What is the advantage of a roux over a raw flour slurry, in sauce?

Question

My standard Bechamel sauce recipe used to be:

1. Stir together flour and oil into a paste
2. Fry for a short while
3. Add a small amount of milk
4. Heat and stir until incorporated
5. Repeat steps 3-4 with increasing amounts of milk, until the mixture is a thick liquid
6. Add rest of milk and boil until thickened

But recently I've got lazy and been doing it like this:

1. Whisk flour with enough cold milk to make a thin paste with no lumps.
2. Add to pan of cold milk and stir.
3. Bring to boil, stirring occasionally
4. Boil until thickened

The roux method requires a lot of care and attention. The second method just requires half an eye on the pan.

But roux is a mainstay of classical cooking. What is its advantage?

Answer 1

Roux Method

The advantages of the roux method:

1. It can be prepared in advance
2. The raw flour taste is cooked out when the roux is prepared, so the sauce is ready as soon as it is thickened; this also makes it easier to add more roux to adjust the thickness of the sauce.
3. It actually requires less supervision. You are actually being overly fussy with your roux based sauce. You could add all of the milk at once, although starting with one smaller batch just to dissolve the roux is a good idea.
4. The butter coats the flour particles, making lumping quite unlikely
5. Can be browned for additional flavor at the cost of thickening power

It also adds oil or butter to the recipe, which may or may not be an advantage.

Slurry Method

The advantages of the slurry method (which is what the second method is, although it is more typically done with water or stock than milk are):

1. It is fast and convenient, if you don't have roux prepared ahead
2. No oil or butter is required, so it doesn't have to be accounted for in the recipe.

Disadvantages:

1. It is easier to get lumping if you don't thoroughly whisk the slurry before heating
2. It must be brought to the boil for at least a couple of minutes to eliminate the raw flour taste, and harder to adjust thickness.
3. Harder to prepare ahead

Conclusion

Use whichever you are comfortable with. For fine sauces, roux based may be superior (and certainly more buttery), but you can have excellent outcomes with a slurry. For casual cooking , I tend to use a slurry, saving roux for more formal dinners and fancier dishes like Thanksgiving gravy.

Answer 2

The advantage can be reduced to one word: taste.

A slurry based sauce is not the same thing as a roux based sauce. Milk pudding is not a Bechamel in the same way that a baguette is not a brioche, margarine is not butter, and 'cocoa-containing fat glaze' is not ganache. It has a different taste, and cooks over the generations have preferred the Bechamel with its rich taste.

Texturewise, the slurry based sauce is a good substitution for practically all uses of bechamel. If you personally find the taste good enough, then go ahead and use it. The world is full of examples where people are very happy with substitutions made for speed or economy reasons. I'd say that cooks at decent restaurants shouldn't use them, because they hurt customers' expectations and can be construed as borderline fraud ("I ordered a roast and you are giving me meatloaf?!") but in home cooking, you (and your family) decide what you like for dinner.

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A small technical note: If you decide to go with slurry, it will be easier to use pure starch, not flour. It has better solubility and you don't run the risk of a raw flour-y taste.

Answer 3

It's also possible to make an uncooked "slurry" of softened butter and flour (beurre manié). That's kind-of handy when you're finishing a sauce and need to add a little body. I've never seen a recipe start with that however.