A lot of Indian curry recipes have a step where you're told to cook an onion-tomato-spices mixture "until the oil separates". Despite having tried cooking such recipes a number of times already, I still haven't really figured out what is meant by this. I have several questions:
How can I tell that the oil is separating? I'm never quite sure whether I'm seeing oil or water coming out of the mixture while it's cooking.
How long on average do you need to cook the mixture until the oil separates?
What causes the oil to separate? Is it simply that all the water has been cooked out of the mixture?
Oil and water have different boiling points. Oil has a higher boiling point as compared to water.Spices and aromatic release their flavors only in oil because the compounds in them that are responsible for aroma/flavor are oil soluble.However, they can burn easily in very hot oil.
Most Indian recipes require that they are cooked in a mixture of water and oil (typically onion + tomato or in some regions, coconut or sesame seeds). Water evaporates, raising the temp of the mix slowly and spices, vegetables, meat etc. release their flavors slowly. When the oil separates/is visible, the temp is the highest. Depending on how you want spices to release flavor, you add them before or after oil separates. These practices vary from recipe to recipe and region to region in India as the oils, spices and other ingredients are varied in nature. Hope that was useful.
Nadia is right. Use your nose and ears too. After a while the sauce will lose its watery appearance and the rawer flavours in the air emanating from the pan will begin to be repalced with more moorish aromas and a greater sizzling. By these stages you'll see angry looking bubbles of oil around the suace and need to keep stirring. I tend to stir a little longer to prevent sticking while getting my (meat/stock/lentils - whichever- etc) ready to pour in.
I find the stage is easy to identify by colour in the case of Tomatoey sauces. these tend to develop a browner appearance by the oil-separating stage.
Oniony-garlicky-gingery mush sauces lose their raw aroma and then their wateriness and start to develop the oil bubbles after a while - about 10-15 as Nadia says. Some great meat curries add the meat and dry spices to the still cooking onion and it gets a refry from the maet juices until drying again. When its really sizzling and sticking to the pan in goes the water, stir until boiling, and on goes the lid for slow cooking
Tomatoes contain insipid ( raw) tasting glycoalkaloid saponins like tomatine which are found in highest concentration around the seed pulp. These emulsifiers create an emulsion (colloidal mixture) with the cooking oil/fat, water and sugars ( from onions/garlic and tomatoes themselves) at low temperatures, but when properly cooked these emulsions break down and the fats/oils separate out due to the thermal degradation of the saponins. Its these saponins that contribute to the perceived 'raw' taste and the visual separation of oils and tomato base indicates significant degradation of the saponins and hence the completion of the cooking stage to achieve desired taste/texture. At this stage most of the fat soluble carotenoids/lycopene from tomatoes and organo-sulfonic acids in onions have been dissolved into the oil fraction and are more bioavailable for assimilation, and more free savory tasting glutamates are released into the cooking medium from breakdown of small amounts of proteins present in the original mixture .
In my experience most recipes call for too much water and therefore will take a very long time to evaporate. Just add enough water to make a creamy sauce (or barely cover the meat)and if the meat is not cooked add more water a little at a time, even if it is in a casserole pan with the lid on top, check it every 15 minutes. In a casserole with a lid on top it retains a lot of water but cooks the meat which I find with a kilo of meat it takes approx 2 hours at 140 degrees centigrade. You can finish it on the stove top on an open flame.
Regards ... Mike