Solar Dried Foods

We love dried foods but don't particularly like noisy dehydrators so I have been experimenting with solar drying. I constructed several frames using 3/4" x 1 1/2" cedar boards. To the bottoms I stapled window screen. I place this frame on a piece of metal roofing and cover with a piece of clear plexiglass. The ribs on the metal roofing allow air to circulate through the screen.

This worked well and 1/4" slices of cucumber and squash were done in 2 days, about 16 hours however, they were bleached white, I suppose from the direct sunlight. I then tried blackening the clear plexi and now it has been 4 days and the veggies are still not ready.

My question is the direct sunlight doing anything other than bleaching the veggies? Would this reduce the nutrients in any way? I can't really notice any difference in the flavor.

In the picture is potato, cucumber, and squash chips that I did last week. I sliced them very thin and they were done in about 10 hours. These are delicious and go very fast.

If I can get this figured out we are hoping to use this as an alternative to canning.

Nobody has answered.  With zero knowledge, I'm gonna take a stab. 

I think that that the sun will help to sterilize the food.  It might not just bleach the color - but I wonder if it does anything to the flavor.  How do they taste?

Other than that, I would think that direct sun on them would be fine.

I have noticed a slight loss in flavor, the biggest obstacle so far is time. As long as my slices are less than 1/8" thick they are ready in two days. Recently I tried some 1/2" slices that weren't ready in four days, and they picked up some sort of mold. We had a series of cloudy rainy afternoons.

I am going back to the drawing board but for now I have gone back to the electric machine. 

You might want to take a different approach.  Instead of putting the food directly in the sun where it will bleach out, you could put them in a box, and let solar heat dry it all out kinda like those electric ones.

This project is for a solar garage heater, but if you put the output hose in the bottom of a box, with racks of food chips in it you might get a good result.  Not sure how long it would take though.


http://blog.hemmings.com/index.php/2007/04/26/almost-free-garage-heat-just-drink-a-lot-of-soda/

Hmm...I'm not sure about drying things directly in the sun. I suspect that the bleaching of the fruit may also decrease the nutrition (that's just a guess, though). Everything I've seen regarding drying herbs recommends drying them out of direct sunlight. Perhaps sunlight can breakdown some of the valuable chemical constituents that you want in your medicines/food (enzymes too?). I'd recommend experimenting with a passive solar dehydrator that uses a thermo-ciphon to circulate warm air through the screens in a dark space.

Conversely, I know that drying mushrooms partially in the sun can actually increase their vitamin D content remarkably. See Mycelium Running by Paul Stamets for more info on this.

So, what about sun-dried tomatoes? Obviously, the sun is part of the process. Has anyone had any luck making them in the Pacific Northwest?

I just cruised around for some info, and it looks the drying process is supposed to take anywhere from 4 days to two weeks. Good luck!

But maybe with the right rig it could work here.

\That is what has stopped me from trying to sun dry anything. Its just too iffy. It probably works great in the dessert sw but humidity and dew make it a long drawn out process around here and I imagine that since your area is known for cloudy rainy weather you would have more trouble than I. I think a possible alternative would be cold smoking them.

I just ran across an article about building your own solar dehydrator.  Might not work so well with rain on the way, but something to look into next year.

http://www.homegrownevolution.com/2008/10/build-solar-dehydrator.html

I really like the design of that one. I debate wether I should buy a typical dehydrator or build a solar one. That one is inspiring, maybe this winter I can work on one. I need to make it collapable for storage purposes so I need to do some thinking.

I just now uploaded this video:

[youtube]http://www.youtube.com/paulwheaton12#p/u/0/5U1JgAkW-xY[/youtube]

We made this one last summer.  We dried kale, apples, pears, prune plums, walnuts, finished dried beans (those all worked really well) and burnt the only batch of tomatoes we put in it.  (we grew brandywines and got some romas from a friend to try out for drying)  I think that's because it was 110 degrees that day....ambient temperature.  It gets hot and dry here in the summer, this design doesn't really work without direct sun.  Kale took about two days, the fruit usually three to four days (slices were quicker than halves - the prunes took a week).  We left the beans in for a week cause it was cloudier at that point in the year. 

