On a recent trip across the Far North, we came across large black freshwater mussels. As we are avid shellfish eaters, we collected a bucketful and headed home eager to cook them.
It is common knowledge that shellfish should be "purged" in preparation for eating. This purging expels the sand, grit, and mud from inside the shells. This process typically takes 30 minutes, in a bucket of clean water.
What isn't common knowledge however is that purging also helps to remove bacteria and other microorganisms within the shells – this is why purging for 12-24 hours is ideal, with frequent water changes.
One misconception about the act of purging is that it makes shellfish sick, so they "purge" out their guts. Operating under this logic, many people add salt to the purging water. Unfortunately, this only works for saltwater clams, mussels, oysters and other shellfish. The reason this works is because they are native to salt water. If you don't have clean sea-water to purge your saltwater catch in, then add salt to the purging water.
When it comes to purging freshwater mussels, do so in clean freshwater. Don't add salt! Freshwater shellfish really do not like salt, they just clam up even more tightly and won't allow the salt water in. This is what they do when exposed to air as well. It is a survival mechanism.
Warning – if any of the mussels float in the water, do not eat them! Dead mussels release gases causing them to float when they die, so throw these ones out. Discard shellfish with broken shells as well.