Freeze Dry At Home

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In response to an Instructables thread from 2009 that I ran across, I decided to test if any idea for an at home lyophilizer (freeze dryer) could work? I didn't have a vacuum pump or any sophisticated condensing or cooling equipment. I've worked with desiccators in the lab and I thought that if I could get a vacuum chamber at the vapor point of water, I might be able to keep it under that point with anhydrous salts.

The Post:

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Step 1: The Science Behind The Suck

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Through my research I learned two interesting facts that led me toward my final project design:

1. Freeze drying is the result of both negative pressure and temperature.The rate of drying depends on both.
2. Freezer burn, and that annoying build up of ice in the back of your freezer, are the result of the same process--namely water vapor leaving or forming solid ice. This freezer-burn vaporization is called sublimation and its opposite freezer ice buildup is called deposition.

Now this led me to think that if I could lower the pressure in a sealed container to the vapor pressure point and keep it there, I could achieve freeze dried fruit goodness in the back of my freezer.

Scientifically, the problem is vapor pressure. Vapor pressure relies on temperature and pressure. The vapor pressure of our berries will tend to equalize with the pressure and temperature inside the chamber. This means roughly that a food being passively dried will only get as dry as its environment.This makes it more difficult to dry something further. (The water keeps coming out of the fruit and raises the pressure.)

This can be solved at home by putting a desiccant in the low-pressure container to absorb the vapor and to actively pull it from the food you want to dry.

However, this means that the rate at which the vapor leaves the food and enters the desiccant is much slower than an active vacuum system (traditional vacuum lyophilzer).

After testing how much vacuum it could pull, I chose to use a wine saver hand pump for my first experiments (see picture below). There are many other similar systems on the market--usually labeled as instant marination containers or vacuum bag systems.

could I use a food saver as a vacuum? would that build up enough pressure?

Yes, but you want to plumb in a check valve between the pump and the jar, you don't want to try and pump the jar for a week straight, get it to pressure and shut the machine down, let the check valve ensure you don't leak air back in when the pump is off. Not sure how big of jars they make for them, but you need 4x the size of what your drying (1 unit for the food, 2 units for the calcium chloride, and 1 unit of space for "room")

I do this by running my chamber at -60C, I spread the food thin on a tray and put it in the chamber, wait 20 minutes, then I bring in my drying pot with the calcium chloride in the bottom, drop the food from the tray into a screen colander and set that in the pot, I lock the top on and draw it down with a harbor freight pump set to 20inHg and leave it for a week. It seems to work good. I've been told I need to intermittently warm my chamber slightly so I bought some pelter devices to put in the pot and try and warm the air slightly every few hours to see if I can dry things even further

nathb1028 (author)  nydiver8 days ago

Thanks so much for this reply!

RS143 months ago
Good work! Just wanted to correct one thing. A phase change of a vapor to a solid is depositon. Sublimation is a phase change of a solid turning straight into a vapor.
nathb1028 (author)  RS143 months ago

Good stuff! Thanks! My first usage of sublimation was flawed I edited it.

roubadg9 months ago

this was awesome... thanks for sharing...

iamaqtpoo1 year ago
Awesome, super, excellent!!! Wow that was a great read, thank you so much!
oldmicah1 year ago
Nicely done, and thank you for the explanation. Alton Bown (of good eats) flash froze berries by rolling them around in a small cooler with chunks of dry ice to flash freeze them. And our local grocery store actually carries dry ice..... (Bwa ha ha ha ) Anyway, cool idea. Thanks for sharing!
They look like preserved rose petals! A beautiful (and tasty) ible!
nathb1028 (author) 1 year ago
Yes you can! It cakes with water and this reduces the surface area of the crystal reducing its efficiency. (Why it's sold in pellets) Simply break it up after drying it to remedy this.

As a side note: There's a pretty cool saturated calcium chlorine water fountain that reduces humidity in your house somewhere online as well.
Thanks for this. Just a question,I know with silica gel you can gently heat it to drive out the moisture & reuse it. Can you do the same with calcium chloride?
Yay for fruit and science! Good job!