I guess the switch statement does not what you expect it to do (you must hit exactly 60°F to reach your while loop) and the while inside is absolutely not necessary anyway. Writing the acRelay pin to HIGH a few thousand times a second doesn't help, it's at that state after the first call.
I think what you really need is something like this:
void loop() {
int grainTemp = thermocouple.readFahrenheit();
Serial.print("Grain Temp: ");
Serial.println(thermocouple.readFahrenheit());
if (grainTemp >= 60) {
digitalWrite(acRelay, HIGH);
} else {
digitalWrite(acRelay, LOW);
}
delay(1000);
}
That still has potential for improvements (p.e. switching the relay off only if the temperature is below 55°F to not continuously switching the relay if the temperature is around 60°F) but it probably does what you expected your sketch to do.