Structures of Some Food Dyes
What About the Structure Gives the Color?
As you look at each of the structures of the dyes included here, you will
see something that they have in common. Each structure has conjugated double
bonds. That is, there is a series of alternating single and double bonds with these
structures having either 9 or 10 bonds in the "line". Since they are conjugated
(the electrons are delocalized in the line of bonds), there are not really
alternating single and double bonds but the pi system extends throughout the
entire conjugated system. In such a system of this length, the difference in
energy between the ground state and the first excited state falls in the visible
region of the spectrum. The groups attached to such a conjugated system affect
the energies and thus the position within the visible spectrum. That is,
wavelengths of light are modified by the electron donating or accepting properties
of the groups attached to the conjugated system.
Red 40, Yellow 5, and Yellow 6 are called azo dyes because they contain
a N=N grouping which is called an azo group. Blue 1 and Blue 2 contain nitrogen
atoms but they are widely separated.
These dyes are water soluble because they are salts. They contain
sulfonic acid groups which are a part of a sulfuric acid molecule bonded to a
carbon. These sulfonic acid groups are strong acids which have been neutralized
to give the water soluble form of the dyes.
What About the Names?
These dyes have a variety of names which can make it rather confusing. A table
is given listing some of the more common names for the dyes considered here.
F D & C Number
Red 40
Yellow 5
Name
allura
tartrazine
Yellow 6
sunset yellow FCF
Blue 1
brilliant blue FCF
Blue 2
indigo carmine
indigotine
Color Name
food red 17
food yellow 4
acid yellow 23
food yellow 3
acid yellow 6
food blue 2
acid blue 9
food blue 1
The numbers don't match. The numbers for the blue dyes are reversed. FD&C
Blue 1 is food blue 2 and FD&C Blue 2 is food blue 1.
What Happens to the Dye I Eat?
If you eat a lot of red M&Ms or drink a lot of red Kool-Aid, why isn't your
urine pink? Some of these dyes do pass through the digestive track unchanged
but most are metabolized. The molecules are broken down into simpler
molecules which are not colored. Some dyes are banned for use in food. Even
those which are allowed can cause problems for a small number of people. It isn't
the dye causing the problems but the metabolites. This situation is not
uncommon. My wife is not allergic to quinine (a component of certain soft drinks
primarily used in the preparation of drinks called Tom Collins. But after a couple
of hours she breaks out in a rash. She is allergic to a metabolite of quinine so the
allergic reaction doesn't show up until the quinine is being metabolized. More
information about possible health problems from dyes can be found on the web
but read most of it with caution. There are lot of people who put things out there
without a whole lot of fact or research behind it.
The Structure of FD&C Red 40
Structure of FD&C Yellow 5
Structure of FD&C Yellow 6
Structure of FD&C Blue 1
Structure of FD&C Blue 2
Web Links
http://www.red40.com/pages/chemistry.html
This site has a brief discussion of dyes but the most impressive aspect of this site
are the structures. There are ball and stick models of Red 40, Yellow 5 and
Yellow 6 plus a banned dye, Sudan 1. If you roll the cursor over the structure of
Red 40 it turns into Yellow 6. This allows you to see the differences in the
structures of the two dyes and how similar they really are. The same is true for
the other structures.
Wikipedia has pages on the various dyes. There is little chemistry given except
for indigo which is not a food dye. If you want to know all the various names for a
particular dye, you will find it. Beware of any information about health hazards –
the only references given are to newspaper articles, not to genuine scientific
studies. Even when Wikipedia sounds good, it may be incorrect.