im not sure but i think while cooking rice after it starts boiling its temp will not increase beyond 100 degrees it will remain constant at that temp
suppose you want to keep it at a temp lower than this or if you want to lower the intensity of the heating system when it reaches this temp you can do the foll.
you can easily devise a circuit using the easily available thermistors
its resistance varies a lot with temp
use it in series with a fixed resitance
hence its voltage varies with temp
find the voltage for the reqd temp and set the heating system to go off at this voltage
u can use a op-amp comparator for this
use a timer to switch off the heating system after a certain amt of time.
Added after 18 minutes:
hey use this ckt instead