Oil separating from fried onion/spice mixture, why does it happen?

Question

I was always taught when cooking Indian dishes to fry spices in a chopped/pureed onion and garlic mixture to release their flavour and essential oils.

I was also taught that when the oil separates from the mixture, the spices are done and you can carry on with the next stage of the dish.

That's the principle technique I've always used.

Can someone please explain why the oil separates and what this signifies? Have the spices absorbed as much of the oil as they can and release the rest?

I'd really like to understand this process.

Answer 1

When you cook a vegetable, such as a cut-up onion, it will release water. The water initially will create an emulsion with the oil in the pan, so you won't see them as clearly separated elements, but it will also be evaporating. When enough (perhaps all) of the water has dissapated, the emulsion breaks and you see the oil separate from the rest of the ingredients.

The breaking of the emulsion doesn't tell you anything in particular about the doneness of the vegetables, nor about the extraction of the spices' oils into your cooking fat, since the time it takes for the water to evaporate will depend on the strength of the burner, the geometry of the pan, and the amount of onion and other vegetable that is present.

If you are always using the same stove, pan, amount and cut of onion, and amount of spices, then you will be able to guage the readiness of the mixture from cues like the water having evaporated. In general, though, it would be best to evaluate the mixture's flavor by tasting it.

Answer 2

@JoshCaswell's answer is right on the money, you see the oil because the water has evaporated. That actually has nothing to do with the spices being "done". Spices don't cook so they can't be done or not done, the question is whether or not they have released their flavors, and whether there is water in the pan or not is no indication either way.

The water from the onion mix is what is keeping it all from burning, so when the water has evaporated you need to move onto the next step whether the spices have released all their flavors or not, otherwise the mix will burn and stick in the pan. I use a similar method for most of my curries and if I'm not ready for the next step at this point I'll add a small amount of water to keep the mix moist.

Curry spices are all actually oils trapped in bark, pods, seeds, roots, and the like. Because they are oils they will mix with, and be released into oil which is why you get much better results frying the dry spices off at the beginning, and adding more spices to the sauce later never gives you the same punch.