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Author Topic: How Is Freeze Drying Done ?  (Read 3695 times)

Offline neilep

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How Is Freeze Drying Done ?
« on: 17/08/2007 23:31:24 »
Dear Freeze Dried Fanciers,

These are White Chocolate Raspberry Crumb Coated FREEZE DRIED Raspberrys !



Hmmmmm !!..Nice aren't they ?....Notice how freeze dried and non wet they are ?

How do they do that ?...how do they remove the moisture retaining the fruit ?

I just don't know.....what I do know is that they taste nice !!

whajafink ?

« Last Edit: 18/08/2007 00:29:39 by neilep »


 

Offline Cut Chemist

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How Is Freeze Drying Done ?
« Reply #1 on: 18/08/2007 01:11:53 »
We have a freeze dryer in my lab.

Freeze Drying, (also known as lyophilizing or lyophilization,) involves freezing a food and then placing it into a container which is hooked to a vacuum.

In my lab, first we place the compounds in liquid nitrogen, this freezes the water contained in the compounds.  Then we place them into a lyophilizer, which is hooked to a high powered vacuum.    Since the container is at a very low pressure, the frozen water sublimes, changing it from a solid directly to a gas.  The gaseous water then condenses and immediately freezes onto a "cold finger." A "cold finger" is just a frozen tube of metal that is kept at -78oC.  This is much too cold to allow the frozen water to sublime again.

Apparently, the phase transition from liquid water to water vapor will destroy the structure of cells, and proteins.  Sublimation is a much gentler phase transition.  This makes freeze drying a very delicate way to remove water from fragile compounds, or foods.  It leaves nice fluffy compounds and intact fruits and vegetables. 

By the way, those look delicious!!! 
« Last Edit: 18/08/2007 01:40:03 by Cut Chemist »
 

Offline neilep

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How Is Freeze Drying Done ?
« Reply #2 on: 18/08/2007 02:14:49 »
YAYYYYYYYYYY !!

THANK YOU THANK YOU for the wonderful response Cut Chemist.
I so want to come and see your lab !

I had this thought that freeze drying was like blowing the object with a very very cold wind but I can see that my ' incapable ' thought process came up trumps once again by creating the illusion that I might actually know something.

Your wonderful post is most enlightening....and appreciated.....If you were nearby you'd be most welcome to as many of these choccy-fruity delights as you wish.

THANK YOU AGAIN.
 

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How Is Freeze Drying Done ?
« Reply #3 on: 18/08/2007 03:53:53 »
Apparently, the phase transition from liquid water to water vapor will destroy the structure of cells, and proteins.  Sublimation is a much gentler phase transition.  This makes freeze drying a very delicate way to remove water from fragile compounds, or foods.  It leaves nice fluffy compounds and intact fruits and vegetables. 

I thought it was the phase transition from liquid to solid that caused the damage - or more precisely, the crystallisation of water.  The point about the liquid nitrogen is that it freezes so fast that it does not have time to crystallise, and that is where the protection comes in.
 

Offline Cut Chemist

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How Is Freeze Drying Done ?
« Reply #4 on: 18/08/2007 06:13:38 »
When the liquid nitrogen runs out we use dry ice in acetone (-78oC).  Is that cold enough to protect from the crystallization of water molecules??

I admit it does make sense that water crystallizing to a solid would break open cells and possibly damage proteins and peptides. 

However, Wikipedia says that it's the sublimation that's important.  The key is to get the samples at a temperature below the euclectic point, and actually some samples are annealed, which means they are melted and frozen several times before freeze drying.   

http://en.wikipedia.org/wiki/Freeze_drying

I suppose that it may be that its leaving out the liquid phase alltogether that preserves the fruits.  But, since "larger crystals are easier to freeze dry and to produce larger crystals the product should be frozen slowly or can be cycled up and down in temperature."  It would appear that sublimation is the key to freeze drying.
« Last Edit: 18/08/2007 06:40:42 by Cut Chemist »
 

Offline eric l

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How Is Freeze Drying Done ?
« Reply #5 on: 18/08/2007 09:57:36 »
The sublimation is the key point indeed, although...
In our la we modified starches (and other products) (for different applications)and often we freeze dried lab scale productions (say up to 1 kg dry substance) that would be spray dried when it came to industrial processes.  Both these processes had the advantage that they would not create lumps that would be extremely difficult to dry, and that the result was more readlily mixable with water.
 

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How Is Freeze Drying Done ?
« Reply #5 on: 18/08/2007 09:57:36 »

 

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