INSIGHT - Comments
Vol 1 No. 9 Overview of the Lyophilization Process
Thank you so much for sending me this edition of INSIGHT. I truly found it to be quite helpful. I am going to distribute it to some of my co-workers so that we can all learn together. I believe that this will help me gain a better understanding of the other editions I have received in the past.
Colette Keng - November 1998
pkhotstuff@worldnet.att.net
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Dear Dr. Jennings,
I am just starting to work in the field of lyophilization and I am in problem with understanding the process.
I read all your "INSIGHT" on your web site and learned lots of things that I don't know before but, still I have questions about freeze drying. Firstly, how can I understand from product temperature changes that the primary drying is completed?
I know the product temperature is dependent on shelf temperature and pressure in the drying chamber but I have no device measuring the drying chamber pressure. Our freeze drier has product and shelf temperature display and of course vacuum display. I can preset the shelf temperature before process but, I don't know what will be the product temperature actually.
The second question is that, if a product having partially wet site is further freeze dried, what will happen to the final product?
Will there be any deformation in freeze dried part of this product?
These are not end of my questions. Another question is, how can I understand that the product have 5% moisture? For example, I weigh a part of material before freeze drying and it gets 4 gr., then I weigh it after freeze drying and it gets 2 gr. So, what does it mean? Does it mean that it has 50% moisture? I don't think so but, I don't know the answer.
So Dr. Jennings, I know these are very stupid question for you but, I wrote this letter by getting courage from your lovely style of writings in "INSIGHT".
Ms. Hilal Haznedaroglu June 1999
Cekmece Nuclear Research and Training Center
P.K 1 Havaalani Istanbul-TURKEY
E-mail: hazneh@cnaem.nukleer.gov.tr
Response by T.A. Jennings, Ph.D.
Thank you for your e-mail message and the kind words that you said about my writing style.
First I would like to put your mind at ease about questions. There is, in my opinion, no such thing as a stupid question. There are only dumb answers.
You first asked how you can tell from the product temperature that the primary drying is complete.
The primary drying is that portion of the drying process when there is sublimation of ice from the frozen matrix. During sublimation there is a loss of mass (generally water) and energy. As a result of the loss in energy, the product temperature will be lower than that of the shelf temperature. When the loss in mass from the matrix approaches zero, the loss in energy will also approach zero. Consequently, the product temperature will now approach that of the shelf temperature. Thus, an indication of the completion of the primary drying is knowledge that the product temperature approaches the shelf temperature.
You state that you do not have a device for measuring the chamber pressure but you do have a vacuum display. It is not clear from your statement whether the vacuum display is either not functioning or it is not measuring the chamber pressure. In order to understand what is occurring during the drying process, it is essential to have an accurate knowledge of the chamber pressure. Without such knowledge, then one is employing, in my opinion, the “art” rather than the “science” of lyophilization.
Your second question is concerned about the deformation of the product during the lyophilization process. One of the characteristics of a lyophilized product is that the configuration of the dried material is the same as that of the frozen material. In order to maintain that configuration, the material must be in a completely frozen state, i.e., there must be no mobile water present in the interstitial regions between the ice crystals. If such mobile water is present, then upon removal of the ice crystals, the material can become deformed or collapse. If you are seeing deformation of the material, then that would be an indication that your material was not completely frozen during the drying process.
In answer to your last question, if the only loss of mass from your product was water, then the loss in weight would be a measure of amount of moisture that was removed from your material. This last statement does not imply that you removed all of the water.
In some products, the final dried material may not form a self-supporting structure. As a result, the flow of water vapor from the product may carry with it product material so that there may be a loss of product (mass) . You can see an example of this by examining the various forms of cakes ( see figure 66)
So if there is only a loss of water from your material, then 50% reduction in mass represents the amount of moisture removed as a result of the drying process.
