How much roux for 2 gallons of gumbo?

I'd go with 2 cups flour

Depends on how "roux-y" you want it. Recipes I've surveyed vary from 6-1 up to 20-1 ratios of stock to roux. I've looked at dozens of Louisiana cookbooks.

Notice you should just concentrate on the liquid to roux - the meat, fish or other ingredients don't need to be accounted for. You should also be aware that if you make a roux with 1 cup of flour and 1 cup of oil, you will get 1 1/3 cups of roux.

I don't really measure, but you should know that the darker the roux, the thinner it will be which is fine. I like a dark roux. It all depends on how much body you want with the gumbo. Some like it thick and some like thinner. Paul Prudhomme's recipe for fried chicken gumbo suggests 1/2 c oil:1/2 c flour to 7-10 cups liquid. You can go heavier on the flour in that ratio if making a dark roux. However, remember that when making fried chicken gumbo, you will also have the fried flour from the chicken dissolving into your gumbo when it simmers which will also add more body and it's not going to be fried as dark as your roux, so it will definitely add to the texture.

I always make extra roux. I add roux to the liquid so I can stop when I'm got the texture I want especially if I'm making fried chicken gumbo.

The darker the roux,the less thickening power. It's scientific and I'm no scientist. It's just fact. I cook roux dark, so I probably use more flour and make more roux than some for gumbo. Like you, I don't measure. I just sort of know what I need.

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You shouldn't because she is right.

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Yeah like I said I just have never paid attention because I always just add until I think it's about right. The old tried and true method....about that much.

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I wasn't doubting you. I was just asking for an explanation was all.

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I knew that. It's just something I learned from who knows where. I can't explain it though! That's what I trying to tell you.

A chef friend of mine explained it to me this way:

I tried to find out what's really happening with the chemistry of roux and what seems to be happening is that the flour dissolved in oil keeps the starch molecules separate and prevents clumping which would happen if you just add flour to a liquid stock. As you heat the oil/starch mixture (roux) in a stock, the starch molecules absorb the liquid and eventually some of the starch molecules burst and release amylose (which is a simple sugar found in flour. The Amylose, in turn, bonds with the starch molecules to restrict movement of the starch in the liquid. The starch thickens the liquid because it bonds physically with both oil and water to make a type of emulsion.

The more you cook roux, the more polymerization of starch molecules occurs and this lessens the thickening action of the roux because more starch molecules are polymerized (become part of a longer chain) and fewer are available for emulsifying the stock (bonding with both the oils and other organic compounds in stock as well as with the oil).

So lighter roux ends up thickening more than a darker roux. And I'll bet that gluten proteins are also degraded with the high heat of cooking too...decreasing the thickening (emulsifying) action of roux.

So what does this all mean?
1) lighter roux thickens more than dark roux...so if you use dark roux and want a thicker gumbo, thicken it with some blonde roux as well or use a cornstarch slurry, or file , or okra.

2) besides thickening, another equally important function of roux is flavoring the stock...and the more you cook the gumbo, the more the flavor changes. The taste also changes with the amount of roux that you use.

3) There are so many variables in cooking gumbo that affect the final taste that make it very difficult to reproduce. Your method becomes very accurate when you can get a consistent taste at the end. Cooking really is an art that involves some science.

Cooking definitely involves science. There are things I know, but I may not necessarily know the why. Thanks for the explanation.