The drying cycle comes after vacuum freeze and this is where the majority of the sublimation takes place. As long as the vacuum pressure is in a low enough range, the heating elements will turn on. Applying the heat to the shelves increases the rate of sublimation which causes lots of moisture to be removed from your food as a gas, re-freezing to the chamber walls which stay cold throughout the entire process.

During the drying cycle, the temperature will fluctuate a lot. Don’t be alarmed if you see temperatures in the single digits at times as well as temperatures in the triple digits at times (depending on what your tray temperature limit is set to). Because applying heat increases the amount of water being released from the food, the freeze dryer will turn the heat on and off throughout the drying process to ensure that the water is released from the food at a manageable rate for the vacuum pump to keep up with, so that the water vapor can freeze to the chamber walls without getting sucked into the pump.

This is why foods with high water content take longer to dry – the heat is applied in increments according to how well the vacuum pressure is keeping up and how low the mTorr is getting at that time. If the vacuum pressure is not low enough, the heat will not trigger to turn on. If the vacuum pressure IS low enough and the heat is not coming on, this will cause the ‘Mid Batch Heater’ failure to trigger, which often means that you need a new relay board. Assuming no errors, the heat will cycle on and off over and over again until the heat can stay on without the vacuum pressure rising. When this happens, the freeze dryer detects that there is no more or very little moisture being released from the food, signaling that the food is almost dry.