Freeze-Drying

[George Schmermund]

Steve, I decided to check out the freeze-drying book that you recommended. I got a copy from Amazon.com for just a few bucks. It's got some really good information in it. After reading the first couple of chapters I'm now thinking about blasting the hinges off the door to my microscopy room and seeing if anything in there still functions. I haven't played with that stuff for quite a few years. This new book might push me over the edge toward some fun toys that I've almost forgotten about. We'll see.

[Steve Hansen]

Hi George,

That is an absolutely great book. While it's too bad that it's out of print the good news is that a fair number are available through Amazon's third party sellers and the prices are excellent. I just ordered a copy for an internet deprived fellow in Ohio in just about perfect condition for $5. I hope that you purchased the book through my Amazon link!

What do you have in mind for projects?

I presume you've looked up the article in Vol 7 #2 where I reference a "micro-freeze dryer" that was described in Review of Scientific Instruments where the authors developed a little chamber that could be used with an optical microscope to study the sample during the freeze drying process.

For others that are reading this, the book is "Freeze-Drying Biological Specimens: A Laboratory Manual" by Rolland O. Hower. Hower did many animal specimens for the Smithsonian. The Amazon link is here:

http://www.amazon.com/dp/0874745322?tag=thebelljar-20&camp=14573&creative=327641&linkCode=as1&creativeASIN=0874745322&adid=1GHT5WAK6Q4Z49J2ZYXX&

[George Schmermund]

Steve - I've been looking around for my stash of old tBJ issues. They're here somewhere. Maybe you could do a recap of the article so that I can stop digging for a while. One of the devices I've used in the past was built around a small molecular sieve pump. I'll try to find the pieces and get some pix of the setup.

I'm attaching this link: http://www.fusor.net/board/view.php?bn=fusor_vacuum&key=1155962228 as a preliminary view of the freeze-drier components. The top image show how a microscope cover slide can be easily placed inside the chamber. A freeze-dried sample can be produced on the cover slip and then vacuum coated with gold for use in an SEM. The pumping module has a very high capacity for these small samples.

[Steve Hansen]

Hi George,

I think that this is what you want. It's the microfreeze dryer that was described in Vol. 7 No. 2.

This apparatus was inspired by a recent article in Review of Scientific Instruments [J. Bayard, H. Cléris and S. Varnier, Microchamber for optical observation of freeze-drying phenomena, Review of Scientific Instruments, 68 (12), pg. 4593, December 1997]. Here the authors describe a chamber in which the sample is viewed by an optical microscope. Light is transmitted through the chamber to the microscope objective. This permits microscopic observation of sample throughout the drying process.

The instrument, which they term an OTA (optical thermal analyzer), is fairly sophisticated. The microscope is connected to a video camera where the images are captured on a PC. Sample temperature is controlled and the PC is equipped with image analysis capabilities.

My version is shown in the figure. The chamber is fabricated from a 2-inch bore stainless steel shaft collar (Grainger 1L653, about $20). A piece of Plexiglas, 3 inches in diameter and 1/8-inch thick serves as the bottom window. A 3-inch diameter x 6.5 mm  optical window (Edmund Scientific F43894, about $16) serves as the upper window. I redrilled and tapped the set screw hole to accommodate a 1/4-inch pipe thread to 3/8-inch hose adapter.
   
The lower window was drilled in four places for two 1/4-20 threaded standoffs and two 1/4-20 bolts. A neoprene gasket is sandwiched between the collar and the window to provide a vacuum seal.

The upper window rests on another neoprene gasket. Gravity and atmospheric pressure hold this window in place and maintain the seal.
   
My microscope arrangement consists of an adapted rack & pinion mount, surplus objective (5x), a coupling tube, and a video camera. Fixturing inside the chamber consists of a transparent stage that is set high enough to permit the  microscope to focus on the sample. The 5x objective, with a 3/4 inch working distance, is compatible with the current design.

I found that the best way (for me, anyway) to freeze small samples is with a can of  component cooler (AKA Quick Freeze, etc.) which can be obtained from Radio Shack and other electronic outlets. This canned refrigerant will quickly get the sample to about -40 °C and will keep the sample cold enough to allow you to get the specimen into the chamber and pump it down.

I have included no provision for monitoring or controlling sample temperature but these features could be easily added by anyone with reasonable cleverness. Some sort of small thermoelectric heater/cooler could be used for this.   

Given the small size of the sample, there is no need for a condenser on this chamber. The evolved water can be easily tolerated by a good mechanical pump.

Since I'm no biologist, this is about as far as I'm going with this device. Perhaps someone with interests in that area would find the microscopic study of freeze drying an interesting pursuit.

[glugsleK73]

Just wondering if freeze-drying is considered raw?  I mean I know fresh is best and step down but still better than cooked is frozen or dehydrated, but this is a combination of both. NOt sure what "heat" is applied to the frozen product. 

Curious because my son loves freeze dried fruits as a snack.

[Steve Hansen]

A good example of freeze dried food is the product from Mountain House. It's fully cooked and then freeze dried. Obviously things that can be eaten raw are freeze dried from the uncooked state.

The evaporative cooling that takes place during the process slows down the sublimation rate. Product is often warmed to speed up the process but it is essential that the product stays below the triple point, otherwise melting will occur.