The construction is the same as the one posted in the homegrown evolution link, it's just bigger.  We had the black metal panels (were solar hotwater panels but were old and leaky) which ended up dictating the 3ftx3ft size of the box.  Found the glass on craigslist (have 30 more of them) and used wood odds and ends for the doors.  Had to buy the chimney and the stainless steel mesh (that was pricey) for the trays.  We lengthened the chimney and that helped speed the drying time. 

Paul, that is a really neat contraption in the video. I really liked the idea for automating the rotation of the dryer. I think instead of water, that sand could be used, as water is precious and susceptible to evaporation. Use two buckets with a calibrated hole drilled in the bottom of each, put a plug in one and use it to catch the sand from the first which would be the counterbalance against the bungee. Just swap the buckets and the plug to reset the system. Or pour the catch bucket back into the weight bucket.

For sun dried tomatoes, there is a variety cultivated just for this purpose in Italy and available here in several seed catalogues. Principe Borghese, is a small, low water type tomato that has a flavor that is apparently improved by sun drying. It is a determinate plant, so the crop yields in a short window. They also hold very well on the vine, as long as it is still attached to the root system. When most of the fruit has ripened, simply cut the entire plant at the base of the vine and hang in the sun. The plant acts as a moisture extractor. Since it is no longer in contact with ground water via the roots, the water stored in the fruits reverses flow and exits back through the stems and leaves of the still attached plant. Pick the fruits once dry. I grew this variety last year, but only learned of the drying technique too late in the season. I did save the seed, but I'm not 100% sure of the purity of the strain as I didn't bag flowers and grew several other varieties nearby. I bought my seed from Territorial Seed Co., but other vendors sell it as well. I imagine this drying technique would work with other fruits as well.

Not solar but if you have a wetter climate it may be useful.
There is a dryhouse at Quiet Valley Historical Farm in Shroudsburg, PA.
It has cheesecloth lined shelves, tiny woodstove.
I plan to build one, but want to have a dual use of solar and wood heat, whichever the conditions demand.
website: http://www.quietvalley.org/

ETA: Sorry about how large the pic is!

This is a food dehydrator that works even in winter in Missoula

That's interesting, in both Mark's and the one in the video that Paul posted, the design for airflow is upside down (from how we made ours).  What's the advantage to that set up? 

The one made from a fridge solves our next problem - how to seal the thing up from insects. 

In our dryer, the collector being lower and at a flatter angle fits with our high sun angle in the summer.  No need to turn the thing throughout the day (it's immobile), the sun always hits it. 

Marina, I think the solar collector above is designed to create an up draft inside of it as the air is warmed, then the air enters the top of the dehydrator and falls through to the bottom of that as it cools slightly. Then the exhaust stack is solar heated (painted black) to create an up draft in it to draw the air out the bottom of the dehydrator.

Has anyone tried using a simple dehydrator box painted black (collects solar heat from all sides) with a vertical stack off the top (like a rocket stove)? The intake would be a simple opening at the bottom or installing a low angle solar collector at the bottom like marina's could be an option. Would this make the conditions too hot, more like a slow cooker?

Hi!
i've build a solar deyd too; link to youtube:


the language is italian

paul wheaton wrote:
Nobody has answered.  With zero knowledge, I'm gonna take a stab. 

I think that that the sun will help to sterilize the food.

A bit nitpicky perhaps, but sterilize is not the word you want here, to sterilize is to kill everything living, and that would be very difficult to do with the sun, unless you were to get much much closer.

Emerson White wrote:
A bit nitpicky perhaps, but sterilize is not the word you want here, to sterilize is to kill everything living, and that would be very difficult to do with the sun, unless you were to get much much closer.

the sun has "antibacterial properties" the heat and the rays' radiation kills stuff. People used to drape their sheets on bushes in the sun to bleach as well as zap them. Yes the sun "will help sterilize food".

