Home Freeze Drying

[AaronM]

I'm extremely interested in freeze drying food. The process involves freezing food and then placing it in a strong vacuum. I know you can do a work-around by freezing slices of things and placing them on a perforated tray in your freezer for about a week. The idea being that the water sublimates without passing into the liquid state.This takes way too long and can only be done practically with very thin slices. The question is: Can something be worked out with a chamber vac? Holding it under pressure for awhile perhaps? Is the vacuum strong enough? I don't have a chamber vac, but I know a few of y'all do - are you interested in trying out an experiment?

[Scout_21]

I've tried looking into this a few times. The issue with using a chamber vac is that you need to hold the food under a vacuum for an extended period of time while the food and chamber holding the food is cold. You also need to deal with the water vapor that will condense in the vacuum pump unless you have some sort of catch (either a trap inline with the vacuum that will condense the water before it hits the vacuum or a block in the chamber that is colder than both the food and the temperature of the chamber...I think). You could possibly repurpose a freezer that can withstand the rigors of maintaining a vacuum inside of it and purchase vacuum pumping equipment that can handle water vapor....but I have found no way to do it easily or cheaply.

I would love to hear a solution if you find one though.

Cheers

[AaronM]

Negating the vacuum issue for a moment: What about a cooler with some dry ice in it to freeze the food, and 2 holes drilled in the sides, with an aquarium pump to allow airflow over the food? Sort of like a turbo-charged version of the plain freezer method.

Maybe something would happen?

A lyophilizer is just way too expensive!

[Scout_21]

Well you could just add a fan in the freezer. Having the warm air move across the food may just allow for issues of bacterial growth. By having the contents under a vacuum you skip the liquid phase of water and it just sublimates when you bring the temp up above 0 C...which is then pulled out by the vacuum pump.

Though it is an idea and perhaps you could try it with something that does not lend itself to bacterial or fungal growth.

Cheers

[AaronM]

The purpose of this is to dry apples and watermelon cubes for use in a watermelon soup. I know this'll taste good, but I see other culinary applications that I'd like to try - and I don't think people are freeze drying all the stuff I'd like to try.

Normal dehydration ruins the texture I'm looking for in the cubes. Is there a way to achieve freeze dried texture without going all the way? As in, not meant for long term storage, but mostly dried?

[Msk]

Why don't you try using dry ice and some form of fan. Alton did an episode beef jerky using a fan and some hepa filters. Just an idea.

[dcarch]

Freeze dried and dehydrated, as you know, are completely different.

Also, when you vacuum water, it starts to boil and then it gets frozen.

dcarch

[Digijam]

If you don't want to hand over ten grand for a tabletop dryer you could always try using a couple of dog food bowls.

No, seriously.

I wouldn't expect this setup to make an appearance in the next edition of Modernist Cuisine (and it does ignore the ideal pre-freeze temperature range for fruit) but for an entertainingly low-tech, MacGuyver-style attempt at freeze drying check out this video from BBC show 'Jimmy's Food Farm'. The freeze drying stuff starts at around the 22 minute mark.

Mark

[Scout_21]

The issue with that example is not that he used cheap containers...it is the several hundred dollar vacuum pump that he used to create the vacuum. The important part of freeze drying is having the vacuum pump that can handle the water vapor that will condense after it comes closer to 1 atm.

[AaronM]

I like it though - we're getting somewhere. Thanks for that link.

The problem with that one for me isn't a couple hundred bucks for the pump, it's that I live in an apartment and that pump is loud.

And your restaurant looks really cool btw.

Edited March 22, 2011 by AaronM (log)

[AaronM]

I need to find exactly how much of a vacuum is required for this.

Is it just relatively low pressure, or a near total vacuum?

The quest continues!

[dcarch]

A used vacuum pump is not very expensive.

Vacuum pump is not noisy.

You can probably use dry ice in a can and let air from the vacuum chamber pass by the cold can before the vaccum pump. moisture will be condensed on the can surface.

Refrigeration is one of the methods to extract water from air.

dcarch

[Scout_21]

You just need to look at a water phase diagram to determine the level of vacuum needed. I've seen "refrigeration vacuum pumps" that would probably work for what you need. The setup on the video was pretty much what I mentioned before...you just need a trap to collect the water before it enters the pump.

Something like this might work but I can't find the vacuum level it can attain Robinair vacuum pump

Edited March 22, 2011 by Scout_21 (log)

[technophile50]

At 0 degrees C, the vapor pressure of water is about 4.5 mm Hg, so you need a vacuum pump that will achieve at least this level. In practice, temperatures for lyophilization are much lower - -10 to -60 degrees C, so the corresponding pressures are lower, beyond the reach of a single stage vacuum pump. Two stage pumps can easily reach 10 microns, and blanked off many go to 0.1 micron or less. A cold trap between the stuff being freeze dried and the pump intercepts most of the water vapor, and its temperature determines the actual operating pressure. Water has a low molecular weight(18) and is hard to pump, and tends to stick in the pump oil, so the pumps used in freeze drying have a gas ballast valve adjusted to leak a small quantity of air(mol. wt ~29) which sweeps the water vapor from the pump oil. A used Welch 1402 vacuum pump can be had for around $300 from e-bay, and there are a couple of cryo fridges for under $200. The Robinair pump goes to 20 microns and would do the job.

BTW, the cold trap - a couple of turns of tubing in the cryofridge, a bucket with dry ice/acetone, or liquid nitrogen - will keep hydrocarbons from the pump oil from evaporating in the warm pump and condensing on your cold food.