Carbohydrate is the nutrient that changes its composition during cooking that gives natural sweetness to the food that you eat. Carbohydrates are composed of starchy and dairy materials that naturally secrete sucrose or sugar in certain amounts. When cooked, these sucrose are secreted and the food gets it naturally sweet taste that is also pleasant to the sense of smell. This is the reason why when you smell food rich in carbohydrates, your brain signals you to eat and you get excited.