Imagine your favorite cheese: perhaps an aged, sharp cheddar, or maybe a blue Gorgonzola or a gentle Monterey Jack. Wouldn't it be wonderful to use those really good cheeses you love on nachos or as a sauce on macaroni or steamed vegetables?
But if you have ever tried melting high-quality cheeses, you've experienced the problem: the cheese separates into a greasy oil slick that no amount of stirring will restore.
One traditional workaround is to a Mornay sauce, which combines the cheese with a cooked mixture of flour, butter and milk. But a Mornay sauce can taste as much of cooked flour as it does of cheese. The starch in the flour masks some cheese flavors, so the sauce loses vibrancy.
James L. Kraft discovered a much better solution around 1912. He found that adding a small amount of sodium phosphate to the cheese as it melted kept it from turning into a clumpy mess of cheese solids swimming in a pool of oil. Kraft made canned, shelf-stable cheese and the technique led to processed cheese products.
You can apply the same chemistry to achieve much higher culinary purposes. The chefs in our research kitchen have made mac and cheese with an intense goat Gouda and cheddar sauce, for example, and build gourmet grilled cheese sandwiches using cheese slices that melt like the processed stuff, but are made from feta or Stilton.
In place of sodium phosphate, we use sodium citrate, which like sodium phosphate is an emulsifying salt that helps tie together the immiscible components of cheese: oil and water.
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, that net breaks apart, and the oil and water tend to go their separate ways. Sodium citrate can form attachments to fat and water molecules, so it holds everything together. The end result is a smooth, homogeneous sauce.