Gently stir the curds for 30 seconds every 5 minutes or so as the temperature of the curds slowly rises. When the curds reach 100°F, increase the heat under the canning kettle until the curds reach 120°F. Hold the curds at this temperature for 25 to 30 minutes, stirring more vigorously every 5 minutes. Most of the curds will be firm now. You can squeeze a few curds to see if they are still soft in the center. (A little soft is OK; runny is not.) If curds are not firm enough, continue to hold at 120°F for an additional 5 minutes.