You can definitely cold crash mead without an airlock. In fact, once the fermentation process is complete, there is no longer a need for an airlock as the primary purpose of the airlock is to allow the release of carbon dioxide during active fermentation and prevent oxygen from entering the fermenter.
When it comes to cold crashing, the goal is to clarify and stabilize the mead by lowering the temperature and encouraging the yeast and other particles to settle at the bottom of the fermenter. Cold crashing is typically done after fermentation is complete and the desired level of sweetness and flavor has been achieved.
To seal the fermenter for cold crashing, you can use a solid stopper instead of an airlock. This helps to maintain a closed environment and prevent any potential contamination from outside air. The stopper should fit tightly and create a good seal to keep the mead free from any unwanted oxygen exposure.
It's important to note that the temperatures at which you cold crash are generally inhibitive for yeast activity, unless you are lagering your mead. Most mead recipes recommend cold crashing at temperatures below 50°F (10°C), which is below the ideal temperature range for yeast fermentation. This means that the yeast will remain dormant during cold crashing, and any remaining fermentation activity should not be a concern.
I personally seal my fermenter with a solid stopper before cold crashing and have never experienced any issues. The mead clarifies beautifully, and the settled yeast and particles can be easily left behind when transferring the clarified mead to another vessel or for bottling.
Cold crashing mead without an airlock is absolutely possible and commonly done. By sealing the fermenter with a solid stopper, you can create a closed environment and allow the mead to clarify and stabilize without the need for an airlock. Just ensure that fermentation is complete before cold crashing, and choose a temperature below the yeast's activity range to prevent any unwanted fermentation during this process.