Q. Is butter a thickening agent?
A. Oh yes, it certainly has thickened me.
Oh, in cooking? Yes, butter is a modest thickener. Think of it in the sauce beurre blanc, which is nothing more than a bit of wine or vinegar reduced over heat with some shallots before butter is whisked in to complete the sauce. The finished texture is a little thicker than heavy cream.
Other sauces are made the same way: the pan juices from cooking fish or meat are reduced, and then butter is added. Butter contains a fairly high ratio of fat to water, and also a small number of emulsifiers, which account for its thickening ability. But, as we mentioned, itís modest, which is why flour is so often added to butter to make a roux (cooked first) or beurre manie (uncooked),
where the flour's starch provides a significantly greater ability to thicken liquids.