Have you ever taken trays out of your freeze dryer and noticed a lot of white crystalized bubbles coat your food? Or have you ever freeze-dried ice cream sandwiches and end up with a bubbly mess? This is common and there is nothing wrong with your freeze dryer. What is happening is just lyophilization, or the science that is freeze drying.
You usually find the white crystalized bubbles on foods that contain a lot of natural sugars. For example, fruits often experience this. During the freeze-drying process, food is first frozen to extremely low temperatures. Once completely frozen, a vacuum pump kicks on to lower the pressure. This causes sublimation to occur. The frozen water in the food is removed from the food, going directly from a solid to a gas state. During this process, some of the crystalized natural sugars are also removed to the surface of the food, where they remain until the freeze drying is complete.
While this same process can also cause ice cream sandwiches to have some bubbling (especially depending on the brand of ice cream you use) there is something else that can cause ice cream sandwich mishaps. A common mistake people make is putting ice cream into a unit that has not been completely pre-frozen. If ice cream is not completely frozen (or any liquid for that matter) when the vacuum kicks on to start the drying process, the change in pressure can cause the liquids to expand – making a mess.
While this doesn’t hurt your freeze dryer or is any sign of problems with your unit, it can be a pain to clean. Ice cream often causes problems when placed into a unit that is not pre-cooled since the ice cream will actually be slightly melting for part of what should be the freezing stage. This makes it hard for the unit to not recognize that the ice cream is completely frozen and the pump will kick on. Then, when you open your door, you will find super light and fluffy ice cream bubbles instead of the fun, crunchy ice cream sandwiches that you wanted.