Cooking spice pastes for a longer duration generally mellows out the initial sharp heat and allows deeper, more complex flavors to develop. The volatile compounds responsible for the initial 'burn' dissipate, while other flavor compounds meld together.
When you initially add a spice paste to hot oil or another cooking medium, the high heat releases the volatile oils and compounds that contribute to the initial spiciness. These compounds, like capsaicin in chili peppers, are responsible for the immediate sensation of heat. However, these compounds are also relatively unstable and break down over time with prolonged exposure to heat.
As the paste cooks longer, these volatile compounds evaporate or degrade, reducing the intensity of the initial heat. Simultaneously, the longer cooking time allows other flavor compounds within the spices to develop and meld together. This process, known as Maillard reaction and caramelization, creates deeper, richer, and more complex flavors. The result is a spice paste that is less aggressively spicy but more nuanced and flavorful overall. The exact change in spice level depends on the specific spices used in the paste and the duration of cooking. Some spices, like garlic and ginger, can become sweeter and milder with prolonged cooking, while others, like cumin and coriander, can develop more earthy and nutty notes.
If you want to retain a brighter, sharper spice flavor, add a portion of the spice paste towards the end of the cooking process. This will provide a burst of fresh spice alongside the deeper, cooked flavors from the paste added earlier.