People often e-mail or call me with their cooking troubles. Many of these questions are ordinary, and not worth repeating here. But I recently got a query that piqued my interest as a kitchen scientist. An Alibi staffer wanted to know why her leftover salmon in sundried tomato cream sauce turned into salmon in half a cup of oil when she reheated it in the microwave. The simple answer is that zapping it broke the emulsion of the sauce. If you know what that means, then skip along to the film times; if you're clueless, read on.
An emulsion is a blend of two liquids that become combined when one forms tiny droplets that are evenly dispersed in the other. An emulsifier, like egg or mustard, bonds to both water and oil molecules, holding them together. Vinaigrette salad dressing is the most common example. You take oil and vinegar, which normally don't mix, and with a lot of whisking, they form an emulsified sauce, albeit a temporary and unstable one. Vinaigrette's unstable nature is evident when five minutes' time is all it takes for the oil and vinegar to separate again, leaving distinct layers in the complimentary Hidden Valley Ranch cruet. If you add an emulsifier like mustard to the oil and vinegar, the dressing will take longer to separate.
Mayonnaise is also an emulsion. Made with oil and egg yolk, it's much more stable because the mixture contains so much egg. Milk and cream are also emulsions, in which water and butterfat molecules are held together by emulsifying protein. Milk is largely water with a little butterfat (you know, about 2 percent?) evenly dispersed throughout. It's easy for the protein to keep the water and fat together, so milk rarely separates. Cream contains anywhere from 20 to 40 percent fat and it's much harder for the emulsion to hold.
Cooks commonly refer to the failure of an emulsion as a “break”, as in, “Oh shit. The Hollandaise broke and my arm still feels like a flaccid salmon filet from whisking it for half an hour. That's it. I quit!” The proper name for breaking is curdling. Sometimes curdling is good—without it we wouldn't have cheese—but mostly it means you screwed up somehow.
When an emulsion curdles, the little droplets of liquid that were once evenly dispersed start merging into big droplets. In milk or cream, curdling can be caused by changes in temperature, or by the addition of acid to the emulsion. If you've ever put lemon in your tea and then added cream, you know what this looks like. It's an ugly, patchy mess. The fat, water and protein molecules all separate from one another and nothing can put them back together again.
Molly's microwaved salmon in sundried tomato cream sauce was a disaster for several reasons. As I mentioned, cream is already a relatively unstable emulsion, and the addition of acidic sundried tomatoes probably made it more so, but the rapid heating of the microwave was the last straw. The cream curdled, the water evaporated, and she was left with nothing but fat and a little bit of protein solids that probably weren't even noticeable.
The only way to preserve a cream sauce on leftovers is to heat it very, very slowly, something that's hard but not impossible in a microwave.