Mussels attach themselves to submerged rocks and other underwater surfaces using strong excreted fibers called byssal threads, also known as their beards. Most preparations call for removing these tough filaments from the shells before cooking, but doing so incorrectly can kill or damage the bivalve inside. It's important to buy and prepare mussels while they're still alive. Tightly closed shells are often a reliable sign of freshness; discard any broken shells or open shells that don't close promptly when lightly tapped. Also throw out any shells that don't open during cooking. Follow the easy steps below to prepare mussels for dishes such as sole au vin blanc.
1. Add ¼ cup kosher salt to 3 cups cold water and whisk to combine.
2. Add mussels to the saltwater bath. This maintains the saline environment they’re accustomed to, helping to keep them alive.
3. Using your hands, agitate mussels gently to remove any debris clinging to the shells. Let mussels soak for 15 minutes. During submersion, mussels filter water in and out of their shells as they breathe. Soaking encourages them to expel any sand or debris remaining inside.
4. Holding each mussel firmly in one hand, use the thumb and forefinger of your other hand to tightly grasp the fibrous tuft emerging from the side of the shell and tug it sharply toward the hinged end of the shell (pulling the beard toward the lip end may tear the bivalve inside, killing it). Discard the beard and repeat with remaining mussels as needed.