Freeze drying, which is also known as lyophilization, is the process of removing water from a product by freezing it then subliming the ice to vapor. Sublimation is a physical phenomenon by which solid ice is converted directly into vapor without it passing through the liquid state. Removing water from food, by sublimation, protects the material against loss of important constituents and against chemical reactions that are associated with withdrawing or vaporizing liquid water.

Freeze drying occurs in nature through the combined effects of solar heating, cold dry winds, and rarified atmospheres of mountainous regions. These natural conditions are used to produce freeze-dried ‘stock’ fish in Norway and a dried potato product called chuno in Peru. Freeze drying is also used in a peculiar way by the North American red squirrel which is known to spread pieces of food in the forks of trees at the beginning of winter, thereby freeze drying its food supply.

Dried foods offer convenience of storage and transport derived from their long shelf-life and low weight. Freeze-dried foods enjoy these properties and are generally of higher quality than products dried by other processes. As a result, freeze-dried foods tend to be preferred in culinary arts over other dried products.

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Laboratory Vacuum freeze-drying technology, referred to as freeze-drying, also known as sublimation drying, is a method of pre freezing water containing materials and sublimating them in vacuum. After freeze-drying, the original biological and chemical characteristics of the articles are basically the same, easy to long-term preservation, and can be restored to the form before freeze-drying after adding water, and can maintain its original biochemical characteristics.

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Freeze Dryer adopts vacuum freeze drying technology to remove the inside water on condition of no damage to the raw material’s taste and nutrition.Pharmaceutical Freeze Dryer removes water from plants and organic compounds while they are in a frozen state. This locks in the freshness of herbs and plants. Their appearance, nutritional content, and potency remain the same as when they were fresh.Freeze drying completely removes water in about 24 hours.

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The freeze-drying machine is a technical method in which the aqueous article is previously frozen and then the water is sublimated in a vacuum state to obtain a dry product. The freeze-dried article is easy to store for a long period of time, and the water can be restored to the pre-freeze form Keep the original biochemical characteristics, suitable for laboratory samples of freeze-drying experiments and a small amount of production.

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Vacuum freeze drying is the use of physical sublimation principle, in a high vacuum environment, the frozen food in the water (ice) without melting, directly sublimation from ice to steam, so vacuum freeze drying is also known as freeze sublimation drying.Freeze dryer is use Imported compressor, stable and reliable performance;Widely used in food, biological products, chemical, Chinese medicine, health products and other fields.

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Freeze Drying’s 3 Primary Stages

Freeze drying occurs in three phases, with the first and most critical being the freezing phase. Proper freeze drying can reduce drying times by 30%.

Freezing Phase

There are various methods to freezing the product. Freezing can be done in a freezer, a chilled bath (shell freezer) or on a shelf in the freeze dryer. Cooling the material below its triple point ensures that sublimation, rather than melting, will occur. This preserves its physical form.

Freeze drying is easiest to accomplish using large ice crystals, which can be produced by slow freezing or annealing. However, with biological materials, when crystals are too large they may break the cell walls, and that leads to less-than-ideal freeze drying results. To prevent this, the freezing is done rapidly. For materials that tend to precipitate, annealing can be used. This process involves fast freezing, then raising the product temperature to allow the crystals to grow.

Primary Drying (Sublimation) Phase

Freeze drying’s second phase is primary drying (sublimation), in which the pressure is lowered and heat is added to the material in order for the water to sublimate. The vacuum speeds sublimation. The cold condenser provides a surface for the water vapor to adhere and solidify. The condenser also protects the vacuum pump from the water vapor. About 95% of the water in the material is removed in this phase. Primary drying can be a slow process. Too much heat can alter the structure of the material.

Secondary Drying (Adsorption) Phase

Freeze drying’s final phase is secondary drying (adsorption), during which the ionically-bound water molecules are removed. By raising the temperature higher than in the primary drying phase, the bonds are broken between the material and the water molecules. Freeze dried materials retain a porous structure. After the freeze drying process is complete, the vacuum can be broken with an inert gas before the material is sealed. Most materials can be dried to 1-5% residual moisture.

Common questions about freeze drying 

What is freeze drying used for? 

Freeze drying has many applications including:
  • Long shelf life pharmacutical products.
  • Creation of dehydrated powders for tablets or capsules.
  • Production of chemically stable bulk raw materials.
  • Probiotic powders, protein powders, other food supplement powders.
  • Extended shelf life, low weight food rations such as military rations, climber rations and astronaut meals
  • Coffee
  • Fruit
  • Purification and concentration of heat sensitive materials or low molecular weight chemicals
  • Advance ceramic powders
  • Recovery of water damaged documents and books
  • Achaeological recovery and conservation
  • Taxidermy

What advantages does freeze drying have over other drying processes? 

Low processing temperatures ensure that nutrients and colour are not lost and proteins are not denatured.
The original shape of the product is kept after the process without shrinkage or toughening.
The product is easily rehydrated due to micropores created by the sublimation of ice from the product.

What capacity do I need? 

Your required capacity depends on how much ice you will be removing over a set period of time. When contacting us you should state the “wet” (undried) weight of your product and the timeframe for drying. Our food and industry machines are rated for “x”kg of ice per 24hours, though cycle times are easily adjustable by the user.

How can I tell when the primary drying stage is complete? 

1) Product temperature monitoring – monitor the product temperature with a thermocouple – when primary drying is complete the product temperature will rise and approach the shelf/chamber temperature due to the loss of sublimation cooling. When the product and shelf temperature are roughly equal (temperature difference, ΔT around 3-5°C), the primary drying phase can be assumed to be complete. The user should be aware that due to heat conduction through the thermocouple, the area around the thermocouple will dry faster than the bulk of the product and a safety margin for additional primary drying should be included. The thermocouple should also be placed at the bottom of the product as the product will dry from top-down.
2) Pressure Rise Test – If the freeze dryer has an external condenser, a pressure rise test can be used. The isolation valve between the condenser chamber and the product chamber is closed and the pressure rise in the product chamber is measured. If the pressure rise approaches zero, then there is little to no vapour produced by sublimation and the primary phase can be considered complete.
3) Capacitance manometer vs Pirani guage convergence test. This requires both a capacitance manometer and pirani guage to be fitted, as a pirani guage shows erroneously high readings in the presence of vapour, the difference in readings between the two can be used to determine the pirmary drying endpoint. When the two readings have converged (and no further change in the readings is noted) the primary drying phase is complete.

How cold do I need my freeze drier to be? 

Extra cold condensers are only required if using solvents with a particularly low freezing point. A phase diagram for your solvent can be used to determine an appropriate condenser temperature. Bear in mind that some organic solvents may cause serious issues with standard sealing materials and should be mentioned in a list of requirements. An excessively cold condenser will increase the cost of the machine as well as it’s complexity!

Where can I keep a freeze drier? 

A freeze drier with air cooled condensers should always be kept in a cool, well ventilated room. High ambient temperatures will reduce performance and efficiency and possibly lead to the failure of the unit!
A freeze drier using chilled water for cooling can be located anywhere indoors with adaquete space, though excessive ambient temperatures should still be avoided. The chiller should be located as per the manufacturer guidelines – typically in a cool, outdoor area with plenty of ventilation.

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