Solar Dehydrating in a Humid Spot

In a previous post several of my commenters observed that they lived in places too humid for solar dehydrating, and had been using electric food dryers. We have similar issues here - the summers are very humid, and there is a tendency for things not to dry rapidly enough, while reabsorbing moisture overnight and molding. Fortunately, the great Sue Robishaw, homesteading Goddess, connected me to her variations on Larisa Walk and Bob Dahse's design for the "Midwest Solar Food Dryer"

We have a small version and plans for a much larger one, and have had a great deal of success with this - a small but important way to get off electricity! It also allows for much greater capacity than even the largest electric dehydrator.


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Sheets of craft store plastic canvas with 7/in. squares make good drying screens. I believe the plastic is polyethylene so is the similar to milk jugs. At least I have no fear of it :-) Before using with food, I "baked" the sheets in the dehydrator at max temp to drive out the minimal odor. It seems plastic always requires a bit of baking to shed odors, etc. So far things don't seem to stick to it but give it time - I am certain something will sooner or later.

Has anyone thought about building a home freeze dryer?

Freeze drying is a much better way to dry, and it is much more efficient than an electric hot air dryer. A solar powered one, I don't know how the efficiency would be for that, would have to do some calculating.

There are many ways to remove the water vapor and to provide the vacuum pump. Basically you could take an old pressure cooker, put the food in, pump it down to remove most of the air, then remove the water vapor either by condensing it (requires a heat pump like a deep freeze to provide the low temperatures) or again with a vacuum pump.

There are many options for the vacuum pump, rotary pumps are probably not the best because the water mixes with the oil and oil mist might travel back up the line and get on the food.

One option is aspirators, which are very cheap, and water or steam can be used as the fluid.

If steam a trough collector might provide the steam... so basically a trough collector, old pressure cooker and a steam aspirator (also called a steam ejector, these are used in commercial freeze drying units).

IIRC one of the problems is getting energy into the food at a good rate, as well. Radiant and conductive transfer is the only way since convective transfer is very limited due to the low density of the gas in the chamber.

It would produce better food, and the food safety issues largely go away as the food is at freezing temperatures during the process. Meat etc. would be easy to dry. Like any food drying method you'd still need to verify that enough water was removed to prevent microbial growth, though.

bob: "There are many ways to remove the water vapor and to provide the vacuum pump. Basically you could take an old pressure cooker, put the food in, pump it down to remove most of the air..."

I'm having a little bit of a hard time holding my tongue- you haven't actually ever done any of these things. That's not a question; it's an obvious fact.

One simple proof- no "old pressure cooker" is capable of holding vacuum. They are designed to hold high internal pressure- only. Not a chance they'll hold any vacuum at all.

You might want to be a little careful about casually giving advice- about things you've never done.

When I had a hoophouse (simple plastic covered greenhouse), I dried food in it, using a hanging dryer. It was very the humid Northeast.

Coincidentally, I was looking at an online site today, which has a very simple hanging dryer (similar to the one I used), and for $17. That's an excellent price for those who don't want to DIY!

I expect the exact URL would be stripped out, but it's, then go to the Kitchenware section (or words to that effect), and it's on Page 3 of the Kitchenware section.


By Pat Meadows (not verified) on 15 Jun 2011 #permalink

I'm using the solar dehydrator model featured in the book The Solar Food Dryer by Eben Fodor. I live in the St. Louis, MO area. My DH used the plans in the book to build our dryer. It works well - it does take two or three days to dry the food if you don't use the option of adding light bulbs to provide heat at night. In fact, it works so well that my food dehydrator gets up to 160F at this time of year and I have to put a doubled bedsheet on the glass to cut the temp to 110-120F to dry herbs in it.

I had a problem with ants getting into the dehydrator at night. That was easily solved by putting the legs into empty tin cans and filling the cans with water, thus creating a moat the ants couldn't cross.

ET: "If anyone builds this, please let me know." :-) Ah, the if. I have similar plans for converting seawater to gasoline. If anybody actually does it, do let me know.

Sorry, but this whole thing is speculation, again; full of "probably" and "should" - the writers claim to have used a pressure cooker as a vacuum chamber "and it worked perfectly" is very suspect to me; particularly in light of the rest of it.

I confess, this kind of thing is a pet peeve of mine, ever since in the late 70's I read so many Mother Earth News articles that gave SO much incredibly useless advice. This is a major problem for those seeking directions; so may are eager to help!

I made up an faux article that summarizes the precise trend; vast enthusiasm, actual construction of the thing in question; article written after using the thing for a month- and zero follow up. Pretty much never do you find articles on "cool stuff" after 5 years of use. Or even one. "Build your own backyard nuclear breeder reactor! We did, and it's great!"

I guess I also ought to confess: I, and my PhD engineer son, have BUILT, and used, vacuum based food driers- both in the USA and in China. So I do have background here. We do not have one working at the moment. That's indicative.

For a serious review of this exact problem, take a look at this discussion on Rob Hopkins' blog "transitionculture dot org"; tinyurl dot com/5orkub (just replace the _dot_ with the actual dot, and they work) This has to do with the mythical "chicken greenhouse" - taught by permaculture for years- but it turns out, almost nobody has one that works.

Sharon; I made a comment to this thread yesterday, with one active URL in it- and it went to "your comment has been saved for review by the blogger" - if you could dig that out it might be useful. Doesn't have to do with freeze drying, though.

Uh, greenpa, I got some news for you. I have used a pressure cooker as a (low) vacuum vessel before. It DOES work fine. The force involved is not that much compared to the thickness and strength of the walls. Obviously you have to patch holes in the top etc. first. Maybe you are thinking about some sort of vessel that has thinner walls. I have seen some pressure cookers that would not work, yes, but there are many that would.

If you are such a pro, why don't you go try making one? That would be useful, instead of griping. You may be surprised at what you didn't know.

Hi. Do you think that this sort of thing would work in Western Washington? We have (somewhat) humid summers but the real problem I think would be the unpredictability of sun and heat. For instance, today less than 1 week from solstice, there is a high of 61 and 100% chance of rain predicted, and this is not unusual for the type of summer we are having. Any Pacific Northwesterners out there who have used solar food dryers?

Uh, bob, I got some news for you; "low vacuum" is not interesting at all, and pointless in regard to freeze drying. As Greenpa clearly understands; and you don't. Everybody on the planet has heard their pressure canner suck air in when you have let it cool more than necessary. That doesn't really qualify as "vacuum" - the specs for freeze dryers say they need pressures of "a few millibars" - a millibar being 0.001 atmospheres of pressure.

Nor is the strength of the pressure cooker walls to the point- the point is the gasket. It's designed to seal tightly when the inner pressure is higher than outer. Yup, it holds a little when the differential is the other way; but it's not designed to really seal against forces working backwards.

You also seem to be a little delusional about where responsibilities lie- you think someone else should build a device to prove your fantasy? Good luck with that! You want to prove it- go right ahead. Frankly, since Greenpa (and his PhD engineer son) have built "vacuum based food dryerS" - plural, you notice - in several locations, apparently- I'm a little more inclined to take his word.

By Ranklebiter (not verified) on 19 Jun 2011 #permalink