Sometimes nothing will do but a crispy, savoury, oozy toastie. But which are the best cheeses to use? Well, science can help you out…
First of all, to get the gooey centre, you want a cheese (or combination of cheeses – let’s go all out here) that will give a ‘good melt’. There are some cheeses that would definitely not be good for this, particularly the acid-coagulated cheeses like paneer, ricotta and cottage cheese (see my post on why they don’t melt well).
The most important factor in melting is water content. High water content in the cheese dilutes the proteins that make it solid, meaning they are more weakly bonded together and so will flow past each other and melt at a lower temperature. Moist cheeses like fontina, gruyere and monterey jack are therefore good to provide the gooiness.
Watch out for using high fat cheeses – they are more likely to leak melted fat as the protein network breaks down with heat, and can make the toastie greasy. Though to be fair, that is often part of the enjoyment. It’s not a health food.
Now to stringiness. For some, another important aspect of a toastie. The reason a cheese goes stringy is that the little balls of milk protein (called casein micelles) in the cheese get linked up into long chains by calcium.
Cheeses tend to get less stringy with age. The first reason for this is that as a cheese ages, more and more lactose sugar is converted to lactic acid. Acid dissolves the calcium holding the micelles together, stopping them from forming stringy ropes and making them fall apart and flow more easily. The second reason is that as a cheese ages enzymes start to break down the micelles into small pieces that are too small to link up into chains.
So if you’re a string fan, then cheeses like emmental, mozzarella and young-ish cheddar are good. And if you prefer just a gooey centre, just use the moist cheeses above, and maybe an aged cheddar too.