After purging mussels 6 times in salt water, 20 mins each, why they still exude much grime?
I've bought mussels at Whole Food Market for a year. They sell them on ice and dry, not in running water.
At 10 s this video, Bart teaches add salt to cold water and soak for 20 mins.
Aliza Green's The Fishmonger's Apprentice : The Expert's Guide to Selecting, Preparing, and Cooking a World of Seafood, Taught by the Masters. p 142.
To begin, place the clams in a sink or large pot of cold saltwater (use 1/2 cup [150 g] salt to 1 gallon [3.8 liters] of water). Add 1/2 cup (70 g) cornmeal to help whiten the meat if desired. Leave the clams to purge for 30 minutes to 1 hour. Scoop the clams from the water, leaving the sand and grit behind. Have ready a clam opening knife (see "A Few Good Tools"). It is highly rec- ommended that you wear a protective glove on your nonwriting hand.
But I repeat 6 times...so 30 mins x 6 times = 180 mins of soaking!!! Mussels exudate less black grime every next time, but even after 6 times, water still has much black grime! See my pic under! Why???
Part 2, Step 2 has GIF that shows you what I mean.