Every time I try to make a Beurre Blanc, it separates into a greasy mess.  What is happening, and how can I prevent it?

--Meegan

 

A Beurre Blanc is a sauce made by reducing a mixture of wine, vinegar and shallots.  Butter is whisked into this to form a sauce about the consistency of heavy cream.  Other acids and flavors can be substituted for the vinegar and shallots, respectively, to make a variety of sauces.  For example, lemon juice, garlic and a pinch of saffron can be used to make a very tasty sauce for shrimp or other sea foods.

The name is pronounced "Burr Blonk" and literally means "white butter" in French.

What is happening is likely one of two things, either you are using clarified butter in place of whole butter, or you are allowing the sauce to become too hot.

To understand, you need to know that Beurre Blanc is an emulsion of fat globules (small spheres of fat) evenly distributed in water.  Normally oil and water don't mix.  However by breaking the fat up into small globules and then coating those with a compound called an emulsifier, you can get them to combine into a homogeneous whole.  The emulsifier in Beurre Blanc is a milk protein that attaches to the fat globules and causes them to repel each other.  Because they are forced apart, the fat globules don't combine into bigger and bigger globs.

The reason you need to use whole butter is that the naturally occurring emulsifiers in whole butter are found in the non-fat part of the butter, which makes up about one fifth of its total weight.  If you use clarified butter, you have removed the emulsifiers which make a Beurre Blanc work.

The other detail which you need to know is that the fat globules in butter are surrounded by a thin protein membrane.  At 136°F (58°C) this membrane breaks down, allowing the oil inside to leak out.  If you overheat a Beurre Blanc, the oil inside the fat globules will escape their protein/emulsifier cage and separate out.  The good news is that if you allow the sauce to cool to about 110°F (43°C), and then add a small amount of water and whisk it in, the sauce will re-form.  However since the oil is now released from the enclosing protein membrane that it previously had, it will slowly seep back out of the sauce.

The easiest way to avoid this problem, I find, is to start with chilled butter.  Once the wine and vinegar mixture has reduced sufficiently, I allow it to cool slightly off heat and then add a couple of tablespoons of the cold butter and whisk it in.  Once that has melted in, I add a couple more tablespoons of butter and repeat until it is all incorporated.  If the sauce cools off enough that the butter doesn't melt in, then I return the pan to the heat just long enough to get the heat up enough to start softening the un-melted butter.  By working back and forth like this -- on and off the heat -- I can regulate the temperature enough to keep the sauce from breaking.  Once all of the butter is incorporated, I return the sauce to the heat one final time while still whisking it, to warm it through, being careful to not allow it to get too hot.  If the sauce starts to get a sheen or I see any drops of oil start to appear, then I immediately take it off heat and whisk it until it cools down a bit.  It is then ready to serve.