Assuming the condition has to be checked I dont know how to tell the arduino to check for that condition for the while statement).
int grainTemp = thermocouple.readFahrenheit();
if(grainTemp >= 60)
{
// Turn the AC on
}
while(thermocouple.readFahrenheit() >= 60)
{
// Do nothing until the grain cools down
}
// Turn the AC off
But, you can see that this is blocking code. It doesn't need to be.
On every pass through loop(), read the temperature. If the value is greater than or equal to 60, turn the AC on. If it is below 60, turn it off. loop() can spin millions of times doing nothing while the AC runs. Or, it can be doing other things while the AC runs.