So I bought a jar of instant coffee not a year ago and it's not reached its expiry date. It's stored in the kitchen cabinet. Recently I opened it just to see its color had turned from coffee brown to something shiny black, solidified at the bottom. I tried scooping it out with a spoon, but it turned out to be hardened as granite. I managed to scrape a spoonful off, but it's still hard as rock. What do I do?
The most sensible solution is to throw it out.
Instant coffee is hygroscopic, it will absorb moisture from the air if possible. I guess either someone was sloppy when scooping out the instant coffee and used a wet spoon or the container was not airtight (either bad packaging design or because a careless user didn’t close the pack properly). Now add high humidity to the mix and you will get the lump as seen in your post.
There are at least three reasons1 to discard this container.
A) It’s a pain to pry it out and to dissolve it, as you have already observed. Pouring hot water straight into the container may help, but then there’s:
B) The flavor is almost certainly not pleasant any more (see 1).
C) But the most important factor is food safety. If the granules got humid, there is a certain probability of bacterial and/or fungal growth and I would really recommend to discard the (ex-)coffee. I wrote more about food safety considerations in this post, explaining the FAT TOM principle of food safety.
1 There are some coffee snobs that would claim that instant coffee per se is disgusting enough to throw away, but we won’t go down that road here.