Food particles diffuse faster in air when hot and we smell it immediately even we are far . food particles diffuse slower when cold ,so it reaches us slowly or we have to go close .
When we heat up something the particles of any kind of matter except BEC Gets heated up and the kinetic energy increases very much. Due the increase in kinetic energy the particles move vigorously and Spread quickly.In this case its gaseous matter. We already know that gas hv very high level of kinetic energy so cold food also reaches our nose but only when we get close. Once the food is heated up the kinetic energy increases and the smell spreads throughout everywhere