What does flushing mussels in water with flour do?

Question

On Serious Eats, I read the following comment under a recipe:

Why not flush before cooking? The correct spelling and translation, meuniere, means mussels a le miller’s wife. Because they add a bit of flour to the flush water before cooking.

For clarity, this is the comment I refer to:

If I understand correctly, the comment refers to soaking mussels in water with flour before cooking them. The water is often used to get the mussels to release some unwanted things in their shells, but I'm not sure what the flour does.

After searching a bit, found that Meunière is a French technique for cooking fish. It involves drenching the fish in flour to fry them afterward. That makes sense as the flour, some fat, and the high temperature make some sort of crust or roux.

When cooking mussels, however, there's liquid during the cooking to prevent reaching that high temperature. I've found that a roux cooks at temperatures upward of 300°F (~150°C), which the steam will not reach.

So my question is, what does the flour do when adding them to the mussels' soaking water? Is there any benefit, perhaps to the resulting stock (the linked recipe uses the stock for the sauce, but doesn't use flour)?

Answer 1

I think you are confusing Meunière (mostly for pan fried fish) and Marinière which is one way to prepare mussels.

Cornmeal is often used to clean mussels; it is supposed to help remove grits and dirt from the mussels.

Answer 2

Apparently, the flour is supposed to help flush the sand out. It supposedly works by having the mussels take in the flour so it spits out the sand. I found this on an old blog post in Dutch, which actually recommends not doing this as most mussels are already flushed on the fisher boats.

The blog does not say whether it actually helps if there is still sand in the mussels.

The relevant quote (in Dutch) from Smulweb:

Veel consumenten bewaren mosselen in een bak met zout water. Soms wordt er zelfs bloem aan het water toegevoegd. Mosselen zouden zichzelf dan reinigen van zand en het vlees zou mooi blank van kleur worden. Tegenwoordig is niets van dit alles nodig. De Zeeuwse mossel is absoluut zandvrij. Direct na de vangst worden de mosselen aan boord gespoeld.

Translated:

Many consumers store mussels in a bowl of salted water. Sometimes, they even add flour to the water. Mussels supposedly cleanse themselves from the sand and the meat supposedly gets a nice white colour. Presently, one of this is necessary. Zeeuwse Mosselen are absolutely free of sand. Directly after they're caught the mussels are flushed on board.