Why you can't reverse cheese curdling in fondue? I was wondering why you can't "fix" or reverse a "curdled" cheese fondue sauce?

As some of you know - I am taking a scientific approach to cooking and am now fiddling with cheese fondue.

I got three small saucepans, poured some wine to simmer and tried adding in different cheeses to see which would melt in a fondue-favourably fashion, while stirring constantly.

I'm surprised to say two of the three cheeses melted with no problem while the third - Emmentaler, no less, became a stringy, lumpy mess from the start.
Well, since the same happened to the other cheeses the minute I took the saucepans off the heat I thought the reverse reaction should be possible if I return them to the heat, but it did not.

Irreversible reactions aren't something new or strange to me, but I wondered what IS it in this cheese-wine sauce that would cause such an irreversible reaction?
Most proteins undergo irreversible structural changes when heated. The exact temperature where this happens depends on the protein. In most cases, no covalent bonds are broken or formed during this 'denaturation' process.
But the irreversible nature of the changes means that there's no good way to rescue a curdled fondue sauce (or a curdled egg custard).

Other components present in the environment of the protein (like alcohol, salt) can influence the denaturation process.

In your case, when you first get the wine to simmer, your temperature might just be too high from the start to prevent denaturation of the cheese proteins, suggested temperatures for melting cheese are around 150F (~66°C). And of course, different cheeses have had different treatments during production (some are already heated to 50-55°C), and can have different behaviour when heated. And duration plays a role (the denaturation isn't instantanuous).