Old books yellow and develop a distinct smell due to the presence of lignin, a compound in wood used to make paper. Lignin, which gives paper its yellow hue, breaks down over time, releasing compounds like benzaldehyde (smelling like bitter almonds) and furfural (with a scent of rye bread). Modern paper production removes much of the lignin, which is why newer books don’t yellow or produce the same smells as older ones. Earlier production methods couldn't remove lignin as effectively.

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