I'm in Western Oregon and I had great success sun drying tomatos last year. I used Romas, sliced them 1/4 thick, sprinkled them with a little salt(salt helps draw water out too) and put them on window screens I took off my windows. I put them out in the sun on my front deck in the afternoon on hot days--at least upper 80's was enough, obviously hotter will be quicker. I put them under the big window(thinking rays would be bouncing down from that too) and the front deck just is a big heat bubble. On a 95 plus degree day the tomatos would dry in an afternoon, other wise it took two days tops. At night I just covered them with a clean kitchen towel. I then put the tomatos in a bag in the freezer(just to be absolutely safe). They are delicious!! You can also dip them in vinegar before drying.

None of this two week stuff

Also above a fired up woodstove is an excellent place to dry stuff also.

There is no such thing as partially sterile, so unless you do sterilize then nothing can have helped sterilize. Do you understand what I'm saying?

Emerson White wrote:
There is no such thing as partially sterile, so unless you do sterilize then nothing can have helped sterilize. Do you understand what I'm saying?

there is such a thing a "good enough". check out exposure to dirt building the immune system. People have been sun drying stuff for ages and yes, sunlight killing bacteria is a pleasant "side effect".

Of course I understand what you're saying. "Most" of the issue of drying stuff outside is having birds poop on it or walk on it(with dirty feet) or mice checking stuff out and pooping on it at night.

you can google uv kills bacteria etc, lotsa info on da web

The deydrator in the last photo is called a down draft dehydrator.I have built one before.It utilizes 3 passive forces.1-the collector heats the incoming air pushing it up.2-the air enters at the top and cools as it gathers moisture pushing it down.3-the cool air exits out a chimmney in the back.This desighn was developed due to flaws in the rodale desighn.In that one the heated air enters from the bottom but ends up fighting itself as it gains moisture and cools.

I was not saying that UV didn't kill bacteria, or that it was not safe to eat solar dehydrated food, only that the word sterile means something different, and specific, that doesn't apply in this context.

We have experimented with about every solar dryer design out there in the past 35 years and came up with our own design back in the mid-1980's that is actually pretty similar to the original posting. But it uses clear glazing and either black cloth or metal over the screen of food. And the roofing is at an angle to improve thermo-syphoning of airflow.

You can see the details on our webite at http://www.GeoPathfinder.com/9473 . That page gets about 150 hits/day, and we do workshops on the design, self-published a booklet about food drying, and do dryer building workshops with groups of folks in our area where we build 10, 4-by-4 foot dryers in a day. Our design was used by the U.N. in a publication they did back in the late '80's since it works well in humid regions, can be modified easily to adapt it to any climate, altitude, or latitude, and it's easy to use found or local materials as long as you stick to the basic physics of the design. It's now used from Alaska to Mexico, at least.

The box-type dryers are all based on adding a solar crutch to an electric box dryer. They work but you have to track the sun, they're slow, they're harder to build, and you can't leave food in them overnight.

We try to keep the direct UV of sunlight off everything but mushrooms. Some varieties, like shiitake, can have 10 times or more their normal vitamin D level if dries gill side up in direct sun. Fruits and veggies all get done in the dark to preserve nutrients and color.

Bob Dahse.

Walk wrote:
We have experimented with about every solar dryer design out there in the past 35 years and came up with our own design back in the mid-1980's that is actually pretty similar to the original posting. But it uses clear glazing and either black cloth or metal over the screen of food. And the roofing is at an angle to improve thermo-syphoning of airflow.

You can see the details on our webite at http://www.GeoPathfinder.com/9473 . That page gets about 150 hits/day, and we do workshops on the design, self-published a booklet about food drying, and do dryer building workshops with groups of folks in our area where we build 10, 4-by-4 foot dryers in a day. Our design was used by the U.N. in a publication they did back in the late '80's since it works well in humid regions, can be modified easily to adapt it to any climate, altitude, or latitude, and it's easy to use found or local materials as long as you stick to the basic physics of the design. It's now used from Alaska to Mexico, at least.

