Why does white paper turn yellow with age

Paper turns yellowish or brownish for a couple of different reasons. For example, dust and oil from fingertips will discolor paper over time.
But a big part of yellowing in paper is a chemical called lignin. The amount of yellowing and the time it takes for paper to turn yellow is determined by the amount of lignin in it. When paper pulp is created from a tree, the tree contains cellulose and lignin. The highest quality paper has the lowest lignin content.
So, newsprint is low quality paper. It typically contains 20% lignin or so. If you leave newsprint out in the sun for a day or two, you can see its color changing. The lignin reacts with light and turns yellow.
Typical white copier paper, on the other hand, has less than 1% lignin. It fares much better in sunlight, but given enough exposure it too will turn yellow.
The smell and the yellowing are both mainly due to the presence of hemicellulose and lignin.
Paper is essentially formed from cellulose fibers, which are produced from wood, from cotton or from annual plants like flax (or even from straw). In these plants, the cellulose is generally accompanied by hemi-cellulose (roughly short chained cellulose) and lignin, which both act as binders to constitute cell walls and such things.
Before you make paper, you make paper pulp from these plant sources. Paper pulp is actually “industrial cellulose”. There are two main procedures, with subdivisions but also with the possibility of intermediate procedures. They would be classified as mechanical pulping and chemical pulping.
mechanical pulping in its simplest form means that you take a piece of wood (a log actually) and press it against a very wet turning grindstone. The product is also known as “groundwood” and contains not only the cellulose but all of the hemicellolose and lignin. Groundwood used to be the raw material for “newsprint” (= the paper your daily newspaper and similar articles are printed on)
chemical pulping consists more in dissolving and/or decomposing the binders. There are different procedures, some of which give a typical brown pulp (like “kraft”). Very often the chemical pulping is completed by bleaching. Paper made exclusively from bleached chemical pulp is known as “woodfree” even if the raw material for pulping was wood.
Matters are complicated a bit because more and more “secondary fiber” is used nowadays. (“Secondary fiber” is simply a more expensive term for “recycled waste paper”).
(see also the Wikipedia article on wood pulp : http://en.wikipedia.org/wiki/Woodpulp or http://www.paperonweb.com/
Anyway, cellulose itself has little tendency to turn yellow with age, UV-light or chemical reaction with acids, but lignin will do exactly that. Up to a point this can be compensated by adding optical brightening agents, which reflect UV light as visible light. And a brown paper may become clearer under influence of the UV in sunlight, (although that would be more the case if the brown colour is due to addition of a colouring agent.)

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