On Sun, 25 Oct 2009 09:03:29 -0000, john royce wrote:

> When I steep chopped garlic pieces in Japanese rice vinegar for a few days,
> they stay the same colour. When doing so in Tesco's Chinese rice vinegar
> they turn bright green blue. What would be the likely difference in the two
> vinegar's?

have you done this many times? the difference could be in the garlic:

The discoloration is due to pigments that form between sulfur compounds in
garlic and amino acids. When the garlic tissue is disrupted, as happens in
processing, an enzyme is liberated and reacts with it to form
thiosulfinates compounds that then react with the natural amino acids in
the garlic to form blue pigments. The age of garlic determines how much
isoalliin there is in the first place, and the nature of the processing
determines how much enzyme is liberated.

Original thoughts and ideas:

Garlic is known to contain sulfur compounds which can react with minute
traces of copper to form copper sulfate, a blue or blue-green compound. The
amount of copper needed for this reaction is very small and is frequently
found in normal water supplies. Raw garlic contains an enzyme that if not
inactivated by heating reacts with sulfur (in the garlic) and copper (from
water or utensils) to form blue copper sulfate. The garlic is still safe to
eat.

* If fresh garlic is picked before it is fully mature and hasn't been
properly dried, it can turn and iridescent blue or green color when in the
presence of an acid. It may be caused by an allinin derivative.

* A reaction between garlic's natural sulfur content and any copper
in your water supply, or in the cooking utensils your are using (such as
cast iron, tin, or aluminum) can sometimes change the color of garlic.

* The other sources of copper might be butter, lemon juice, or
vinegar.

* Garlic will also turn green (develop chlorophyll) if exposed to an
temperature change or is exposed to sunlight. Some people say it can be
stored for 32 days at or above 70 - 80° F to prevent greening (but I'm not
yet sure that is true).

* Are you using table salt instead of kosher or canning salt? That
can cause the garlic to turn blue or green. Table salt contains iodine,
which discolors whatever you're pickling. Use kosher or pickling salt.

* Different varieties or growing conditions can actually produce
garlic with an excess natural bluish/green pigmentation (anthocyanins*)
made more visible after pickling.

* Any of various water-soluble pigments that impart to flowers and
other plant parts colors ranging from violet and blue to most shades of
red. This pigment is produced after chlorophyll is destroyed due to
environmental changes. This is a variable phenomenon that is more
pronounced for immature garlic but can differ among cloves within a single
head of garlic. If you grow your own garlic, be sure to mature it at room
temperature for a couple of weeks before using it.

Don't worry, greenish-blue color changes aren't harmful and your garlic is
still safe to eat. (unless you see other signs of spoilage).

<http://whatscookingamerica.net/garlictips.htm>

your pal,
blake