By Harold McGee
Butter is remarkable for being a convertible emulsion. This offspring of cream can be turned back into cream! Its convertibility is what makes butter so useful as a finishing enrichment for many sauces, including simple pan deglazings, and it’s what makes possible the sauce called beurre blanc, literally “white butter.” There’s only one requirement for converting butter into the equivalent of cream with 80% fat: the process must start in a small amount of water. If you melt butter on its own, the fat phase remains the continuous phase, and the water droplets settle out of it. But if you melt butter in some water, then you’re starting with water as the continuous phase. As the fat molecules are released into the water, they’re surrounded by water—and by the substances contained in the butter’s own water droplets, which merge right into the cooking water. The droplets contain milk proteins and remnants of the emulsifier membranes that coated the fat globules in the original cream. And those protein and phospholipid remnants reassemble themselves onto the fat as it melts into the water, coating and protecting separate fat droplets and forming the fat-in-water emulsion. However, the droplet coatings in this reconstituted cream are sparser and more fragile than the original fat-globule membranes, and will begin to leak fat if heated close to 140°F/60°C.