Exploring the Science of Sublimation

This post is brought to you through the kindness of Mercer Foods.  Mercer Foods are manufacturers of wholesale frozen fruits.  They are a wonderful company and product a great product.  Now on to the post…

 

One of the fascinating physical properties of freeze-drying is a process called “sublimation”, a little-known but important process that allows freeze-drying to happen. Most people don’t walk around seeing sublimation in action, but this unique “phase change” where water changes from ice to vapor deserves a little time in the spotlight, as it’s the basis for all of Mercer Foods’ offerings.

Our freeze-drying process produces superior products that maintain their nutritional value from the moment they are frozen, and it wouldn’t be possible without the help of sublimation. Try not to evaporate in excitement as we take you through the science, as well as how it fits into our process.

The basic states of matter

First, some physics class terminology. Everything around us consists of tiny particles in five states of matter: solids, liquids, gases, plasmas, and Bose-Einstein condensates. Forget about those last two because they are way outside the realm of freeze-drying — we will focus on the first three. You are probably familiar with all sorts of different solids, liquids and gases, so let us move on to phase changes. A phase change is when a state of matter transitions to another state of matter.

For example, water turning into ice is an example of freezing. This seems pretty basic, right? The other phase changes are evaporation, condensation, melting, sublimation, and deposition. The only two you are probably not familiar with are sublimation and deposition. Sublimation is the phase change where a solid turns directly into a gas without ever melting to a liquid. Deposition is the reverse, a direct phase change from a gas to a solid. Deposition is rarely seen in the natural world, but the manufacturing of dry ice directly from carbon dioxide gas is a common example. The reverse of this process, when dry ice evaporates directly to carbon dioxide gas, is the most well-known example of sublimation.

Sublimation in freeze-drying

Now, meet another example of sublimation: freeze-drying!

One of the lesser-known qualities of water is that it doesn’t exist as a liquid in the presence of a vacuum. In a vacuum, ice “sublimates” directly from solid to vapor at room temperature without melting. This process is hugely beneficial in freeze-drying because it allows frozen foods to be dried directly by vaporizing all water without ever having the water melt. It also avoids a common problem with melting frozen fruits and vegetables — the breakdown of cell structure into a mushy product. Most people have defrosted a bowl of strawberries and been greeted with this. Freeze-drying skips the melting phase and removes frozen water from frozen food in gaseous form, leaving behind perfectly preserved food.

Original post January 20th, 2017

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