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In the past I have made up to 2L of coffee using a french press and stored it in a flask that I have brought to lectures. Sometimes I will continue drinking this after 5 days, but it tastes pretty bad.

Two related questions: Is it bad for my health to drink after two weeks? Does it lose caffeine over time?

  • Tangential to your question, but you may want to consider using cold brew methods such as the toddy to make a concentrate you can keep in the fridge for a couple of weeks. Add about one quarter concentrate to three quarters boiling water and you're good to go. – Evan Feb 9 '15 at 22:53
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There are a few factors at play here. The temperature that the coffee is brewed at and how well the flask holds that temperature are primary.

While the coffee stays hot the caffeine level will decrease. The speed will depend on the temperature and the amount of caffeine to start with (which is thoroughly discussed in other questions here).

When the coffee cools bacteria growth will start. At what point this bacteria become dangerous not only depends on temperature but also how clean the flask was to begin with and what quality water is being used. Clean tap water in the US, brewing coffee in a very clean french press, and placed into an incredibly clean flask could last two weeks (although it would taste terrible and have little caffeine) but if any of those steps is less than ideal there is a good chance of bacteria growth. Some people's guts will handle that bacteria without a problem, other people may get sick.

Edit: I should point out that as I have researched more I'm less certain about the statement regarding caffeine levels over time. Caffeine should not burn at any temperature coffee is left at, and it does not evaporate. So either experiments that say caffeine levels decrease are wrong or the reason it decreases is not clear. Some suggest that bacteria eats the caffeine, which does tie into why it becomes less safe to drink over time.

  • Great answer. At the very least the coffee will be terrible and not worth drinking. At the worst you're drinking contaminated water. – Mayo Feb 2 '15 at 17:31
  • Thank you, valid points. I guess I won't do this anymore! – user223 Feb 3 '15 at 3:46
  • There are bacteria strains that will metabolize caffeine, so that may contribute. Otherwise caffeine is fairly stable in solution and would not degrade (think energy drinks and soda pop). Assuming you put the coffee in a sealed container (flask) while it's fairly hot (with no or little bacteria load) if might be safe to drink for some time, granted it would probably taste terrible. – Suspended User Feb 5 '15 at 17:26
  • @ChrisinAK - I had read about some of the bacteria strains but found very little information on how fast they grow and how much caffeine they consume. I'm sure it's highly variable but some range would be interesting to know. – Justin C Feb 5 '15 at 19:54
  • I think the one I read a study on was a pseudomonas strain and I believe it was actually metabolizing the caffeine for oxygen. – Suspended User Feb 5 '15 at 20:03
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Would you drink plain water from a container 2 weeks later? If not then don't do it with coffee.

EDIT:

I have finished half drunk water bottles and found the water to be stale. However, based on Chris in AK's comment I'll have to rethink my answer.

I very regularly drink "old" water that is in a sealed container. In fact, the growler sized flask on my desk was probably last filled over a month ago and I'll still sip out of it today. It definitely doesn't taste fresh, but it's fine.

This news changes the equation.

  • 1
    I pretty much assume any "plain water" you buy bottled is older than two weeks. – Suspended User Feb 3 '15 at 0:55
  • It answers the first, but not the second. Regardless, I hadn't thought of it in such a way(which is rather embarrassing)(+1) – user223 Feb 3 '15 at 3:45
  • @ChrisinAK- Everytime I started drinking a half drunk bottle of bottled water I noticed the difference in taste. Two weeks later there is definitely a difference. If you ever left a cup of coffee on the counter for a week (go on vacation) you'll come back and see bacteria growing on it. – Mayo Feb 3 '15 at 11:39
  • A cup of coffee open to the air is much different than a sealed flask or container. I very regularly drink "old" water that is in a sealed container. In fact, the growler sized flask on my desk was probably last filled over a month ago and I'll still sip out of it today. It definitely doesn't taste fresh, but it's fine. – Suspended User Feb 5 '15 at 17:33
  • Anyone else reminded of "Signs", the M. Night Shyamalan movie? – Kyle Macey Feb 5 '15 at 19:06

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