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Old 16-03-2015, 06:46   #1
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Doing your own freeze-drying

Hi there,

So in preparation for a roundtrip Atlantic, I'm looking at ways to preserve as much food as I possibly can. I can get very cheap fruit/veg/meats here at home (as in, almost free), and would like to take full advantage of this.

For this I've mostly looked at dehydrating, and recently freeze-drying (do chime in here if I'm missing some obvious things!). Dehydrating is easy enough to do yourself, but obviously freeze-drying has some significant advantages over dehydrating...

I know it can be done at home in a regular freezer, but only in smaller batches and it would take a very long time. Imagine trying to freeze-dry fruit/veg for a full year like that... I've also just learned we have a blast-freezer at work (I work in a restaurant). I've been scratching my head for a while now on how to take advantage of this to do some freeze-drying, but I can't really figure it out...

Any ideas?
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Old 16-03-2015, 07:05   #2
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Re: Doing your own freeze-drying

I used a pressure cooker attached to a vacuum pump. Put frozen food in and pull a vacuum. Take it out after about 8 hours. No need to put the vacuum vessel in the deep freeze. The whole point is to sublime the ice crystals into vapor so that the food retains its size and appearance. By the time the food warms above freezing the deed is done. The rest of the time is just drying out to make it long lived. I don't know what the shortest time that could be used.
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Old 16-03-2015, 07:21   #3
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Re: Doing your own freeze-drying

So freeze it, put it in the pressure cooker (no moisture absorbers in the pressure cooker?), pull vaccuum and let it sit at room temperature for 8 hours...?

That's it?
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Old 16-03-2015, 07:36   #4
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Re: Doing your own freeze-drying

Yes, that is it. I never did do a series of tests to determine the shortest time I could leave it. Do put a filter to prevent vacuum pump oil from back diffusion into the food.
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Old 16-03-2015, 07:52   #5
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Re: Doing your own freeze-drying

Any idea what the pressure dropped down to? As in, how big a pump would one need?

And how did you make the connection between pump/filter/pressure cooker?
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Old 16-03-2015, 10:41   #6
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Re: Doing your own freeze-drying

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Yes, that is it. I never did do a series of tests to determine the shortest time I could leave it. Do put a filter to prevent vacuum pump oil from back diffusion into the food.
Jerry, What have you used for a vacuum pump and does it not require some sort of accumulation to catch the withdrawn moister? I won't ask how much vacuum, I have forgotten it's measurement. Inches of mercury I think?
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Old 16-03-2015, 12:16   #7
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Re: Doing your own freeze-drying

Go Here Freeze Drying at Home | Harvest Right Mac
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Old 16-03-2015, 12:39   #8
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Re: Doing your own freeze-drying

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Jerry has the right idea. Damn near 4K for one of these?
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Old 16-03-2015, 13:41   #9
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Re: Doing your own freeze-drying

Neat little thing, that, but not at all what I'm looking for . That thing almost costs as much as I paid for the bloody boat

Vacuum pumps also seem to be a tad expensive if I'll just be using it once and only for this purpose. What pressure are we looking for and are there other ways to achieve it? I have a big compressor for example...

Cheers!
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Old 16-03-2015, 16:52   #10
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Re: Doing your own freeze-drying

I had a vane type vacuum pump for evacuating air conditioners to remove all of the moisture from the system. It got down to pressures of about 1 torr (mm of mercury) if I remember. This is low enough to cause ice to sublime. Air from a calibrated leak helped carry the water vapor out. I added a molecular sieve to prevent backflow of the oil vapor. I let it pump for about half an hour then closed the valve and turned off the pump. I remember it still had vacuum when I took the food out 8 hours later.
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Old 16-03-2015, 16:59   #11
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Re: Doing your own freeze-drying

I used a low vapor pressure hose to connect the pump to the pressure cooker. This has been 30 or 40 years ago! I particularly liked doing meat and strawberries. Later I mostly bought vegetables and MRE's. I just liked my home grown beef better than bought beef.
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Old 17-03-2015, 08:28   #12
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Re: Doing your own freeze-drying

I can't speak to "freeze drying", but have had great success extending the life of many foods by vacuum sealing. You can buy an inexpensive seal-a-meal type device almost anywhere and then the items go in a conventional freezer. Simple. It's also something you might use again. I buy large quantity, family pack meats and then seal them in meal size and freeze.
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Old 20-03-2015, 09:49   #13
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Re: Doing your own freeze-drying

Okay I get that if you have a high vacuum and a calibrated leak it will carry the moisture out. But I don't understand how if you turn the vacuum off and still have vacuum 8 hours later where has the moisture gone? Thanks
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Old 20-03-2015, 10:11   #14
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Re: Doing your own freeze-drying

The liquid moisture has converted into vapor. The volume of the little tank was big enough that the pressure did not come all the way to atmospheric. I guess that it comes to the vapor pressure of water at room temp. The food That I took out was not completely dry but it had the appearance of fresh instead of dried. I was not trying for 20 years of storage. I only wanted to go backpacking for a couple of weeks.
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Old 20-03-2015, 10:15   #15
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Re: Doing your own freeze-drying

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The liquid moisture has converted into vapor. The volume of the little tank was big enough that the pressure did not come all the way to atmospheric. I guess that it comes to the vapor pressure of water at room temp. The food That I took out was not completely dry but it had the appearance of fresh instead of dried. I was not trying for 20 years of storage. I only wanted to go backpacking for a couple of weeks.
It's been a good brain teaser anyway. Thanks!
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