Why did my cold oolong tea turn black when I added ginger-vinegar syrup?

October 28, 2014 4:13 PM

My partner made Oolong tea two days ago, cooled it and put it in the fridge to drink as iced tea. Today we poured a cup, and then added a splash of homemade ginger-vinegar syrup and the liquid (the tea was a light clear brown, the syrup a darker clear brown) immediately turned into an opaque dark greeny black. Why?

The ginger-vinegar syrup (a homebrew version of this stuff) was made with apple cider vinegar, and a boiled down ginger-sugar syrup, and is probably a couple months old (and may be going off/fermenting as a result. I just smelled it and it does have a boozy odour)

Partner (who likes to live dangerously) drank the black concoction and 6 hours later is fine, apparently.

So what happened?

posted by stray to Food & Drink (27 answers total)

Was there metal involved at any point? A metal container or a metal stirrer?
posted by BrashTech at 4:16 PM on October 28, 2014

Response by poster: No - the syrup would have been made in a steel pot, but it is stored in a glass bottle with a swing top wire bale seal (google tells me that's what those are called!). The tea is in a erlenmeyer flask, and the glass we poured it into was actually a beaker because we are horrible nerdy hipsters.
posted by stray at 4:20 PM on October 28, 2014

That it turned black makes me think oxidation. I'd guess some kind of reaction between the acid in the vinegar and the old tea. Can you try it with fresh tea and see what happens?
posted by phunniemee at 4:26 PM on October 28, 2014

Response by poster: Hrmm. Just tried it with fresh oolong, fresh orange pekoe and fresh peppermint tea - a little bit of syrup and they all turned black! Guess it's the syrup - but what's the chemical reaction?
posted by stray at 4:43 PM on October 28, 2014

This is a total guess, but the acid in the vinegar could be precipitating something in the tea - particularly if there was something protein-y already in the tea, or (unpleasant to think about) growing in the tea. For example, when you add an acid to milk, you get clear liquid and lumps of very opaque curds. Something like that could be happening in your tea. I would see if you can repeat the reaction with fresh brewed tea. If so, it's something inherent to the tea. If not, some bacteria or fungus probably grew in your tea.
posted by fermezporte at 4:58 PM on October 28, 2014 [1 favorite]

Oolong tea is really acidic, isn't it?
posted by Nevin at 5:17 PM on October 28, 2014

This sounds exactly like an acid-base indicator kind of thing. There are many chemicals that change color depending on the pH of their environment. There's a bunch that are naturally occurring, Hydrangeas change based on soil pH, and the juice from red cabbage changes color from like a bluey purple to a light pink thing if you add in vinegar. I'm supposed to be working so i can't do any research on the topic, but if you google "oolong tea acid base indicator" or maybe just "tea acid base indicator" I bet you'll get some hits that'll set you on the right path.
posted by DGStieber at 5:30 PM on October 28, 2014 [1 favorite]

2nding DGStieber. The extra H+ ions from the acid attach themselves to one of the complex organic anions in the tea, and it changes colour. If you just add vinegar (acid), it should also happen. If you add bicarb (base), the colour change should reverse (and it'll fizz, for extra fun).
posted by kjs4 at 5:40 PM on October 28, 2014

Response by poster: Well, I just added white vinegar to the oolong tea, and nothing happened.

The syrup is apple cider vinegar and sugar ginger which may have fermented and produced alcohol at this point - would that mixture be much more acidic than white vinegar?
posted by stray at 5:43 PM on October 28, 2014

Response by poster: Stoneweaver - it really does turn black - the change looks like precipitation.
posted by stray at 5:44 PM on October 28, 2014

Response by poster: Bicarb doesn't do anything to the black liquid, as far as I can tell.
posted by stray at 5:46 PM on October 28, 2014

Is there vanilla in the ginger-vinegar mix? Vanillin will react with the tannin to form a dark precipitate (seen as brick red curds when you add some brands of soy milk to tea).
posted by ambrosen at 5:57 PM on October 28, 2014

Oooh, complicated. There goes that theory then.

Can you let the tea sit for awhile and see if the precipitation settles out?

The fact that this happens to peppermint tea is interesting, as I don't think peppermint tea has as many tannins as black tea. Does this happen if you add the syrup to hot water? Could be a heat thing.

