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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.

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What is semi-skimmed milk? –  SAJ14SAJ Sep 18 '13 at 12:23
    
The English name for reduced fat milk. –  Tom Robinson Sep 18 '13 at 12:26
    
From "condensed milk" to "dulce de leche" you only have to heat, wait and stir –  belisarius Sep 18 '13 at 17:20

2 Answers 2

up vote 4 down vote accepted

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.

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Thanks for your answer, but I suspect there's something else going on here. The milk had begun to separate quite dramatically when it was only warm to the touch. I've heated and boiled milk before without it curdling...so either the sugar is having an unusual effect or some other variable we don't know is causing the separation...right? –  Tom Robinson Sep 18 '13 at 21:44
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What kind of pot was it in? It's possible that the bottom of the pot was hot enough to curdle the milk before the rest of the milk was heated. –  sourd'oh Sep 18 '13 at 21:47
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I could see brown sugar being acidic enough to make heated milk curdle, but white sugar has a neutral pH so there isn't really anything but the heat to make it curdle. –  sourd'oh Sep 18 '13 at 21:59
    
That may well be it. I used a quite heavy cast iron pot. –  Tom Robinson Sep 23 '13 at 8:16
    
Thanks a lot for your comments, sourd'oh. I'll try again some time, with fresh, whole milk or cream, and perhaps a recipe. Your points were really helpful! –  Tom Robinson Sep 23 '13 at 8:21

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.

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The question mentions that white sugar was used so I can't imagine this being much help. –  Chris Steinbach Jun 13 at 20:09

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