Why can't whole milk be substituted for cream in caramel sauce? I learned to make caramel sauce pretty recently and for a while was on a serious sauce-making kick.

Anyway, the day after Christmas I went to make a batch and noticed I didn't have any cream. I did a quick Google search and a few sites said I could substitute whole milk for the heavy cream in my recipe. When I got to that stage and added the milk it very quickly became apparent that those sites were wrong. Instead of thickening and becoming creamy the melted sugar simply dissolved away and I was left with a really nasty smelling pot of sweet warm milk.

So obviously I can't use whole milk instead of cream, but I'd really like to know, for curiosity's sake, why it doesn't work.
Quite simply, it's the fat content.

Whole milk or "full-fat" milk is 3.25% fat by weight. Heavy cream is 36-40% fat by weight. These two products are at opposite ends of the fat spectrum, and there's very little difference between 1% and 3% when it comes to an item such as caramel sauce, for which the optimal ratio is about 50% fat. (A little butter can boost the fat content from 40% to 50%).

You might be able to substitute standard/single cream (18-20%) or maybe even coffee cream/half-and-half (10%), but any lower than that and you're just making sugary milk.

Other alternatives to (possibly) get it thicker:


Use (much) less milk; I'd advise not attempting a direct substitution, just find a recipe based on milk. Even the best milk-based caramel sauce will still be substantially runnier and/or grittier than a cream-based sauce.
Considering that butter is 80% fat and homo milk is 3.25%, you could use a mixture of (approximately) half milk and half butter that would emulate the fat content of heavy cream. I've never tried this personally, and I suspect that the flavour might be a little off, but at least it would be closer to the expected texture.
Try a reduction (simmer off the water in the milk). You'll be simmering a long time, and you'll have to watch it very carefully to make sure it doesn't burn, and you'll probably have a nasty stuck-on mess to clean up in the pan afterward, but it will thicken.