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The practice of freeze drying, or lyophilisation, has radically changed the way we store and preserve everything from food and pharmaceuticals to flowers and even museum artefacts, but how does a freeze dry machine work? How does sublimation actually occur in a vacuum?

The Science Behind Freeze Drying

Learning about freeze drying teaches you something of the very centre of physics and thermodynamics. Two things to get to grips with are sublimation and the use of a vacuum in the process.

What is Sublimation in Freeze Drying?

In a nutshell, sublimation is the direct conversion of a solid into a gas, without ever becoming a liquid. How does this occur? The answer lies in applying heat under extremely low-pressure conditions.

The Key Role of Vacuum in Freeze Drying

That’s where the vacuum comes in. Applying low-pressure vacuum conditions exerts less force on the atmosphere surrounding the frozen product. At these low atmospheric pressures, the frozen water in the product is able to sublime directly from solid to vapour, since sublimation naturally occurs faster than evaporation of a liquid.

The Freeze Drying Process

The freeze-drying process involves three main stages: freezing, primary drying, and secondary drying.

The Freezing Phase

The preparatory method starts with freezing the material, since that guarantees that the structure and properties of the material (which is in an aqueous state) will be maintained during drying.

The Primary Drying Phase

Following which, the product is cooled to a freezing point, the material is put in a vacuum, and then it is subjected to heating, at which point, the water inside the product sublimes to the vapour state, and the vapour is removed from the system.

The Secondary Drying Phase

Secondly, under moderate vacuum conditions, the liquid turns into steam and, since it cannot escape, it condenses into a thin film on the material surface. Finally, the product temperature is increased further and any remaining water bound to the material is removed, making the product bone dry.

The Freeze Dry Machine: A Closer Look

The frozen vat would be placed along a raft, below a mechanical device known as a ‘Torrinhaus’ (a vacuum system), which would allow warm air (not as hot as a home oven, but hot enough to blow off the water) to circulate, in a powerful, quick vortex through the vat. Next would come the dwelling box itself, functioning as an incubating and controlling cooker, allowing you to watch the process by double–glazing.

Essential Components

A conventional freeze dry machine consists a drying chamber, a vacuum pump that draws the vapour to a low-pressure region, and a condenser to condense the water vapour.

Operations and Maintenance

Using a freeze dryer requires setting the correct temperature and pressure parameters, periodic checks on the integrity of the system, and regular maintenance.

The Benefits of Freeze Drying

Preservation and Food Storage

This is why the perfection of food preservation techniques such as freeze-drying (which is superb at preserving food, retaining most of the nutrient content, colour and texture) is such a bonus for keeping food for long periods.

Rehydration: Bringing Food Back to Life

One of the miracles of freeze-dried food is the fact that it rehydrates – which is a funny word for it because the food isn’t really rehydrated. It is now capable of absorbing liquid. When you do decide you want to eat it, the only thing you have to do it add water. The food will absorb the water and once again become the meal you remember.

Real-life Applications of Freeze Drying

Freeze Drying in the Food Industry

The method is also used in the food industry where you can find freeze-dried products such as fruits, vegetables and even milk or cheese.

Other Notable Uses of Freeze Drying

Apart from foodstuffs, it is also used to preserve pharmaceuticals, flowers, and in the production of instant coffee.


Put another way, freeze-drying has literally become a part of us – from advancing food storage to preserving life-saving pharmaceuticals, the effect it has had is powerful.

Frequently Asked Questions

1. How does freeze-drying work?

Freeze-drying is based on the principle of sublimation, which means changing a solid straight into a gas, bypassing the liquid state.

2. How does a freeze dry machine work?

A freeze dry machine works under vacuum conditions by heating the material and then allowing water suspended in the solids to sublime – and then exhaust the vapour.

3. What does it mean to food and its preservation through Freeze Drying?

Through Freeze Drying, we can preserve food by removing water at sub-zero temperature so the food would still have its nutrition, colour and texture while making the food shelf-life extends.

4. Then, will you be able to use it?

Yes [laughs]Oh, you can use the freeze-dried caviar?Yes [laughs]Yes [laughs], well, the rehyrosation part ­– Yes! It can be rehydrated! ­– is the tricky part. It’s perfectly edible as it is [laughs]Rehyrosation: how do you do it?Then, will you be able to use it?[interrupting once more] You have to add water. Then, then! [laughs] ­– then you’ve got to wait. You can’t just add water and eat it immediately ­– you have to wait.Collazo: Oh, so you have to keep it hydrated.La bouche: Hydrate it?Yeah.If you snap it up dry, it might never become edible.

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