I heard that if I put the curry powder early, the cooking heat will destroy the flavour, and that it should be put at the end, before turning off the heat
but I can't find anywhere on the internet confirmation for this statement.
is that true ? when is the best time to put the curry powder ?
None of these is categorically true. Adding it early or late will give different flavor, adding it at the very end of cooking will usually leave you with an unpleasant raw spice taste unless it is a roasted variety of curry powder.
It does make a big difference whether the powder is added into oil, water, or an emulsion.
For the most "complete" flavor, add it twice - part goes in the oil when your aromatics are mostly done, before adding bulk liquids; part goes in a few minutes before taking the dish off the heat (optionally bloomed in oil).
Most spices in curry powder are oil soluable, not just in oil, but oil that you are actively heating, ie: frying. the trick is not to burn your spices. You will need to place your curry powder in with your browned onions, on a slightly lowered heat, ensuring that you have sufficient oil for the job in hand. If your spices start to clump or are cooking too uickly, use small amounts of water and continue to cook, the water will soon evaporate. If you add your powder at the end of your cooking process it will result in a bitter, grainy and unpalatable product.
I'd say "it depends". It depends on the time and temperature.
Usually you use high heat to brown your food and then you use a lower heat to finish cooking. Add the curry when you lower the heat to avoid burning the curry.
It's probably similar to how you would treat saffron
