What causes curding in this case: heating milk with large quantities of sugar Yesterday I decided to try to make dulce de leche or caramel. I did not follow a recipe: I was as curious to see what would happen as I was hungry for sweet, caramelised, milky sugar.

I mixed semi-skimmed milk with a large quantity of white sugar in a saucepan and heated gently. The ratio was roughly 1:1 by volume.

Within minutes (and long before the mixture was at all hot) smooth, silky shapes began to form in the mixture, which at this point was still roughly half milk and half undissolved sugar.

The silky shapes soon coagulated and grew harder and chewier over time. The milk had obviously curdled.

I haven't come across any other references to sugar curdling milk in this way. I have only seen it referenced as a 'stabiliser'.

I assume that the curdling was made more likely by my milk's low fat content. I was not stirring constantly. But I'm not looking for a fix, just for understanding. I didn't really want to get it 'right'.

So, could someone please explain what most likely happened here? Is it simply that milk heated with a high enough proportion of anything that dissolves in water will separate and curdle within minutes? Or is is something specifically to do with sugar?

Specifically, what was it about the sugar that I added that caused the milk to curdle (the amount? something else?), and what is the nature of the curdling process? (The second question has been answered elsewhere, but it may be relevant in answering the first, which has not been asked elsewhere).

Thank you for any advice. I know that next time, when I follow a recipe and want to get it absolutely right, I'll have a better understanding of what's going on, thanks to your answers.

P.S. In the end I strained the mixtures, reserving the very sweet solids, and continued to reduce the sugar/milk liquid mixture until I ended up with something very similar in taste, colour and consistency to condensed milk.
It's not the sugar that caused the milk to curdle, it's the milk itself. Dulce de leche and caramel are both usually made with either cream, condensed, or evaporated milk. The issue with regular milk (especially skim), is that it has such a high water content and low fat content. The fat in cream buffers the protein, helping to prevent curdling, and the canned milks have already had most of the water removed, making them less likely to curdle. 

The sugar could also have a buffering effect on the milk protein, unfortunately you have a catch 22. In order to buffer the milk form curdling, the sugar would have to be dissolved in the milk. In order to dissolve sugar in the milk at a 1:1 ratio, the milk would have to be heated past the point where it would curdle.

You might be able to prevent this by using a double boiler until the milk has reduced a lot. Making a caramel requires almost all of the water to be cooked out of your syrup, so this will take a really long time and isn't advisable.