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Author Topic: Why does old tea change colour?  (Read 12588 times)

Offline Stephen Tucker

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Why does old tea change colour?
« on: 02/11/2011 15:01:07 »
Stephen Tucker  asked the Naked Scientists:
   
Dear Chris & Co.,

As a relatively new newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive], I must first thank you for your newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive].  As a doctoral student in the field of mathematics education and leadership, I love to learn, particularly about things I know very little, which leaves a plethora of choices.

Every morning I drink a mug of loose-leaf tea.  (We Americans can't handle small amounts of things... it is 16oz instead of 8oz... we can't even use the metric system, though I did impress it upon my students when I taught primary school.)  I vary my pre-dawn pick-me-up between several (Teavana) flavors, but they are usually white, herbal, or rooibos teas.  Some have fruit or floral bits, others herbs or spices, a few are just the tea leaves.  The brew's color usually ranges from nearly clear to yellowy or greenish, or even pink, but never brown except the rooibos, which is reddish-brown.  (No black
 teas; I don't need the caffeine.)  One morning I left my tea unfinished and did not make it back to my desk until the next morning.  The leftover tea was dark brown.  The next week, with a different type of tea (and a different mug), the same thing happened; the tea started pale green but turned brown by the next morning.  I have done this several times, starting with a clean glass mug and usually leaving just a few ounces of tea to see what happens, and I always return to a brown liquid, even when I haven't had rooibos tea.  I don't recall this ever happening before.  If it makes a difference, I now drink my tea in Logan, Utah (elevation 4,534 ft or 1,382 m) instead of Charlotte, NC (elevation 751 ft or 229 m). Given that I pour the steeped tea through a strainer so few if any bits ever make it into the mug, why does my tea turn brown?  And no, I have not yet tried to drink the results.

I  know that rambled for quite a bit, but I wanted to make sure I included all the details I could.  Thanks, and keep up the good work!

Stephen Tucker
Utah State University

What do you think?
« Last Edit: 02/11/2011 15:16:59 by BenV »


 

Offline Don_1

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Why is leftover tea always brown?
« Reply #1 on: 02/11/2011 15:20:41 »
I'm going to plump for oxidisation. Your herbal infusions may not be fully oxidised when you first brew them, but left long enough, the oxidisation will continue and turn the brew brown.

Let's see what others think.
 

Offline MikeS

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Why does old tea change colour?
« Reply #2 on: 02/11/2011 15:43:47 »
Well, you've convinced me Don.
 

Offline Stephen Tucker

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Why does old tea change colour?
« Reply #3 on: 05/11/2011 14:52:10 »
Thanks, but here's where I admit that chemistry is not my forte.  How does the process work in this context, and why does it cause darkening?  And why didn't it seem to happen, at least not as noticeably, at the lower elevation?

Bonus points if you can explain it using a diagram or other (semi-)abstract visual, albeit one that might be hard to convey on a radio podcast.  My math focus is pedagogy at the elementary (primary) level, and since I've already tried the concrete phase on accident, a connection to the abstract terminology would be quite helpful.  I often find that explaining a process as one would for a child--not patronizingly, rather by bridging concrete to abstract and/or physical to visual to symbolic models--helps both student and teacher better understand the process.
 

Offline RD

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Why does old tea change colour?
« Reply #4 on: 05/11/2011 15:48:27 »
The boiling point of water is dependent on altitude ...

Quote
These boiling point elevation examples from cities across America, shows how different parts of the country have different boiling points due to their elevation. Since elevation within each city varies, these figures are mainly based on the elevation of the city's airport.

    Albuquerque, New Mexico (5,352 feet): 202.3° Fahrenheit (94.6° C)

    Denver, Colorado (5,280 feet): 202.4° Fahrenheit (94.6° C)

    Minneapolis, Minnesota (841 feet): 210.5° Fahrenheit (99.1° C)

    Birmingham, Alabama (644 feet): 210.8° Fahrenheit (99.3° C)

    Detroit, Michigan (639 feet): 210.8° Fahrenheit (99.3° C)

    Miami, Florida (11 feet): 211.9° Fahrenheit (99.99° C)



as is the amount of dissolved oxygen.




