UKH

I recently switched from drinking coffee to tea so have started taking a flask of hot tea out on walks. I brew my tea in a glass teapot so I can see it is lovely clear ( thankyou soft Scottish water) amber colour when I pour it into the flask, but a few hours later it is a much darker brown almost black. Originally  I thought it was because I was using loose leaf tea and that tea dust passing through the strainer was leading to the tea brewing all day long in the flask, but having switched to using teabags where surely  no tea leaves are escaping I'm still ending up with brown/black tea after a few hours. Can anyone explain what is happening?

In reply to Jon Greengrass:

No idea of the science behind it or if it tastes any different. I do remember on long car journeys back in the sixties that my parents would make black tea and put it into the flask but then decant some milk into a small bottle to add at the point of drinking.

Mother swore that it tasted better.

In reply to Prof. Outdoors:

I would agree that not putting the milk in with the tea tastes better.  Not sure why.  I usually can't be bothered to do that, though.

In reply to Neil Williams:

Same experience here, coffee fine but tea seems to ‘stew’

If you going to be carrying milk separately, may as well just fill the flask with boiled water and carry tea, coffee and milk. You can have tea or coffee then with the bonus of not having to clean the flask out.

In reply to Jon Greengrass:

its to do with tannins in tea reacting with heat.

Just a dyes darken with greater exposure to hot water 

In reply to Jon Greengrass:

Glass flask, or stainless?

Stainless includes nickel. Nickel is a common catalyst. Maybe some chemistry is going on with the tannins.

Have you tried leaving a brew in your teapot (removing the teabag), and seeing if darkens? It might be oxidation of the tannins.

Red wine absorbed into a cotton dishcloth darkens to a much bluer colour very quickly.

In reply to Jon Greengrass:

Is your flask completely clean?

If you previously used ground coffee to make your coffee, it is hard to prevent a build up of dark coffee oils in the flask over time. Most washing up liquid isn't great on coffee oils and it can be hard to physically get to the inside of the flask.

If you used instant, this won't be the issue as long as you try to keep it clean.

In reply to SDM:

I fill flask with hot washing soda solution for an hour or so. Seems to work.

In reply to captain paranoia:

I’ve got an old glass flask in the back of a kitchen cupboard, I shall carry out an experiment and report back.

In reply:

When you pour hot water over the tea, the water absorbs more oxygen due to its increased surface area.  Along with the elevated temperature, it all helps to accelerate the reaction between the tannins and that soluble oxygen.  I guess the darkening of the tea reflects the by-products of that reaction.

Nasty stuff anyway, tea.  Soup's the way forwards.

In reply to Jon Greengrass: likewise I have started having tea in my flask for a change from Ribena or coffee . Always carry the milk separately in a smaller bottle in honour of my dad who always did. Made my mate a brew one day this week. He pulled a puzzled face and after a while Said " there's something odd about this tea , it tastes of chillis " . I  m sure that he hadn't seen the milk bottle , an old El Paso jalapeño jar that has been through the dishwasher about 10 times . Anyway kudos . He shurugged and drank it anyway . 

In reply to Jon Greengrass:

Before I has a stainless I tried and tried to get good hill Tea  glass was no better 

loose tea  needs hot water bag dust tea is “drinkable ‘ if the water is long of the boil 

I’ve given up hot hill drinks but before I did it was hot water  and carry a tea bag   Or the Globetroter stove if time would allow. 

Supposes a good bru be it tea or coffee has to be done probably, anything else is no more than a reminiscence   Bit like music on the radio when it should be live   

Or posting on ukc when I should be out

In reply to Dave Cundy:

FTFY

In reply to Jon Greengrass:

Tannin can react with iron to make a dark purple ink - google “Oak gall ink”.

No idea if stainless protects against that or not.

In reply to EdS:

I tend to drink redbush tea, less tannin and actually tastes the same out of a flask as it does from a China mug.

I'm not a fan of milk, but have always found milky drinks develop a weirdly unpleasant taste in flasks.

In reply to Jenny C:

I tend to have hot water and milk in the flask, then pour it over instant coffee in a mug when I want a brew. Tastes much better than letting it all sit in the flask for a few hours.

In reply to Jon Greengrass:

I just asked Elvis for you.

He said 

‘ huh, huh , a huhu. You ain’t nothing but a teabag, drowning all the time, your tea bags got to many tannins and it ain’t no friend of mine. A hu hu. ‘

In reply to Jon Greengrass:

It's trying to turn itself back into coffee.

In reply to Jon Greengrass:

As others have said, it's the tannins evolving.  What we mean by "tannins" is actually a whole class of different molecules, which have a rather flat, plate like shape (they're polyphenols, to be more precise).  When they oxidise, they join edge-to-edge to make bigger, but still flat molecules.  This changes both the taste and the colour.  The smaller molecules are bitter - as they join together the bigger products become astringent as well as bitter (the astringency actually arises from the way they interact with the polymers that make your saliva slimy).  Their colour changes to make the solution browner (this is the same reaction that makes a cut apple brown when you leave it out) - and when the molecules get even bigger they become insoluble and fall out of solution to make the tea hazy and cloudy.

In reply to Richard J:

Thank you, that is a really interesting explanation 

In reply to Prof. Outdoors:

Earl Grey with a slice of lemon is good in a flask.  
in fact milk is tea is just all round bad