Why Do Old Books Become Yellow with Time
We all have seen that old books become yellow and brittle with time.
This is due to the presence of lignin. As we know paper is made from wood, wood in turn is made of carbohydrates like cellulose and lignin. Lignin is very complicated molecule that adds hardness to wood. More the lignin, harder is the wood. However in paper it is a problem. Over time, lignin breaks down to form many phenolic acids, which are yellow, in colour. These acids reacts with cellulose. This causes the paper to become very brittle.
But this can be prevented by removing lignin from the wood pulp before it is made into paper. These require additional chemical reactions. In addition the paper is made alkaline by adding calcium bicarbonate. If any lignin is left in the paper, when it forms acid the calcium bicarbonate will immediately react with it and neutralise it. This kind of paper is called Acid free paper.
All this makes paper expensive. Things like newspaper, tickets, notebooks etc. are therefore not printed on it. But all books nowadays are printed on acid free paper.