Freeze-drying, also known as lyophilization or sublimation drying, is a method of careful drying of high-quality products. In freeze-drying, the ice crystals sublimate without passing into a liquid state. The end product of freeze-drying is known as lyophilization which is completed in a batch process.
Why Lyophilization Is Used?
Lyophilization, or freeze-drying of bacterial cultures, stabilizes the cultures for long-term storage while minimizing the damage that may be caused by strictly drying the sample. Many microorganisms survive well when lyophilized and can be easily rehydrated and grown in culture media, after prolonged periods of time in storage.
Lyophilization is also used in the biotechnology and biomedical industries to preserve vaccines, blood samples, purified proteins, and other biological material.
This short laboratory procedure can be used with any commercially available freeze dryer to preserve your culture collection.
Laboratory Vacuum lyophilization 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.
Lyophilizer 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.
The lyophilizer 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.
Vacuum freeze drying( lyophilization) 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.
Principle of Lyophilization
Lyophilization is based on a simple principle of physics called “SUBLIMATION”. Sublimation is the process of transition of a substance from solid to the vapor state without passing through an intermediate liquid phase. The process of lyophilization consists of:
- Freezing of the product to convert the water in the product to ice form,
- Sublimation of ice directly into water vapor under vacuum.
- Drawing off the water vapor
- Once the ice has been sublimated, the products are freeze-dried and can be removed from machine.
The principle advantages of lyophilization as a drying process are:
- Minimum damage and loss of activity in delicate heat-liable materials
- Speed and completeness of rehydration
- Possibility of accurate, clean dosing into final product containers
- Porous, friable structure
The principle disadvantages of lyophilization are:
- High capital cost of equipment (about three times more than other methods)
- High energy costs (2-3 times more than other methods)
- Long process time (typically 24 hour drying cycle)
Equipment used for Lyophilization
A lyophilizer consists of
- A vacuum chamber containing product shelves which are capable of cooling and heating containers and their contents.
- A vacuum pump, a refrigeration unit, which is associated controls are connected to the vacuum chamber.
Samples are generally placed in containers such as glass vials that are placed on the shelves within the vacuum chamber.
Cooling elements within the shelves freeze the product. Once the product is frozen, the vacuum pump evacuates the chamber and the product is heated. Heat is transferred by thermal conduction from the shelf, through the vial, and ultimately into the product.
The lyophilization process
Lyophilization involves the removal of water or other solvents from a given product by a process called sublimation. This occurs when the ice of a frozen product converts directly to the gaseous state without passing through the liquid phase. This enables the preparation of a stable product that is easy to use and store at ambient temperatures.
A low pressure environment is pre-requisite to allow this process to take place. In order to start the removal of water, the pressure inside the freeze dryer must be below the “triple point value” for the product, whilst also maintaining the temperature of the sample below its freeze point in the lyophilization process.
Pre-freezing – first stage of the lyophilization process
The sample is frozen, which means the water in the product is converted to ice, thereby the phase has changed from liquid to solid.
Slow pre-freezing will produce lager ice crystals, which are easier to lyophilize, whilst fast pre-freezing results in smaller crystals.
Primary drying – Second stage of the lyophilization process
In the second stage of lyophilization the sublimation process starts. The ice formed in the pre-freeze step is removed from the sample by the direct transition of the “solid” ice to a vapour without passing through a liquid phase. The resultant vapour is collected by the condenser, which has a lower temperature and pressure than the product. The vapour is thus converted back to ice on the condenser surface.
The “energy” required for this process to occur is provided by a gentle heating of the sample, which will start the sublimation process and eventually the sample will dry.
If too much energy (heat) is applied to the sample during this stage the condenser of the lyophilizer may not be able to condense the volume of vapours fast enough, the ice condenser temperature will subsequently rise along with its vapor pressure, thus increasing the risk of the sample melting.
Secondary drying – Third stage of the lyophilization process
Finally, any residual water present, which is strongly bound to the molecules of the sample, is converted to vapour and removed from the sample.
This water has invariably a vapour pressure lower than that of water in its “free” form.
Removal of the water in this final stage of lyophilization will be performed at higher product temperatures, consequently, any biological activity of the sample will not be impaired or affected. This usually involves increasing the temperature and lowering the pressure to provide enough energy to break down the molecular bonding. A process called desorption.
Advantages of the Lyophilization
Lyophilization has many advantages over the other drying and preserving techniques.
- It maintains food/ biochemical and chemical reagent quality because they remains at a temperature that is below the freezing-point during the process of sublimation.The use of lyophilization is particularly important when processing lactic bacteria, because these products are easily affected by heat.
- Food/biochemicals and chemical reagents which are lyophilized can usually be stored without refrigeration, which results in a significant reduction of storage and transportation costs.
- Lyophilization greatly reduces weight, and this makes the products easier to transport. For example, many foods contain as much as 90% water. These foods are 10 times lighter after lyophilization.
- Because they are porous, most freeze-dried products can be easily rehydrated. Lyophilization does not significantly reduce volume, therefore water quickly regains its place in the molecular structure of the food/ biochemicals and chemical reagents.
Safety Guidelines for Lyophilizer
One of the most common mistakes when it comes to lyophilization is not knowing your sample’s melting point, which makes it difficult to choose the correct lyophilizer. Your samples may melt during the process. Another common mistake is thinking colder is better when freeze-drying on a shelf-type freeze dryer. During primary drying, you should set the shelf temperature to just below the sample’s eutectic temperature. There should be just enough heat to encourage the molecules of the sample to move — but prevent melting.
A third mistake is using the wrong equipment for your samples. Because freeze dryers are used in a group setting, you should you know the following before buying one:
- How much moisture will be lyophilized
- What the sample is (and the eutectic temperature)
- How to properly use the freeze dryer
If the unit is not used correctly, it could ruin all of the samples. Which brings us to another common mistake: Not maintaining the vacuum pump. The pump must be in excellent working order for lyophilization to work. Running the pump with the gas ballast open 30 minutes before and after the freeze-drying process will increase the life of the pump. Opening the gas ballast purges contaminants out of the pump to prevent damage to internal components. You should check the pump oil often for discoloration and particles, and change the oil as needed. Regular oil changes keep the pump pulling at optimum vacuum during the freeze-drying process.
Lastly, having the wrong freeze drying accessories for your lyophilization process can also be a big mistake. Do you need a stopper sample under your vacuum? Then a stopping chamber is required. Are you freeze-drying in flasks? Then be sure to have a drying chamber with ports.