Before you invest in a new freeze dryer, it is important to understand how this equipment works. Unlikepharma ovens, freeze dryers have a relatively unique working principle, making them a perfect choice for food processing.
In this article, we shall explore all the fundamental factors on how freeze dryer works. From optimal drying temperature, sublimation phase to other secondary process.
Let’s dive right in:
What’s a Freeze Dryer?
A freeze dryer is a machine that dries food or pharmaceuticals by freezing and getting rid of moisture. This helps in retaining flavor, shape, and nutrients much better than other methods of drying.
In most cases, efficiency and reliability will depend on the freeze dryers design. Therefore, at any given instance, only choose reputable freeze dryer brands with known history of efficiency and reliability.
Freeze Dryer Working Principle
The machine works in three main steps: freezing, primary drying, and secondary drying. It’s all about each playing a pivotal role in protecting your product’s quality and shelf life.
A. Freezing Stage
The freezing stage is the first most important step in the process of freeze-drying. At this stage, all the water inside the product is turned into ice that is solid form of water. This step must happen before any drying can take place because only ice and not liquid water can be removed in the next phase of sublimation.
For proper freezing, temperatures should drop below the freezing point of water, 0°C (32°F). Generally, products are frozen to temperatures between -40°C and -80°C. This deep freezing is important because it ensures that all the moisture in the product turns to ice, even the water deep inside its structure.
There are two key points to understand in this phase in the freeze drying temperature:
- Eutectic point: The lowest temperature at which a mixture of substances (like salts and water) can solidify completely. For most food and pharmaceutical products, it lies between -10°C and -35°C.
- Glass transition temperature (Tg’): This is the temperature at which colloidal or semi-solid systems attain solidity and assume a glassy state. Normally, it lies between -30°C and -50°C for most products.
If your product is not frozen below these points, then it may retain partial softness or semi-liquid state and thus compromise the drying process.
Importance of Uniform Freezing
It is important that the whole product gets uniformly frozen. Uneven freezing can lead to issues such as:
- Parts that remain soft and never dry completely
- Collapse of the product during drying
- Nutrients or structure loss
To prevent this, you need to put the product in thin layers (generally 1 to 2 cm thick) in metal dishes or pans. Then, you set these dishes on cold shelves of stainless steel that conducts cold rapidly and evenly.
Some systems have pre-freezing chambers and others do the freeze drying right inside the freeze dryer. In both cases, sensors measure the product temperature to confirm it has reached the correct freezing level.
Freezing Time
The duration needed to freeze a product depends on its thickness, water content, and freezing method. For instance, the temperature for freeze drying fruits, will be quite different from meat or grains. Typical freezing times are:
- 2 – 6 hours for most pharmaceuticals and foods
- Up to 12 hours might be required for thicker or denser products
Freezing stage prepares the product for the next stage of sublimation. When you skip or rush this stage, the whole process of freeze drying will fail. Thus, adequate freezing help in maintaining the product’s shape and ensures that moisture leaves during drying without interfering with the structure or quality of the product.
B. Primary Drying Process (Sublimation Phase)
The primary drying stage is the core of freeze-drying. This stage involves the removal of about 95-98% of water from the frozen product. The ice within the product does not melt, but rather transforms directly from a solid to vaporized state. This type of change is known as sublimation.
Sublimation is a unique transition that occurs only under certain special conditions. For ice to sublimate into vapor without first melting into liquid water, these two are required:
- Low pressure
- Controlled heat
In freeze-drying, a vacuum pump lowers the pressure inside the drying chamber. Then, the pressure falls to between 50 and 300 millitorr (0.066 to 0.4 mbar), which is way below normal atmospheric pressure (760 torr). At the same time, the temperature is raised slightly to make sure the ice can get enough energy to turn into vapor.
Additionally, for sublimation to efficiently occur, the temperature of the product is maintained between -20°C to 0°C. This is to prevent the product from melting or collapsing. Heat is applied slowly and evenly using the internal heated shelves of the drying chamber. If heat applied quickly it may lead to puffing up or collapsing of the product.
Vapor Removal and Condensation
As the ice sublimes, it has to be taken out of the drying chamber so that it won’t turn back into liquid on the product. A vacuum pulls this vapor toward a cold condenser, which is much colder, around -40°C to -60°C. When the vapor hits the cold surface, it solidifies again and turns back into ice.
This process goes on until nearly all the ice from the product has sublimed. The condenser requires regular defrosting during long drying runs to prevent frost from building up.
Drying Rate and End Point
The factors that control the drying rate during primary drying include:
- Ice thickness: The thicker the ice, the longer it takes to remove.
- Surface area: Greater exposure leads to faster drying.
- Shelf temperature: More heat (within limits) facilitates sublimation.
- Chamber pressure: Lower pressure aids in quicker escape of vapor.
On average, this phase lasts between 6 and 24 hours depending on the type and size of the product. For some biological samples and thick materials, up to 48 hours may be required for completion of primary drying.
Sensors measure the product temperature and the pressure difference in the chamber to check if the process is finished. When no more vapor comes off and the product temperature stops rising, sublimation has ended.
C. Secondary Drying Process (Adsorption/Desorption)
After most ice is removed during primary drying, secondary drying begins to target residual moisture at the molecular level bound to the product. Residual moisture of about 1% to 4%, which does not get sublimated during primary drying, has to be desorbed.
Secondary drying involves an increase in temperature of the product at constant deep vacuum. Shelf temperatures are usually raised to between +20 °C and +40 °C depending on the thermal stability of the product. The extra heat gained during secondary drying breaks the physical bonds between water molecules and the solid matrix of the product, so that the remaining water can be removed.
This phase reduces moisture content to below 2% and thus enhances the long-term stability of the product. This low moisture level prevent microbial growth, enzymatic activity, and chemical degradation. The final moisture content is then verified gravimetrically or by Karl Fischer titration, depending on the sensitivity of the product and requirement for compliance.
After the secondary drying is finished, the chamber is brought back to atmospheric pressure using a non-reactive gas like nitrogen.
Conclusion
A freeze dryer protects your product by freezing and drying it without damaging or losing quality. In fact, there are many benefits of freeze drying that makes it the perfect process in specific food processing industry.
At SaintyCo, we offer many freeze dryer machine models for your unique process. For any inquiries, contact us now.
FAQs
1. What are the benefits of freeze-drying?
You preserve nutrients, taste, shape, and shelf life. It’s great for sensitive items like food, medicine or biological materials.
2. What are the disadvantages of a freeze dryer?
Long drying time, high cost, and complex operation. You also need to precisely control temperature and pressure.
