Have you ever noticed that, delicious as pizza made with fresh chevre can be, it's always going to be the weird alternative for those of us who have cow-milk allergies? In between learning I had a cow-milk allergy and learning I could happily consume buffalo-milk mozzarella, I ate a lot of goat cheese pizza. It was always missing something. And the thing that is missing, my friends, is the stretch.
What Makes Cheese Stretchy (or Not)?
The step in cheese-making that turns boring milk into fantastic cheese is curdling, the process which gave Little Miss Muffett her favorite pastime. As I believe I've mentioned before, not counting the unusual properties of some thistles ("Creamy Thistle-Rennet Cheeses"), there are essentially two ways to set curd: rennet and acid. And right there is the critical choice for whether your cheese is going to melt or not.
Rennet's active ingredient is an enzyme called chymosin (also sometimes called rennin). Chymosin, like many enzymes, catalyzes reactions on proteins, specifically casein, the most abundant protein in milk.
In particular, chymosin causes the cleavage of the part of the casein bundles (called micelles, which is a more general biochemical term for the grouping that casein forms) that causes the bundle to repel other casein bundles. The result is that the micelles stop avoiding each other, and form long chains, which are capable of stretching and melting when heated. Some cheeses that are meant to be stringy, like mozzarella, are made in a fashion that enhances these chains, by kneading and stretching the newly formed curds into alignment.
Acid, usually in the form of lemon juice or vinegar (or in large commercial operations, citric acid), works in a much less elegant fashion. In general, acids work to denature proteins—that is, cause the proteins to lose their shape and just sort of stretch out. Thus, the casein micelles deaggregate and form loose, fragile networks of denatured proteins. This new network is fragile and random, not directional like the micelle chains formed by chymosin. Unfortunately, when heated, there is no real way for these networks to stretch, so the curds (and the final cheese) brown when heated, rather than melting and stretching.
So next time you want to try an alternative topping to pizza, think long and hard before you reach for the chevre. There's nothing wrong with it, but it just won't be the same. If you want the stretch, look for cheese that was curdled with rennet, not acid. Also, seriously, try buffalo mozzarella. That stuff is amazing.