I will be making this Gumbo today & reviewing the recipe it states to add in 1/3 cup of veg oil mixed with the toasted flour, it seems like a lot of veg oil. Do you think I have to use all the veg oil?
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Roux is made of nearly equal parts of flour and fat (the French use butter). That's a basic kitchen staple going back hundreds of years.
The roux in gumbo has other items, but it's basically the same concept. You are making a thickening agent for your soup base.
You can usually shade a roux to a little less oil than 1 to 1, but not a whole lot- the flour has to be completely covered with oil to avoid making dumplings. Roux is so fundamental to New Orleans cuisine that you practically have to, but if you're determined you could use, say, cornstarch dissolved in water and get a perfectly good dish. Gumbos traditionally have often used file (a powder made from sassafras, I think the bark) as a thickener in place of roux; shouldn't be hard to find a recipe.
There is also a product called "Wondra", I think from Gold Medal; it's a very fine flour, possibly with anti caking agents, that you can supposedly sprinkle directly into the pot for thickening. I haven't used it myself, but a friend who is an excellent cook swears by it. I also don't know if anybody uses it in this kind of quantity; I think it's more for last minute adjustments.
Considering the recipe calls for 1/2 cup of flour and 1/3 cup of oil, this is about the right proportion. It's two parts oil to three parts flour, a very typical proportion.
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As others have written, the oil is for the roux. In a traditional gumbo, the roux is cooked so dark that the thickening power of the flour is pretty much gone by the time the roux is 'done'. So the roux there is for flavor, which is why file powder is added later to thicken or okra is added. That recipe puzzles me since the flour is already toasted and then mixed with the oil, so I would say that you don't need to use all the oil. One thing to consider though, 1/3 cup is not a lot if you are make a big pot of this Chili Gumbo.