Allura Red AC/FD&C Red Dye #40 (Why you don’t poo pink)
7th June 2006
The next spot on our chromatograph is Red 40, or Allura Red AC. Here is its structure:
Red 40 is another azo dye. Have you ever wondered why you don’t defecate green (or yellow, or red, etc)? I have. This is because bacteria in your intestines cleave that N=N azo linkage. It is, however, possible to overwhelm this system, resulting in excretion of unchanged dye in the feces (as anyone who is coming off an Ecto Cooler binge can tell you).
The intermediate migration of this dye follows our cellulose-is-much-like-silica theory. The molecule is doubly charged (vs triply charged) and has more nonpolar surface area, possibly making it stick more poorly to cellulose (the amphiphilic isopropanol presumably competing for nonpolar sites on the molecule).
One more dye and we’re done!
June 9th, 2006 at 6:20 pm
I’ve wondered why dyes don’t come out the same color they go in as well… and I have a few more biological factoids to add. Bacteria in the gut also breaks down a byproduct of old hemoglobin, urobilinogen, and oxidizes it into stercobilin which gives stool its brown color. When urobilinogen is absorbed by the kidneys and converted to urobilin, it makes the yellow color of urine.
Great blog! Keep them coming!