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There are a great many questions here about roux. This is about the best ratio of flour to butter (fat), and also whether the type of flour (protein content) affects this ratio.

Let me explain. Roux is a thickening agent. The less you have to use, the less fat you ingest. I've seen recipes ranging from 1:1 flour to butter to 1,6:1 flour to butter. That is 60% more flour than butter!

I'm also wondering if the type of flour affects the efficiency of the roux. More proteins (gluten) means higher thickening power (or not), or does it not have any effect?

I've read that heating the roux until it's darker than golden brown will reduce it's thickening power. Is that true?

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This is close to a duplicate of your own question:… Didn't notice this until after I wrote an answer. – SAJ14SAJ Mar 23 '13 at 17:42
Note that flour doesn't actually contain gluten, it contains glutenins and gliadins which develop gluten in the presence of water and heat. This is important because I doubt that a roux actually has enough time to really develop any gluten. Starch thickens, gluten creates elasticity. – Aaronut Mar 23 '13 at 17:47

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According to On Food and Cooking by Harold McGee, roux functions as a thickener due the starches in the flour swelling up and interfering with the flow of water.

In fact, he indicates that technique can be used with any starch and any fat.

This implies that a lower-protein flour (which implies higher starch, weight per weight) like soft summer wheat, or a cake flour will make a more effective roux.

As an aside: roux can be made with oil instead of butter. As we know oil contains no water, and gluten only forms in the presence of water, we know that roux functions without gluten, as there can be no gluten development in an oil-based roux.

McGee explains that cooking the roux initially increases its thickening power by cross-connecting some of the starches. However, as browning occurs, the maillard reactions are transforming starches and proteins into other molecules, and reducing the ability of the roux to thicken.

So yes, it is true that the darker the roux, the less thickening power it has.

In New Orleans style gumbos, for example, the roux is so dark (almost a mahogony color) that it adds no thickening power at all to the stew—it is there for the flavor. The thickening in that dish comes from (depending on the tradition followed) either file powder or ochra, or both (not considered traditional).

I was not able to determine an ideal ratio of flour to fat. McGee indicates that a 1:1 ratio is traditional. However, if that fat is butter, about 20% of it is water, so that really does leave more flour than fat after the water evaporates. In any case, as long as there is sufficient fat for the roux to be cooked smoothly, it will work fine. Additionally, the fat in the roux is also indirectly an ingredient in the finished dish, so it might be desirable in its own right, depending on the outcome.

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what about the use of high protein flour? does that make a difference? – sarge_smith Mar 23 '13 at 23:09
Since flour is pretty much protein and starch, the more protein, the less starch. Starch is the active thickener in roux, so it will have slightly less thickening power. I would still use it if it is all that is on hand--the difference between low and high protein flour is a couple of percentage points. This is in the noise for most recipes, since we are not doing industrial quantities at home. – SAJ14SAJ Mar 23 '13 at 23:19
Great answer SAJ14SAJ, thanks. I'm making quite a lot of roux for croquettes (mostly) and I'm using bakers-flour. Now I can switch to all-purpose. – BaffledCook Mar 24 '13 at 10:15

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