Do you have the ingredients for the original syrup? Have you tried adding them separately to the tea? Or adding them one by one. It may be one of those weird, only if everything is there reactions, which can be very hard to pinpoint.
posted by kjs4 at 6:34 PM on October 28, 2014

And, ignore the heat suggestion, just saw that it was in ice tea.
posted by kjs4 at 6:35 PM on October 28, 2014

Oh, try adding the syrup to just water. It might be a weird dilution thing. (Like pastis)
posted by kjs4 at 6:36 PM on October 28, 2014

So.. wikipedia for pastis says the reaction there is due to terpenes being soluble in >30% ethanol, but insoluble in less.

These people say it happens with grapefruit based things too.

And ginger also has terpenes, although geeze, there are so many different kinds and I haven't gotten more about *which* terpenes have opacity in solution determined by ethanol concentration.
posted by nat at 7:30 PM on October 28, 2014

In case you want to know what the aromatic components of ginger (and anise), this book has them. Not an organic chemist so that's all I've got.
posted by nat at 7:43 PM on October 28, 2014

I think it's a combination of what BrashTech said about the metal -- the "steel pot" you made the syrup in, in this case -- and what stoneweaver said about the tannins:

Complexes of Ferrous Iron With Tannic Acid

In solutions with a pH of 4 or more, a black material containing ferric iron and tannic acid is precipitated. In a solution containing 500 parts per million of tannic ...

You must have picked up quite a bit of iron from that pot with the vinegar-ginger-sugar combo.
posted by jamjam at 7:45 PM on October 28, 2014 [1 favorite]

Response by poster: Back! So far (it's been maybe three hours on the most recent batch, I'll leave it out over night) it has not precipitated out.

The reaction doesn't happen with water, so I'm not sure about the terpenes thing.

The pot was stainless steel...thought that was non-reactive?

This is fascinating, thank you all for your theories! Glad we bought all these hipster beakers (brand new, before anyone jumps on that as the cause).

A bit of googling yielded this thread about a particular brand of honey turning tea black. Maybe it has something to do with the sugars in the syrup?
posted by stray at 8:20 PM on October 28, 2014

Response by poster: I don't have any apple cider vinegar left alas - but if someone wants to try adding that to tea to see if that does it...
posted by stray at 8:24 PM on October 28, 2014

Response by poster: Oh, and no there isn't any vanilla in the syrup.
posted by stray at 8:25 PM on October 28, 2014

The pot was stainless steel...thought that was non-reactive?

Vinegar will corrode stainless steel:

Long-term exposure to table salt and salt and vinegar mixes can damage stainless steel.
...
Do not allow the following food items to remain on stainless steel surfaces for hours (can cause staning): ketchup, mustard, mayonnaise, lemon juice, vinegar, salt, or salad dressing.

and since acetic acid has a higher boiling point than water, by boiling the syrup down you would have, neglecting any effects of the ginger and sugar, only made the vinegar stronger.
posted by jamjam at 9:11 PM on October 28, 2014

I found a school science lab experiment that appears to use the same reaction between tea and fruit juice in order to test for iron (link is for a .doc file - here is Google's Cached version). Of note: "Certain chemicals in tea react with iron compounds in fruit juices to form precipitates."

The PokPokSom doesn't appear to have any iron content, but perhaps your homemade version absorbed some from the stainless steel pot?
posted by 1367 at 9:30 PM on October 28, 2014

Response by poster: Okay! Thanks folks, gonna go with iron barring another suggestion. Very interesting! The vinegar wouldn't have been in the pot for very long, but it must've picked up some.

Does this render the syrup (which is probably past its best-before anyway) unsafe?

Thanks all for the food mystery help!
posted by stray at 10:05 PM on October 28, 2014

Huh, no, I really don't think that the stainless steel leeched a high concentration of iron into your syrup. That's...kinda the opposite of what stainless steel was designed to do.
posted by desuetude at 10:10 PM on October 28, 2014 [1 favorite]

Response by poster: The dregs at the bottom of the glasses once we tossed the black liquid smelled iron-y, like blood, so I still think iron was the culprit. Still not so sure how the iron got in there in the first place.
posted by stray at 2:39 PM on November 1, 2014

Stainless steel can transfer metals (iron, nickel) to the food being cooked, especially if the oxidised layer is removed (metal utensils, cleaning method) prior to cooking. It varies greatly by the design of the pan, so it may not be easy to clearly state whether or not this is the case.

One other thought to consider: How hard is your water? Does it possibly contain some iron?
posted by 1367 at 6:40 PM on November 1, 2014