Freeze Drying Process

A look into how Freeze Dried Fruits and Vegetables is done

Freeze Drying is a process that allows us to remove all water from the fruits and vegetables preserving them and allowing them to be easily transported. The main visual difference you will notice between these two processes is that when a product is Freeze Dried it will still look about the same unlike when you dehydrate a product and it shrinks up. This is due to the process of Freeze Drying. There are four steps in this process and if done correctly the end result will look similar to the original products and hold the same nutrients.

4 Steps in the Freeze Drying process are: Pretreatment, Freezing, Primary Drying, and Secondary Drying.

1. The first step is to preserve appearance, stabilization, vapor pressure and concentration.
2. Freezing is the most important step. This is where you are able to preserve the most nutrients, color, and cell structure. First you place the product in a freeze-drying flask, then place the flask in a shell freezer, which is refrigerated and cooled by dry ice and methonal or liquid nitrogen. (For larger products or quantities, this can be done by using a freeze-drying machine). Products are frozen at they're lowest temperature at which solid and liquid can co-exist. This is so the product does not melt.
3. The Primary Drying phase removes all the ice from the product without re-hydrating it. During this step the pressure, controlled by a vacuum, is lowered and heat is supplied, which allows the liquid to turn into a gas.
4. The Secondary Drying phase is the final step in the drying process that removes all unfrozen water molecules, During this step the temperature is raised even higher to break any chemical interactions that might be happening. Typically the air pressure is lowered at the same time, but some products do benefit from and increase in air pressure.

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