Understanding Freeze-Drying: How Does a Freeze Dryer Work?

Freeze-drying is a clever process used in many industries (it is the process that makes freeze-dried food) but have you ever wondered how freeze drying works? This article is going to unravel the methods of freeze-drying.

How Does Freeze-Drying Work: The Basics

Freeze-drying – or lyophilisation, from the Greek lyo (release) and philia (love) – is a process of dehydration used to preserve delicate materials or make them easier for transport. An item to be preserved is frozen, and then put into a low-pressure environment with heat applied to the area to enable the frozen water in the object to sublimate.

Just how cold does a freeze dryer get, then? During the initial freezing phase, it hits -40°C (-40°F), low enough to make sure any material’s water is indeed frozen solid. Then, as the name suggests, the now-frozen material is placed in a vacuum, where it’s far easier for the water to skip past the liquid phase and convert to a gas (vapour) directly from a solid (ice).

The Steps of Freeze-Drying

The freeze drying process consists chiefly of three steps: freezing step, primary drying step and secondary drying step.

1. Freezing

The first step of freezing in the freeze and dry method is actually to convert the water present into ice at a very low temperature below its eutectic point, which is the lowest temperature at which a substance is still a solid. By freezing the material, the structure is preserved and the moisture is locked in.

2. Primary Drying

Following this, samples are placed in the secondary drying firedamp tank. Here, they are first brought to freeze temperatures and then subjected to what is known as ‘primary drying’. The pressure is reduced (typically to a few millibars) and a little bit of heat is added. Under those circumstances, the ice will go through sublimation – change from solid to gas directly, without a liquid phase. Under such circumstances, up to 95 per cent of the water of the material may be removed.

3. Secondary Drying

The third step, secondary drying, removes any residual moisture. The temperature is raised still further than in primary drying to drive off the water which is hold by the object. This final step is necessary to ensure that the material remains stable in the long term.

How is Freeze-Dried Food Made?

We know how does a freeze dryer work, but let’s see how this process is used in freeze-dried food.

After the product is frozen, it is introduced into the freeze dryer, where it loses water due to the extreme vacuum. In the secondary drying phase, what is left – water still bound to the food – is removed. What’s left is a product with no moisture; however, you will still need to check the expiration date before consuming. The product can be stored under normal atmospheric conditions, which means you’ll need to refrigerate it.

And freeze-dri­nged food: how is that rehydrated? The pores that remain in a freeze-dried blueberry are too small to contain bacterial or mould spores: but they correspond to where the ice crystals were What’s cool about freeze‑dried food is that it rehydrates instantly and fully – just add water!

How to Freeze Dry Liquids

Surprisingly, the technique can also be used to freeze-dry liquids. The basic approach is the same – we simply freeze the liquid as a solid block or frozen droplets. It is then freeze-dried in a similar manner. The final product could take the form of either a powder or granules, depending on the size of the frozen droplets.

Conclusion

It’s used to preserve food for astronauts, salves that medics rely on, even flowers for flower arrangements that last. The ability to ask, ‘How does freeze-drying work?’ gives us an understanding of this miraculous technology that keeps products alive or in near-lifelike form for long periods of time. Whether it’s in your pantry, the clinic or on the kitchen table, freeze-dry makes products for longer.