In solid form, cheese is a stable emulsion. The tiny droplets of dairy fat are suspended in water and held in place by a net of interlinked proteins. When cheese melts, however, that net breaks apart, and the oil and water tend to go their separate ways. Sodium citrate can form attachments to both fat and water molecules, so it holds everything together. The end result is a perfectly smooth, homogeneous sauce. The sauce even can be cut into processed cheese-like slices once it cools.
When making cheese sauce, we add 4 grams of sodium citrate for every 100 grams of finely grated cheese and 93 grams of water or milk. To make cheese slices, we reduce the amount of water to about 30 grams (cold wheat beer works very well, too), pour the melted mixture into a sheet pan, and let it solidify in the refrigerator for about two hours before cutting it into pieces, which then can be wrapped in plastic and frozen.
Because this method of stabilizing melted cheese bypasses all of the flour, butter and milk used in Mornay sauce, the resulting cheese sauce is much richer; a little goes a long way. But the sauce keeps well in the refrigerator and reheats nicely in the microwave, so save any extra and use it to top vegetables, nachos or pasta.
MODERNIST MAC AND CHEESE
We've offered both weight and volume measurements for this recipe. But as with most modernist recipes, a digital scale is best. Sodium citrate is widely available online. Feel free to substitute an equal amount of your favorite cheeses in this recipe. If you have an immersion blender, you can use it to blend the cheese sauce instead of transferring it to a food processor. But this can cause splattering, so do so with care.