I rarely ever think about hydrogen peroxide, but a few times in my
life I've picked up a (sometimes strong) aftertaste of hydrogen
peroxide from chocolate products, most notably chocolate milk.  One
time it was so bad I had to throw the rest of it away.
 Any idea if that particular compound is used in chocolate processing?
 Any explanation as to what I may have actually tasted?

Hi there,

Resources are a bit scarce, but I know a thing or three about
chocolate and I think this will answer your question:

Peroxide is not used in the production of chocolate.  What you're
experiencing is the horror of chocolate that has gone bad.

In chocolate production, milk fats are used to give the chocolate
texture and a good "mouth feel".  Milk fats can oxidise if improperly
stored or handled, however, causing the production of peroxide, which
can lead to unacceptable flavors - the taste of peroxide, for example
- if the chocolate is past the early stages of deterioration:

"Milk fat, however, has a limited shelf-life as it can be oxidised or
attacked by enzymes (lipolysis). The enzymes accelerate the break-up
of the acids into shorter chain free acids, which have a rancid
flavour and make the chocolate unacceptable. When this type of
reaction occurs with cocoa butter, however, the acids formed are
largely tasteless, so the chocolate remains acceptable.

The initial result of oxidation is the formation of peroxides
(containing O2 groups). These have no taste themselves but decompose
to produce unpleasant flavours. A measurement of the amount of
peroxide present is used to detect the early stages of deterioration.
In order to keep the milk fat for longer periods, contact with oxygen
must be minimised. Sometimes the air in the packaging is replaced by
nitrogen and oxygen barrier packaging is used. Chilled storage is
preferred and the presence of copper and iron must be avoided as these
act as catalysts for the oxidation process."

Chocolate Ingredients
http://www.rsc.org/pdf/books/chocsc.pdf

I've experienced this phenomenon, too, and it's not at all pleasant. 
This is the result of chocolate milk beginning to spoil, or in the
case chocolate candy, the result of chocolate being stored improperly
or for too long.

For more information about chocolate, I have compiled a resource guide
that's been used by many over the past several years to find new
chocolates and keep track of chocolate related sites.  **DISCLAIMER** 
My guide was compiled as both an educational resource and a way to
keep my own chocoholism under control.  I hope you find it useful:

Chocolate Resource Guide
http://www.darkfriends.net/princessmoo/chocolate.html

Hope this helps!

--Missy  <-- resident chocolate enthusiast

Search terms:  [ "chocolate making" peroxide ], and much hands on experience.

Good Answer, interesting (if a bit bilious) discussion ensued.
Thanks to all for playing

pcventures-ga: To put it as politely as possible, you are the victim
of your own overactive imagination. 1) Hydrogen peroxide is an
unstable chemical which exists only in water solution and could not
possibly exist in dry chocolate.
2) Hydrogen peroxide in small quantities is non-toxic. There is, in
fact, a food grade hydrogen peroxide. You can look it up on Google. It
is a component of specialty mouthwashes and used in certain food
processing as a safe disinfectant. (I personally use ordinary hydrogen
peroxide as a mouthwash and have lived to tell the tale.)3) What makes
you certain you can identify the taste of hydrogen peroxide? 4) I am a
chemist with 40 years of experience and know whereof I speak. 5) H2O2
is good stuff...be not afraid and cool it.

To monroe22-ga:
Popycocks! 
1) Although H2O2 is mainy handled in solution, it CAN be distilled up
to 100% (I would not recomend it though, it decomposes violently by
catalysis). This solution CAN form hydrates with chelated salts of
rare methals, hence you CAN have H2O2 in "solid" state. Alternatively,
you can freeze it and have it as "ice" H2O2. Furthermore, very
commonly, peroxydes (e.g. derivates of Borate or Benzoil) are
routinely used as "solid" H2O2 (i.e. such substances release H2O2 when
in presnce of H2O by hydrolisis). Such suplements are used mainly in
diswhaser and cloath washing solid soaps (also in some foods as
preservers i.e. antimicrobials).

