Sodium citrate may sound unfamiliar and possibly suspect, but it’s basically just a form of salt that works as an emulsifier. More plainly stated, it’s an agent that reduces the cheese’s acidity, makes the proteins in the cheese more soluble, and prevents it from separating into a greasy mess; instead creating a smooth, creamy texture that will never “break.” You probably won’t find it at a normal grocery store, but it’s available in specialty food stores and online. It looks like salt and tastes slightly sour (and of course salty), and you’ve probably tasted it before in club soda. Just the tiniest sprinkle of it will transform an entire block of cheese into a submissive puddle of its former self, so there’s little need to be concerned that it’ll increase the sodium level of your cheese sauce. (And if you’re that concerned about your sodium intake, you’re unlikely to eat a heaping plate full of delicious nachos anyway.)
A Mornay sauce is a classic French cheese sauce based on Bechamel. The butter (emulsifier) and flour (binding agent) react with the cheese in order to work within a certain range of temperature to get that creamy goodness. It is a great sauce but even under the most watchful eye, it can “break” (meaning when the oil separates) – rendering the sauce useless.
Cheeses have different melting temperatures, and overheating cheese leads to the “breaking”. So the key is to heat cheese to its melting point, without going too far. To make a sauce out of a cheese that is difficult to work with (harder cheeses, like Parmesan or Romano)? Add a little sodium citrate (E331) and a little liquid (water, beer if you want to add flavour) chemically emulsifies the cheese into a sauce that does not break. It’s a foolproof way to nail cheese sauce, without resorting to Cheeze Whiz/etc.