Creamy sauces can be lovely, rich and decadent when executed well, unfortunately, the path to creamy perfection can be difficult to navigate. I find that ensuring the sauce maintains a consistent creaminess without the sauce breaking is probably the most common pitfall I’m weary of. In my preparation of the foie gras sauce in this dish, I found that upon serving, the sauce was certainly not as homogeneous as I’d hoped. So below, I’ll share with you some of the conclusions I’ve found from research and (little) experience.
You’ll find that the most common reasons for a sauce breaking can be narrowed down to the below:
1. Too much fat; a creamy sauce is basically an emulsion, which is a mixture of two different liquid components that normally don’t mix, and so by nature is a bit unstable. Having too much fat (butter, oil etc) will reduce the sauce’s ability to sustain itself as an emulsion. The same applies to adding the fat too quickly. The fat should be added and incorporated in small steps to help ensure the stability of the mixture.
2. Using half cream or light cream; for some reason, I find that it’s necessary to use heavy (double) cream in sauces. A light cream is much more prone to break (I think because the cream here is the emulsifier, and so needs to be thick to carry the fat)
3. Sauce gets too hot; letting your cream sauce boil for too long will certainly lead to it breaking sooner or later.
4. Sauce gets too cold; if you refrigerate a sauce overnight, chances are it’s going to break when reheated.
5. Not serving the sauce straight away (and this is what happened to me here); because of the inherent instability of an emulsion, leaving it warming for too long will lead to the two liquids separating again.
Here’s a very useful link http://www.thekitchn.com/food-science-why-did-my-sauce-46045