How do I keep ground coffee fresh after opening?

Skip gadgets and reuse clean glass jars with good lids for cheap, easy portioning. Keep one jar for the week and freeze the rest, label the tops, store away from heat and steam, and always scoop with a dry spoon to slow flavor fade.

Living in humidity, the fight is against moisture and air. For pre-ground, those one way valves do not add much because the coffee is already done degassing. Get a truly tight container, fill it only with what you will drink in about a week, and stash it in a cool dark cabinet. Split the rest into small freezer bags or jars, press out the air, and freeze them. When you need more, let a portion warm up while still sealed so condensation forms on the outside, not on the grounds. Keep your scoop bone dry and close the lid fast.

I live on a sailboat where the air is soup and this is what works. Treat the coffee like a dry spice. Only expose a small amount to the room, and keep the bulk isolated from the climate. Put weekly rations in a tight jar. Freeze the rest flat in thin packets so they thaw fast. If your kitchen is steamy, store the jar inside a larger sealed bin or drawer to buffer humidity swings. Do not open containers while they are still cold. Wait until they reach room temp so you do not get condensation inside.

Flavor loss is mostly oxidation and moisture pickup. Pre-ground has tons of surface area so giving it fresh air and humidity every day is the main culprit. A one way valve does not address either once the coffee is ground. Choose containers that match your weekly consumption so there is little headspace. Press a piece of clean plastic wrap directly onto the surface before sealing to push out air if your container is a bit large. Store the active container in a steady cool spot away from light. Freeze backup portions in well sealed packs and only thaw them once, while sealed, to dodge condensation.

Humidity and oxygen are the big flavor killers for pre-ground coffee. CO2 valves are more helpful for freshly roasted whole beans than for pre-ground, which has mostly finished degassing by the time you open it. What matters most for you is a truly airtight container, minimal headspace, and keeping it cool and dark. I would go with 21oz Stainless Steel Coffee Canister. Stainless steel helps block light and slows temperature swings, and the 21 oz size is big enough to hold about a week or two of grounds without leaving a lot of air inside. Best routine that works in humid climates is simple. When you open a big tub, split it into several small freezer bags or jars, squeeze out as much air as you can, and freeze all but one. Fill the canister with only what you will use in a week, then store the canister in a cupboard away from heat and sunlight. Pull a new frozen portion when you run low and let it come to room temp sealed before opening. Avoid the fridge, use a dry scoop, and close the lid quickly. This keeps flavor lively far beyond a week.

If you ever decide to change one thing, grinding closer to brew time is the biggest upgrade. If you are sticking with pre-ground, you can still keep it lively. Ignore the valve hype and focus on seal quality and portion size. Small opaque canisters or jars are best, one in use and the others frozen. Keep the working container in a cupboard, not on the counter near warm appliances. Close it promptly every time. You will notice the finish stays sweeter and less cardboardy.