Making a roux has two purposes:
- Coat the flour granules with fat so they are able to dissolve into the cooking liquid without binding up.
- Cook the flour to remove the raw cereal flavor.
When the cooked, fat-covered, flour is introduced to boiling liquid the starch granules swell and explode tangling up the cooking liquid. The cooking liquid is thus thickened and delicious.
If your roux is separating then the starch has not gelatinized. Because you saw this problem in the slow cooker and not in a pot I suspect that you are simply not bringing your liquid to a full enough boil to gelatinize your starch.
I have never seen a roux separate out. The roux should dissolve into the cooking liquid. You might check your ratio and make sure that you don't have too much oil in your roux. Variability in measuring your flour might account for why you see this intermittently.
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.
Best Answer
Technically, you can make roux with any starch and any fat, per Harold McGee.
So use cornstarch or arrowroot or whatever you have. Just avoid something with strong flavor like cornmeal.
Of course, the flavor and thickening properties will be those of the starch you use... And you probably don't want to make a brown roux with anything but flour because of the flavor difference.