How long can powdered coffee remain fresh? So, I hadn't had coffee for a long time. Plus, I was feeling a little sleepy in the morning so I decided to have a glass of coffee. It tasted a little different than the last time I had it, but I thought maybe I just forgot the taste or the amount of sugar was not same.

Right now, I was just taking a look at the small coffee jar and found out that the best before date had gone by in December 2017 xD. Worried, I opened it, and it didn't smell bad. It was a somewhat pleasant smell. Plus, it was mostly powder - not many lumps. Also, it was stored in a corner of the kitchen (not too hot temp, not too cold) in its airtight jar (so there was no moisture in it).

My question is: How long can the coffee remain fresh in such conditions? I would happily throw it away and buy a new pack, but I wanted to know just out of curiosity.

Side question: The jar also said that the coffee (plus some sugar) could be had with water. Does coffee taste good with water?

Thanks. :)

PS: I was confused whether to ask this on health stack exchange or here - please don't roast me if it's the wrong website. ;P
If, as you say, the coffee smelled ok, was stored dry, and showed no signs of deterioration (lumping, strange colours, etc.), it is perfectly safe to use.

The "Best before" date gives you the producer's idea of how long the coffee will last in its original packaging without losing too much of its taste, aroma, and visual appeal. There are also legal limits involved. So, using a product after its "Best before" date doesn't create a health hazard (provided the packaging isn't damaged, and the product is stored correctly)sup>*. 

In the context of your question, 4 months after the date isn't any problem at all, the different taste could just be due to loss of volatile aroma compounds, especially if the packaging was already opened.

Concerning your second question about using it with water. That's actually the standard way to drink it in Europe: just dissolve the amount you want in a cup of hot (not boiling) water, and add sugar and milk/cream to taste. Same as freshly brewed coffee (from ground beans) is prepared with hot water.


*: Note that a "Use by" date is different. That is used for fresh or pasteurised products that mostly need to be kept cool. Passed that "Use by" date, such products can be a health hazard, depending on how the product was stored and transported (usually there is a safety margin)