What is freeze-drying?

Based on years of proven performance and experience, EYELA offers a comprehensive lineup of products related to freeze-drying

Recently, freeze-dried foods have become very popular, ranging from vegetable chips and fruits to instant miso soup, soups, cooking ingredients, and even emergency rations. By freezing food or raw materials containing moisture and then removing the water while still frozen, a freeze-dried product is created with its primary components intact.
Freeze-drying technology is widely applied to pharmaceuticals and industrial materials, making it indispensable in various fields.

Video Showing Freeze-Drying Process

Advantages of Freeze-Drying

Compared to drying methods based on evaporation under atmospheric pressure, sublimation via freeze-drying offers several advantages.

Principles of Freeze-Drying

Freeze-drying is a method where moisture in a sample is frozen below the freezing point (eutectic point), and the moisture is then removed and dried through “sublimation” while maintaining that frozen state.

While water boils at 100°C at atmospheric pressure (1013 hPa), its boiling point decreases as the pressure is lowered. If the pressure is reduced to 123 hPa, water boils at 50°C; at 6.1 hPa, it boils at 0°C. Since 0°C is also the freezing point, water reaches a state where it “boils while remaining frozen.”

In contrast to general drying, which involves “evaporation” from liquid to gas, freeze-drying is a drying method that involves “sublimation” from a solid (ice) to a gas (water vapor). During sublimation, the sample temperature remains at approximately -30°C.

The figure on the left shows the state of water.
The point where solid, liquid, and gas coexist is called the “triple point,” which occurs at a temperature of 0.01°C and a pressure of 6.117 hPa.

Under atmospheric pressure, water changes from ice (solid) to water (liquid) to Water vapor (gas). However, at pressures below the triple point, it can be seen that the state changes directly from solid to gas (sublimation) without passing through the liquid phase.

Steps of Freeze-Drying Process

1.Pre-freezing

The process of freezing the sample in advance.

The sample is sufficiently frozen using a low-temperature water bath or a freezer. Generally, pre-freezing is performed at temperatures between -80 and -30°C. This is a critical pretreatment step that determines the success or failure of lyophilization.

2. Primary drying

The process of sublimating and drying the frozen sample under a high vacuum.

Sublimation initially begins from the surface of the sample, then proceeds at the interface between the dried and undried portions within the sample. Sublimation occurs slowly over an extended period.

3. Secondary drying

The process of removing ※bound water as a final drying stage after primary drying.

At this stage, the sample is heated to approximately 30°C to increase the degree of dryness under a high vacuum, similar to the primary drying process.

※ “Bound water” refers to the water within a substance that is chemically linked to its constituent molecules through hydrogen bonding.

Components of Freeze Dryers

A freeze dryer consists of four main components.

1.Main units of Freeze Dryers

Vapor is collected through sublimation.

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Integrated freeze dryer and drying chamber.

2.Manifolds & Drying Chambers

Vessels are attached, or samples are placed on the shelves.

3.Vacuum Pumps

Reduces the pressure within the system to promote sublimation.

4.Pre-freezing bath &Pre-freezer

Freezes the vessel containing the sample in advance.

In addition, vessels, adapters, and piping materials are required. You can freely select the desired configuration to suit your application.

By using a pre-freezing bath and a pre-freezer, the surface area of the liquid inside the freeze-drying flask can be increased, allowing for efficient freezing.

Selection Points for Freeze Dryers

The following three points are key factors in determining the configuration of a freeze dryer.

１．Sample Properties, Shape, Vessels, and Throughput

Whether the sample is solid or liquid, its corrosivity, and the type of vessel used will determine the loading method and the necessary equipment (manifolds, drying chambers). The number of vessels and the choice of components change based on the substance, application, and throughput. Depending on the physical properties, a specialized vacuum pump may be required.

2. Dehumidifying Capacity

This refers to the amount of moisture that can be frozen and collected by the freeze dryer in a single freeze-drying cycle, which depends on the cold trap’s shape and size. If samples containing moisture exceeding the capacity are loaded, drying time will increase, and the final quality may deteriorate. Furthermore, moisture that cannot be captured by the cold trap will be drawn into the vacuum pump; in oil-sealed pumps, this causes oil degradation and can lead to equipment failure.

3. Cold Trap Cooling Temperature

To collect vapor under vacuum, the cold trap must be cooled below the freezing point of the gas at that specific vacuum level. Generally, traps are available in -45°C and -80°C. A lower trap temperature promotes sublimation, resulting in a dried product with lower residual moisture content.

Operating a freeze dryer requires peripheral equipment and parts in addition to the main unit, so please consider the system as a whole. EYELA provides reliable systems that strongly support various laboratories and manufacturing processes. Please refer to the combination guide in our catalog for more details.

Selection Points for Vacuum Pumps

In freeze-drying, a high vacuum level of approximately 2 Pa to 30 Pa is required to promote sublimation.

*Oil Mist Trap: Captures oil smoke and mist from the exhaust air of an oil rotary vacuum pump using an internal filter to prevent dispersal.

FAQs Regarding Freeze Dryers

Actual Freeze-Drying Data using the EYELA Freeze Dryers

Please click here for the actual data.

An Example of a Freeze-Drying System

A complete system capable of handling sample flasks from pre-freezing to freeze-drying

Product name	Model	Cat. No.	QTY
Freeze dryer	FDL-2000	280920	1
Flask manifold (TS29/38)	PMH-8	119830	1
Sample flask (TS29/38)	—	116190	8
Vacuum pump with oil filtration cassette	GCD-136XNF	191090	1
Oil mist trap (for GCD-136XN)	OMC-200	118690	1
Pre-freezing bath	PFR-1000	197410	1
Pre-freezer	PFM-1000	197420	1
Trap monitor	—	197480	1