What causes curding in this case: heating milk with large quantities of sugar

Question

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.

Answer 1

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.

Answer 2

It might not be the sugar curdling the milk, but just the milk curdling itself. SourDoh mentioned the possibilities linked to the lower amounts of fat, but there's also the possibility of the milk's age playing a role - fresher milk may well solve the problem.

When milk curdles when heated, it is usually linked to acids, or spoilage, but it can happen even in otherwise good, plain milk if that milk is on the older side. This milk is often fine for cold applications, not spoiled or anything, it just can't be heated without separating at this stage - so no good for tea or coffee.

I think it's a bit more likely to happen with older milk, not gone bad but getting near that point and at the end of its shelf life... milk slowly gets more acidic as it ages and bacteria work on it, and there's a point where it hasn't spoiled, but will fairly soon, that can be acidic enough to curdle itself when heated.

You can check if it's the milk by heating a bit by itself, in a pan or even the microwave, to see if the milk proteins clump up.

Answer 3

Salt!!

my caramel started curdling and I couldnt figure it out, then it hit me - I had started adding salt earlier, for ease. I held off on salt til the sauce was done, and boom!, bob's your uncle. lol

From the web: Salt is another ingredient that can cause milk to curdle. But​, obviously, you need to season your sauce. The key is to add the salt at the end, rather than cooking or reducing it with the salt already in it. Apr 4, 2017

Answer 4

I own a restaurant and make tons of homemade chai with brown sugar. For the last 3 weeks we have had problems with the milk curdling (all of a sudden). After much experimentation, we discovered it was the brown sugar. More specifically, the molasses in the brown sugar. Any acid will cause milk to curdle. For example, cheese can be made by adding a few drops of lemon juice to milk. Bleach left over from washing can have the same effect.

In our case, we have used this same recipe for our chai for 15 years now. So why all of a sudden did it start to curdle, I wonder. So: 1. We have definitely pin-pointed it to the brown sugar. 2: In products like Domino's brown sugar, I BELIEVE the molasses is simply added back into refined granulates sugar. If this is the case, perhaps the added-back molasses has changed. 3: I know city water acidity changes. We triple-filter our water before using it for cooking/coffee etc, but perhaps it is more acid and then the added acidity of the molasses puts it over the top. In our case, should I switch to confectioners sugar for the chai, there was no curdling.

Answer 5

It is the amino acids in the molasses that is in the brown sugar that will make milk curdle when it comes to a boil. Now maybe light brown sugar would be less likely to curdle because there is less molasses in it?