book that's surrounded by oxygen will yellow, even if it's kept in a dark room. "Sunlight just speeds up the oxidation process," she said.

Ensuring that our newspaper clippings stay crisp and legible is one thing, but preservationists, archivists and librarians wage a constant war against paper degradation and oxidation. Preserving important historical documents — anything from a nondigitized will to the Emancipation Proclamation — requires an awareness of what environmental factors can damage paper products.

Original article on Live Science.