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| luci |
 Posted: Thu May 08, 2008 8:03 am Post subject: freeze drying |
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Joined: 08 May 2008
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Hallo all,
I understand that when freezedrying stuff, you freeze it and then you use sublimation to get the water out.
But I was wondering the following thing: do you need to put less energy in the product to go from solid to gas phase then when you are working at low pressure and temperature or is the input of energy the same or higher then when you would do it with higher temperature or higher pressure levels? |
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| Harold14370 |
| Posted: Thu May 08, 2008 9:15 am Post subject: |
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Forum Professor
Joined: 13 Apr 2007
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| It takes 740 calories per gram to sublimate ice versus 540 calories per gram to vaporize water. So I think freeze drying would be less energy efficient, not even counting the energy to run the refrigeration equipment. |
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| SteveF |
| Posted: Thu May 08, 2008 1:26 pm Post subject: |
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Forum Bachelors Degree
Joined: 28 May 2007
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Location: NC USA
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Â
Freeze drying is not intended to be energy-efficient. Its purpose is to remove water without damaging the product, which would happen with other water-removal methods. Freeze drying is used because it is the only way to get the job done.
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| raed |
| Posted: Thu May 08, 2008 3:08 pm Post subject: |
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Joined: 05 May 2008
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But why they convert water to ice then sublimate it
I mean they could apply low pressure to evaporate the water without damaging the compound ??
_________________
Raed Ahmed Çeleby
Dont let MEDIA affect you , You have a brain !
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| SteveF |
| Posted: Thu May 08, 2008 4:12 pm Post subject: |
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Forum Bachelors Degree
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Â
I will answer your questions but only if you stop using those silly red letters.
It is very annoying.
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| William McCormick |
| Posted: Thu May 08, 2008 6:02 pm Post subject: |
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Joined: 03 Apr 2008
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| raed wrote: |
But why they convert water to ice then sublimate it
I mean they could apply low pressure to evaporate the water without damaging the compound ?? |
If you treated the food storage area with nitrogen first, so it did not allow the food to go bad, then evacuated to a small degree the nitrogen and expelled water vapor.
You probably could pull the water out, with a weak vacuum that would not hurt the foods structure. But it might take a lot of time, because of the elevated humidity.
If you freeze dry at very low temps the amount of water that will be pulled off is amazing. Because of the very low humidly. Any water vapor even in the air, is solid. Leaving you with zero percent humidity. You just have to keep the food just below freezing, whatever its freezing point is. And then remove the ice crystals from the air. Centrifuge separation probably.
Not sure I would call that sublimate.
My son who goes down and gives them a hand now and then, told me they use nitrogen to keep apples fresh, long past harvest. I volunteer to core apples at our local cider mill. Boy they make a great pie. All you can eat, while you are working. Hot apple cider and cinnamon. Apple turnovers. And Award winning cider.
Sincerely,
William McCormick
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| luci |
| Posted: Fri May 09, 2008 3:22 am Post subject: |
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Joined: 08 May 2008
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Hallo,
thanks all for you answers.
Are there any formulas to use to calculate the amount of energy needed?
Or how did you get that 740 calories/ gram?
And does the pressure and temperature influence this enegery?
And how do they apply the energy then?
They first freeze the product, then start the sublimation, but when starting the sublimation, do they then raise the temperature a bit to have the sublimation? or ?
I understand that at low pressure and low temperature its easier to go from solid to gas, but how does the temperature and pressure really influence it all?
And read:
| Quote: |
But why they convert water to ice then sublimate it
I mean they could apply low pressure to evaporate the water without damaging the compound ?? |
because removing water in the liquid state damages the structures more then when you sublimate it.
(I am working on freezedrying specificaly on the conservation of micro-organisms, so not to preserve food, I suppose its the same with food?) |
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| Harold14370 |
| Posted: Fri May 09, 2008 9:28 am Post subject: |
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Forum Professor
Joined: 13 Apr 2007
Posts: 1675
Location: Pennsylvania
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| luci wrote: |
Hallo,
thanks all for you answers.
Are there any formulas to use to calculate the amount of energy needed?
Or how did you get that 740 calories/ gram?
And does the pressure and temperature influence this enegery?
And how do they apply the energy then?
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I got it here
http://daphne.palomar.edu/jthorngren/latent.htm
But it looks like I made a typo. Should have been 720.
The reduced pressure makes a difference because it reduces the temperature of the boiling point, so the water vapor is at a lower temperature and would absorb more heat to get up to the atmospheric pressure boiling point. You can figure that to go from one state to another requires a certain amount of energy, no matter the path to get there. If you start with water at room temperature and at the end of the process discharge steam at atmospheric pressure it will require 540 calories per gram. Maybe if you dump ice to the environment instead of steam it would save some calories per gram, but I think you will lose out because of the inefficency of the refrigeration, and the need for a vacuum pump. |
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| luci |
| Posted: Fri May 16, 2008 10:00 am Post subject: |
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Joined: 08 May 2008
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Ok I see
Thanks a lot for that link Harold14370 |
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