« Last Edit: 05/11/2011 16:16:14 by RD »
 

Offline Stephen Tucker

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Why does old tea change colour?
« Reply #5 on: 07/11/2011 12:01:05 »
Thanks; that helps quite a bit!  If I understand you correctly, the oxygen dissipates more quickly at the higher altitude.  This may sound like a silly question, but how/why does oxidisation cause the colour change?

Also, could this have anything to do with evaporation?
« Last Edit: 07/11/2011 14:23:47 by Stephen Tucker »
 

Offline RD

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Why does old tea change colour?
« Reply #6 on: 07/11/2011 16:08:01 »
I Googled a bit and the green tea turning brown is apparently normal ...

http://blog.mellowmonk.com/2005/04/why-brewed-green-tea-sometimes-changes.html

The reaction is the same one which turns fruit brown when it is exposed to air ...
http://chemistry.about.com/od/chemistryfaqs/f/brownapplefaq.htm

Heat destroys the enzyme than causes this colour change reaction.
At altitude the boiling point of water is lower, your tea is going to contain more of that enzyme than at sea level, so your tea will turn brown quicker at altitude.

[ if the water at the different locations has different concentrations of iron minerals that would also be a factor which could change the rate of the colour change ]
« Last Edit: 07/11/2011 16:14:29 by RD »
 

Offline Geezer

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Why does old tea change colour?
« Reply #7 on: 07/11/2011 20:10:29 »
Sorry if I seem to be pouring cold tea on the discussion, but I suspect it's what is in the water at the different geographic locations rather than their altitudes that makes the difference.

To make a valid scientific observation, you'll have to eliminate the water as a variable. I suppose you could always make your tea with distilled water, but don't blame me if it tastes lousy ;D

It is a well known scientific fact that it is only possible to make good tea (and good whisky) in Scotland because of the quality of the water.
 

Offline Bored chemist

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Why does old tea change colour?
« Reply #8 on: 07/11/2011 21:09:32 »
"This may sound like a silly question, but how/why does oxidisation cause the colour change?"
It's not a silly question, but it's quite a complicated one.
Tea contains a whole lot of different chemicals.
Some of these, like the tannins
http://en.wikipedia.org/wiki/Tannin
are fairly dark to begin with but, if they are allowed to react with the oxygen in the air, they produce other compounds that are darker in colour.
It's a tremendous oversimplification but, in general, bigger molecules absorb more light and the oxidised tannins tend to stick together to make bigger molecules.
 

Offline Stephen Tucker

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Why does old tea change colour?
« Reply #9 on: 17/11/2011 14:23:53 »
Hate to beat a dead horse here, especially since you have done such a wonderful job of explaining this, but... yesterday's tea didn't change by this morning.  It occurred to me that I hadn't purposely tested each of the teas I have on hand, and that there was one in particular I might have forgotten.  If you follow the links, you'll head to Teavana's page for each tea.  They include ingredient lists as well as pictures of what the tea looks like when brewed.  (They don't have pictures of how dark the teas turn if you leave them out, but why would they?) I know I can't rule out all the variables, as Geezer pointed out, but this is water from the same source, made in the same way using the same equipment, etc., involved in the initial incident, aside from the different tea.  It wouldn't hold up to rigorous scientific standards, but I did what I could with what I had.  Oh, and the tea made from second or third steepings of the same leaves does darken over time, too, but I haven't tried to compare how much relative to the first steeping.  Any theories?

LINKS TO COMMERCIAL TEA WEBSITE REMOVED - MOD.

(I haven't left them out separately yet, as they were a pre-blended gift.)

I'm eager to read your input, and thanks again for your insights!

(By the way, if you like straight up white tea, Silver Needle is great, and while the Pearls are better, I stick to the relatively economical version when purchasing for myself.)
« Last Edit: 18/11/2011 19:23:30 by Geezer »
 

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Offline Geezer

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Why does old tea change colour?
« Reply #10 on: 18/11/2011 07:06:34 »
Shrunk
Very funny. Thread locked, and no more spammy links please.
 

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Offline Geezer

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Why does old tea change colour?
« Reply #11 on: 18/11/2011 18:28:11 »
Shrunk
OK, Stephen understands he can't put in a bunch of commercial links. Thread unlocked.
 

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Why does old tea change colour?
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