Making roux is one of those culinary rites of passage, which I found strange because I thickened sauces happily for years with no roux. The first sauce I learned to thicken about 10 years ago was a cream sauce for poached fish from Julia Child's Mastering the Art of French Cooking. It called for thickening with beurre manié, which was so incredibly easy, reliable, and delicious that I used that method whenever I needed a thickener. Eventually, I found out that beurre manié is supposed to be a lazy way of thickening. One should cook the flour and butter first to "cook out the flour taste." Even Julia herself said that in one of her shows.
There was a good discussion of beurre manié and roux on Michael Rhulman's blog, where no one seemed to agree on anything (these were mostly professional chefs, so I assumed they knew what they were talking about). Some people claimed that if roux is not made with clarified butter, it has less thickening power. Others claimed that sauces thickened with beurre manié would thin out after several minutes of simmering, particularly if you double dip when tasting since your saliva contains a starch digesting enzyme. Proteins, enzymes, catalysts, and other big "sciency" words were summoned to give more weight to the arguments that seems quite empty to me.
Cooks are funny people. They have this awe of "food science." If Alton Brown or Harold McGee said something with the word "protein" in it, it seals the deal. After all, science is always right. Well, that's not really science to me. That's just regurgitation of something someone said. Real science is all about setting up a proper experiment with good controls and that's what I decided to do to answer this thickening question once and for all.
I made 4 sauces:
- bechamel with roux
- bechamel with beurre manié
- velouté with roux
- velouté with beurre manié
- same pot
- exactly the same amount of liquid, flour, butter (I didn't clarify it), and salt
- same simmering duration (3 minutes after combining hot liquid with flour/butter)
This sealed the deal! Beurre manié wins. I am a one pot kind of girl and roux requires a whole other pot, so that you can cook your flour/butter. Beurre manié requires just another little bowl and a fork, which can easily go into the dishwasher.