I’ve started trying Trader Joe’s whole bean coffee that comes in canisters. When I opened one today and poured the beans into the grinder, I noticed there was some moisture inside the can that I had not remembered ever seeing. The can says “nitrogen flushed can to help ensure freshness”. The can was fully sealed so I don’t believe anything happened after-market. Is it safe to assume the wetness inside the can is due to the nitrogen flushing or should I be concerned it’s something else abnormal?


Are you sure it is water? If you bought a dark roast, there is a distinct possibility that there is a bit of oil in the coffee. Another possibility is a minor machine malfunction during the roasting process. With large scale coffee roasters, a small amount of water is sprayed directly into the drum just before dumping to "quench" the coffee and speed the cooling process. Almost all of this water evaporates immediately under normal circumstances. As a technician in the coffee industry, I have seen a couple issues that caused more water to go into the beans than intended, and the whole batch had to be thrown out. Hopefully it ends up being oil. In either case, as long as the coffee isn't moldy or foul smelling, it should be just fine.


I would not suspect the nitrogen. It is used to displace air. Air has oxygen and moisture and both are not good for the beans.

Most likely there was a problem and some moisture was on the beans. Given they are roasted this should not happen.


It's a possibility concerning thermodynamics. Assuming that nitrogen is pressurized into the cans, it is released with a rapid drop in the pressure right after you unseal it.

During this depressurization, nitrogen losses lots of energy, thus the temperature around it decreases*. Probably, the most thermally conductive thing around is the can itself as it is made out of metal**. So, the water vapor in the air condensed on the metal itself***, which remains a wet spot.

  • (*) This is the whole idea how our refrigerators work: we first pressurize a gas outside a chamber, then depressurize it to cool down an isolated chamber, which we call a refrigerator.
  • (**) As metals are more thermally conductive we feel heat better through them. That's why that you can move around an electric oven easily when air, which is thermally isolating, protects you. Yet, you could hardly touch a pan on top of an electric oven as metals conduct heat pretty good.
  • (***) That's why the condensed droplets appear outside of a beer can easily when you take it out of the refrigerator. Cold metal, good thermal conductivity, water vapor condensed on it easily.

Note that, my answer is just speculative as I cannot repeat the experiment.

  • If they are pressurized it is very very light. When I open one I hear nothing. No way the nitrogen is liquid (no evaporation).
    – paparazzo
    Oct 17 '18 at 17:52
  • @paparazzo I'm sure that the nitrogen is not liquid. Still, there should be some evaporation when pressure drops (if there's any). Another possibility is that it may be a few drops of grease coming out of beans mistaken as water. Actually, we're just speculating. Jeff should check the possibilities on site.
    – MTSan
    Oct 17 '18 at 19:42
  • Look up the definition. Evaporation is liquid to vapor.
    – paparazzo
    Oct 17 '18 at 20:09

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