What Are The White Bubbles In My Freeze Dried Food?

Have you ever wondered why there are bubbles in your freeze-dried products? Whether it's ice cream, orange slices, or grapes, you might have noticed that sometimes a fraction of the pieces on the tray have bubbles. In some cases, such as when freeze-drying oranges, you might see all pieces covered in tiny bubbles. You might guess that the cause is sugar, and you'd be right!

But why does this happen?

Formation of White Bubbles

Because freeze drying involves freezing food at enormously low temperatures before a vacuum pump lowers the pressure, water sublimates directly from solid to gas and causes natural sugars in freeze-dried foods to rise to the surface. As the ice sublimates, water-dissolved sugars are left behind and crystallize on the surface.

For example, crystalline white bubbles in freeze-dried strawberries or tomatoes contain concentrated sugars. It doesn't affect the nutritional value but may change texture and appearance. Knowing which foods display these traits helps manage expectations.

Case Study: Ice Cream Sandwiches

Additional Factors Contributing to Bubbling in Ice Cream

Ice cream must be completely frozen to avoid bubbling during freeze-drying. When ice cream is not entirely frozen, the residual liquid can expand when the vacuum pump lowers the pressure. Residual liquid may form bubbles as it swiftly adjusts to the decreased pressure. Different ice cream brands with different sugar and fat amounts may react variably, causing bubbling as well. Another factor is the ice cream surface area. Smaller chunks freeze more evenly to minimize pressure-induced bubbling. 

Consequences of Improper Freezing

Poorly frozen ice cream expands and bubbles when vacuum pump kicks on. Consequently, it produces soft, fluffy bubbles instead of freeze-dried ice cream. These bubbles grow so bigs the entire shelving unit and the drum become a mess to clean. Appropriate freezing and pre-cooling are necessary to minimize these issues and guarantee freeze-dryer efficiency and life.

Prevention and Best Practices

Tips for Preventing White Bubbles

1) Make sure all freeze-dried foods are fully pre-frozen to avoid white bubbles. They stay solid throughout sublimation and decrease natural sugar surface crystallization.

2) Slicing fruits uniformly, and putting similar amount of fruits on each tray facilitates even freezing and sublimation. 

4) Set a lower temperature for drying, e.g. -10F instead of -5F to start drying. A lower food temperature ensure they are frozen before drying starts. 

5) Set a lower target vacuum pressure, e.g., 450mtorr instead 550mtorr.  A lower target vacuum pressure results in a better quality of freeze dried food, and reduces occurrence of white bubbles. 

So, if freeze-dried foods have white bubbles, don't worry; follow best practices for ideal outcomes.