The Discoloration of Furfural

Freshly distilled furfural is colorless, but when it is exposed to the atmosphere for some time, it turns via yellow and brown to black. This color buildup is known to be triggered by oxygen as proven by the fact that when freshly distilled colorless furfural is stored under its own vapor pressure (after pulling a vacuum), it stays colorless indefinitely [22]. Three further facts shed light on the nature of the phenomenon  

Difurfural (5,5 -diformyl-2,2 -difuran) with 6 conjugated double bonds is yellow. 

Against this background, we can revert to the role of oxygen as the proven trigger of the discoloration process. As oxygen is known to be a diradical 0-0  

The relative quantity of color-causing substances in furfural is extremely small.The naked eye can detect colored substances in concentrations as low as 10 M. When a commercial 98 % pure furfural has turned totally black, and is then submitted to a vacuum distillation, it will still yield more than 97 % of pure furfural. In other words, in all but extreme cases, the color-causing substances amount to less than 1 %. Even if furfural is permitted to solidify to a gel after storage for many years, vacuum distillation of this gel will still yield 90 % of pure furfural [28]. 

---