Chilling Out! How to perfect the freeze drying process

Having previously discussed the numerous applications and the importance of spray drying , I would like to take the time to focus on a different method used to remove moisture from a product. Spray Drying involves atomizing a product into a fine mist of tiny droplets; these are exposed to hot air in a chamber which rapidly evaporates the water content, leaving a dry powder. A slightly more complex method of moisture removal is freeze drying, also known as lyophilization. Freeze drying involves freezing the product and reducing the surrounding pressure; this allows the frozen water within the material to sublimate, which means it transitions directly from a solid to a gas state. Numerous factors influence this process and affect the quality of the final product. So, if you want to know how to achieve the best freeze drying results possible or want to learn more about the process, read on!

Finally, the sun is shining here in Switzerland, a country famous for its snow-capped peaks. I was with some friends on the weekend when the temperature reached 26°C which meant we needed something to help us cool off. One of my friends kindly bought some beer, but it had not been kept in the fridge and needed cooling. Being a competitive group, we discussed ways to quickly reduce the beer’s temperature. There were several suggestions, ranging from sensible to ridiculous. We decided to each try our method, and the winner got to enjoy a cold beer first and bask in the glory of having outsmarted the rest of the group. We decided on 6°C as the optimal temperature and tested our theories.

The first idea was to put the beer bottle in the freezer; my friend thought we were overthinking the task, and it was best to keep it simple: put the beer where it is cold! One of the wilder ideas was to chill the beer with compressed air; the thinking went along the lines of “I once blasted my finger with compressed air from a can, and ice formed on my finger and froze it – surely this can freeze a bottle of beer quicker than a freezer.” Off they went to get a can of compressed air from the garage. The next idea had some good thinking backing it up. The idea was to wrap the beer in a wet paper towel before putting it in the freezer to take advantage of a process called evaporative cooling. As the water in the towel evaporates, the evaporation process requires heat which is drawn from the beer bottle, cooling it down faster. It was time for my suggestion, and I decided to put my beer bottle in a bucket of ice water and add salt to the water. We all removed the lids from our beer bottles so that we could probe the beer for its temperature at regular intervals, then started our processes simultaneously. We then all laughed at our friend who decided to use compressed gas. They sprayed the bottom of the bottle creating a localized white frozen mist, then kept moving the bottle around, trying to get the whole thing cold; occasionally, they would freeze their finger, much to our amusement. At regular intervals, we checked the temperature of our beers, and it started to become apparent whose method worked best. Being a very modest person who does not like to brag, I kept quiet as it became apparent that my method was the best, and when my beer was the first to reach 6°C, I certainly didn’t point and laugh at all my friends and mock them and their inferior methods, no matter what you may have heard! Obviously, my years of experience in laboratories helped me gain an advantage. I knew the water would increase the surface area in contact with the bottle assisting the cooling. Understanding the effect of surface area has also been used to increase the speed of the freeze drying process.

Sublimation occurs at the sample’s surface, and the process becomes more difficult as the sublimation front moves downwards through the sample as water molecules must now pass through the already dried product (the “cake”) before leaving the matrix. This led to the development of manifold freeze drying, a technique that improves the process by increasing the surface area. Even freeze drying in vials can be accelerated by using larger containers that increase the surface area, which allows more water molecules to be released from the matrix. If liquids are to be dried in flasks in a layer thicker than 1 or 2 cm, then ‘shell freezing’, whereby the product is frozen under rotation in a cooling bath, would be beneficial. The rotating bath spreads a thin layer of the sample on the wall of the flask, increasing the surface area available for sublimation, thus reducing the time it takes to dry the product. Sublimation rates can be doubled using shell freezing instead of bulk freezing.

‘Sublimation occurs at the sample’s surface, and the process becomes more difficult as the sublimation front moves downwards through the sample as water molecules must now pass through the already dried product (the “cake”) before leaving the matrix.’

Cooling speed is another important factor in the freeze drying process . When trying to cool my beer bottle, I knew the salt would lower the freezing point. The freezing point of water is 0°C; however, with a 20 % salt solution, it is around -16°C. This increases the rate at which the ice melts, absorbing heat from its surroundings – in this case, from the beer bottle. Keen to enjoy our beers, we wanted them cooled as quickly as possible but not to the point at which they froze. However, in the case of freeze-dried samples, the frozen state of a sample is very important for the freeze drying result. Most liquid products freeze by forming crystals, and the cooling speed influences the size and shape of these crystals. Rapid cooling produces small ice crystals, and slower cooling creates larger crystals. As I already mentioned, the sublimation process gets more difficult over time as the water molecules need to escape the already-dried product. Large crystals facilitate the sublimation process by creating a more open structure with less restrictive channels in the matrix. However, slow freezing can cause damage to certain biological products as large crystals can damage cells, and surface-induced denaturation can damage proteins. Small crystals are useful for preserving structures to be examined microscopically; however, they are hard to freeze dry due to the narrower vapor paths. Optimal freezing requires striking a balance between these factors to dry quickly but without damaging the sample.

Rapid cooling produces small ice crystals, and slower cooling creates larger crystals – Large crystals facilitate the sublimation process by creating a more open structure with less restrictive channels in the matrix. However, slow freezing can cause damage to certain biological products.

So, speed is important, but so is ensuring the sample is frozen adequately, which requires analysis of the product’s critical temperatures. Materials must be properly solidified; otherwise, the vacuum applied during primary drying will cause any unfrozen product to boil and damage the sample. Formulations are either composed of eutectic or amorphous mixtures, which tend to freeze in different ways . Eutectic mixtures freeze at lower temperatures than the water surrounding them; therefore, the water will freeze first, but the remaining substance will remain liquid. The temperature whereby the entire substance is properly frozen is called the eutectic temperature and is the critical temperature the formulation can endure. Eutectic mixtures will be damaged if a vacuum is applied to an incompletely frozen formulation.

Materials must be properly solidified; otherwise, the vacuum applied during primary drying will cause any unfrozen product to boil and damage the sample.

Amorphous mixtures form a glassy state when frozen, and the formulation becomes more viscous as it cools and eventually freezes to a vitreous solid at the glass transition point. Regarding stability, the critical point for amorphous mixtures is the collapse temperature in the frozen state, which is typically slightly lower than the glass transition point. Such products are challenging to freeze-dry. As I’m sure you are now aware, freeze-drying is not as straightforward as it may seem. It is important to understand the nature of the sample and the effect of various influencing factors on the drying process to ensure the quality of the end product and the reproducibility of the method applied. I hope this guide helps you understand the process better and improve the quality of your freeze drying process.

Till next time,

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