2) H2O2 is allways toxic, even in small quantities. This is not to say
a person cannot tolerate a small enough dose, BUT there is a
H2O2-specific enzime called superoxide-dismutase that we ALL have and
it was designed by nature, specifically to get rid of H2O2. Cells
CANNOT handle H2O2.

3) If you are  taking/drinking/exposing yourself to H2O2 I would
strongly suggest a remedial course in Lab Safety.

4) H2O2 HAS a distinct metallic taste. YES, you can actually taste
H2O2. The same way you can smell O3 or "feel" humidity. Human seses
are much more developed that you lead to believe. The only caveat is
that NOT all people are sensitive enough to percieve it.

5) With regards to the subject at hand, I know from reliable sources,
H2O2 is NOT used in the making or preserving of chocolate. Simply
because it makes no sense. Chocolate contains fatty acids. When
oxidized, such acids produce an awfull taste. H2O2 would only
accelerate such oxidation. Chocolate (the cheap versions) are loaded
with anti-oxidants such as BHT wich actually reacts with H2O2 and O2
or other oxidants and neutralizes them, so preventing the fatty acids
from gettin oxidized.

6) If it ever crosses your mind to play around with H2O2 I would
recommend caution. It burns and destroys cells. It will affect your
taste buds and smell receptors in your nose if exposed to it. It will
destroy your eyes if splattered.

7) I would strongly suggest you check an SDS (Safety Data Sheet) on
hydrogen peroxide before posting nonsense.

My reply to the last two commentors is this:

 1. As I said in the question, I almost never think of hydrogen peroxide.
    In other words, it's not like I was thinking of that compound while
    eating chocolate or drinking chocolate milk, and my imagination got
    the best of me.

 2. It's only happened a couple of times, usually with chocolate milk,
    which as was pointed out is likely to contain unstable fats which
    can turn into compounds with an odd taste.

 3. My purpose of the question was this: to find out why I (very rarely)
    experience a strong, unpleasant metallic taste when drinking chocolate
    milk or eating chocolate.  The taste was much like that of hydrogen 
    peroxide, so it made me wonder if it's used in chocolate processing.
    My thought was that perhaps that somehow some of it got into the chocolate.

To buckyfuller-ga: You seem blinded by your own expertise. Allow me to
remind you that I am a chemist with 40 years of experience and do not
need your supercilious remarks about lab safety and MSDS data. There
are commercially available mouthwashes containing H2O2...why don't you
go argue with the FDA about that? Have you personally experienced the
taste of pure dilute H2O2? I doubt it. The taste of commercial 3% H2O2
is from the stabilizer. Thanks for the gratuitous remarks about frozen
H2O2 being solid...gee, I would never have known that without your
kind help. Chelates of H2O2 are not H2O2, if I may return the favor. I
am well aware that H2O2 should not be applied to the eyes.
Its use as a disinfectant of cuts and as a mouthwash have not caused
any deaths that we know of. Yes, I know that concentrated H2O2 is
dangerous...it was used to propel the German V2 in WWII. I doubt if
the ordinary citizen will ever come  in contact with it. When a
solution of H2O2 evaporates, the H2O2 decomposes. Possibly you are so
obtuse that you couldn't decipher that in my comment. In any event,
this 76 year old thanks you profusely for your profound wisdom and
guidance.
monroe22-ga

Monroe:

 I appreciate your response to my initial question, and the interesting
 (if volatile) banter with buckyfuller.

 However, I know I'm not imagining this unusual taste.  Looking back at my 
 original post, I said it was usually with chocolate milk and similar 
 products that I experienced this odd taste.
  
 And as for why I suspected Hydrogen Peroxide, it was because I've used
 it a few times after having teeth extracted as a rinse to avoid infection.
 I've never forgotten that odd, unpleasant metallic taste/aftertaste.
 Unfortunately, I've experienced a very similar taste in a handful of
 bottles of chocolate milk and at least one candy bar.
 You seem to think I was worried about this - on the contrary, I was already
 aware of H202's fairly benign nature - I simply hate the taste of it.
 
 All I sought was an understanding as to why I would experience such
 a taste in a chocolate product.  Keep in mind that I love chocolate,
 and almost never think about Hydrogen Peroxide (why would I anyway?).