Ground flaxseeds "can start to go rancid fairly quickly — as soon as a few weeks after they are ground. This is when the fats start to oxidize, and ... is when you can also lose the nutritional benefits of those fats. The taste is also off."

(Source: An article by Huffington Post writer Carly Ledbetter, quoting registered dietitian Kelly Hogan at Mount Sinai Hospital in New York City.)

I would suggest: Once flaxseeds are ground up, it's best to store them in a fridge or freezer, and then to eat them before they become bitter or rancid.

Flax can go bad within a few weeks after it's ground

Ground flaxseeds "can start to go rancid fairly quickly — as soon as a few weeks after they are ground. This is when the fats start to oxidize, and ... is when you can also lose the nutritional benefits of those fats. The taste is also off." (Source.)

Unfortunately, if you buy ground flax from a store, there's a chance that it might be more than a few weeks old. Still, please read on.

Buying refrigerated or vacuum-packed ground flaxseed

"Ground flaxseeds are usually available both refrigerated and non-refrigerated. If you are purchasing ground flaxseed that is sitting on the store shelf at room temperature, we recommend that the flaxseed be packaged in a gas-flushed, vacuum-sealed bag. If you are purchasing ground flaxseed that is found in the refrigerator section, it's not essential that vacuum-sealed packaging be used". (Source.)

Once you open a vacuum-sealed bag of flax, air will enter. Once this happens, it's wise to refrigerate or freeze the product.

Finding vacuum-packed flaxseed in your region

You can do a Google search for [ buy vacuum-packed flaxseed ] to look for companies in your region who manufacture the stuff.

I live in Canada. Here:

• CanMar produces various "Fee Fi Fo Flax" and "Flax For Nutrition" vacuum-packed roasted flax products. These products cost about 5 to 9 CAD per pound, depending where you buy them. The company states that unopened packages will stay fresh for 18 months.
• Puresource produces "Inari" brand vacuum-packed ground flax. It costs about 7 CAD per pound.

Storing and taste-testing your ground flax

Once you have your ground flax, it's wise to always refrigerate or freeze it.

"Because ground flaxseed flows readily even when frozen, many users choose to store ground flaxseed in the freezer for even longer shelf life." (Source.)

You can take a spoonful of your ground flax and taste it. If it tastes good, it's probably still fine. If it tastes bitter, fishy, or rancid, it may be best to throw it away.