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I keep a little plastic container of instant coffee at work and after a week it has either gone into blocks or darkened. I open it a few times. Its location is above a hot/ cold water dispenser (but seems room temperature) just airconditioned. The home supply of instant coffee is perfectly fine. Why is the work coffee in such bad shape?

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In a word, humidity.

Instant coffee absorbs moisture from air rather well. I suspect that your container isn't perfect airtight, and it's in a humid place. It's possible that the container is airtight, but enough water vapour gets in each time you open it to cause problems. This will be more of an issue for small amounts in a big container.

To test and possibly address this, try a small, definitely airtight container, such as a glass jar with a screw top. Even plastic boxes with an O-ring seal and clips aren't always well-designed; especially if the lid had been hot it may not actually seal perfectly.

You can also use silica gel sachets (food grade versions are available) in the jar with the coffee. These don't last forever, but they can be dried in an oven and reused, also they're cheap. Don't forget to store the spares tightly sealed.

Of course, if you're spooning some out with a damp spoon that will get water in. I have seen people sprinkle the coffee directly into a mug of steaming water - and the steam collects in the container. Don't do that - if you want to sprinkle it into your mug, do so before adding hot water.

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  • This will happen eventually in the original jar, but often more slowly because of sealing, environmental conditions and rate of use.
    – Chris H
    Oct 23 '20 at 8:56
  • There is actually a good way to test how tight your gasket-and-clip box is: fill it with water, close it, pat it dry on the outside, and give it a good shake. If no water comes out, I would assume that there won't be much humidity infusing in over the relevant timeframe. And + 1 on not sprinkling directly into the mug - a very common mistake, also with dried spices into a pot of still-steaming soup.
    – rumtscho
    Oct 23 '20 at 10:47
  • @rumtscho that's how I test them, though squeezing with water in there is a good idea too (e.g. for putting a box of leftovers in a backpack). In general, 4 clips good, 2 clips bad (or at least suspect). Some other types are far, far worse (he says, taking hummus from a pot originally used for sauce from my local Indian takeaway, and known to leak with liquids)
    – Chris H
    Oct 23 '20 at 11:13
  • Plastic containers are permeable to oxygen. That's why the coffee goes in a glass jar, a tin or a bag with a foil layer or coating.
    – D Duck
    Oct 24 '20 at 12:57
  • @DDuck, I think the clumping is due to humidity though, rather than oxygen
    – Chris H
    Oct 24 '20 at 13:36

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