The box-type dryers are all based on adding a solar crutch to an electric box dryer. They work but you have to track the sun, they're slow, they're harder to build, and you can't leave food in them overnight.

We try to keep the direct UV of sunlight off everything but mushrooms. Some varieties, like shiitake, can have 10 times or more their normal vitamin D level if dries gill side up in direct sun. Fruits and veggies all get done in the dark to preserve nutrients and color.

Bob Dahse.

Wow - very informative and detailed!  Thanks for sharing.

three solar dehydrators.

The first solar dehydrator is shown by robert and marina at dell artimus farm.  The solar heat comes from a heated panel at the bottom, and there is a black chimney at the top that creates a draw.  They use  a stainless steel screen.  The dryer is a year and a half old.  They have dried beans, flowers, cherries, grapes (raisins), kale, walnuts and apples.    They tried some tomatoes, but those ended up as pig food.

Matt at feral farm shows a "down draft solar dehydrator."  The solar heat enters at the top and then goes down.  Because as it gathers moisture, the solar heated air gets heavier.    He has nettles in there.

Mark Vander Meer, of  wildland conservation service in Missoula, Montana shows off his solar food dehydrator still loaded with dried plums.  Those plums have been in there all fall, winter and most of the spring.  He talks about trying to dry fruit with electric food dehydrators and how expensive that was.  This solar dehydrator also uses the down draft technique.  He says plums take three days and apples take a day and a half.

These are all passive systems.  There are no fans.

Does anyone here know what material window screen is made from? Aluminum? Stainless steel? Galvanized steel? I've always wanted to make the simplest version possible of this dryer for mushroom drying (just a frame-mounted screen), but I've always beem concerned about aluminum or something toxic like zinc being absorbed by the mushrooms while drying. Is there any info on this? 

peachlovingman wrote:
Does anyone here know what material window screen is made from? Aluminum? Stainless steel? Galvanized steel? I've always wanted to make the simplest version possible of this dryer for mushroom drying (just a frame-mounted screen), but I've always beem concerned about aluminum or something toxic like zinc being absorbed by the mushrooms while drying. Is there any info on this? 

Stainless is the safest kind of screen but costs more. I would think if you can get steel with no coating it would be ok. rub a bit of oil on it before use.

Len wrote:
Stainless is the safest kind of screen but costs more. I would think if you can get steel with no coating it would be ok. rub a bit of oil on it before use.

I've seen window screens that are steel, door screens are often aluminum, and they also sell a plastic composite mesh screen that's mostly nylon.  I wouldn't trust it for food, though. 

I think the fancy screens on my Excalibur dehydrator are nylon.  My next solar dehydrator, I think I'll size it to fit the same screens so I can swap them around. 
Might be able to use a loose-mesh nylon cloth, or stretch old pantyhose (sewing skills might be able to make them removable for cleaning).  Nylon intended for clothing, and washable, seems safer than industrial-grade composites.

Traditional material would have been cane or wicker baskets, and you can get chair-caning replacement material all pre-woven, might be spendier than steel but would be pretty.  Willow 'dowelling' racks, or loose-woven mats, might do well for large stuff like apple rings.  I don't know that I'd go for a basket-rack setup unless I had an easy way to get standard-sized baskets, making them takes more time than I have lately.  Pulling a bunch of old chair seats and making a custom dehydrator could be very cute.

You can also just scrub the hell out of whatever you want to use. 

Oiling a metal screen, heating it without food to cure it, and then renewing the oil each time you wash it, can create a sacrificial barrier layer like on cooking pots.  Your food will get some oil on it, but won't get much rust, aluminum oxide, or whatever. 
Oiling isn't a good idea for plastic screens, though, unless you know they are food-grade.  I would use paper over any old plastic screens instead, since oil might potentially lift toxic components from the old plastic into closer contact with the food. 

Paper over any old racks (cake cooling racks, wooden slats, etc) could be a good low-tech solution.  Or leaves.  I've heard that some traditional groups in this region used skunk-cabbage leaves as drying racks, wax paper, etc - their waxy surface keeps foods like mashed berries from sticking as they dry.  Banana leaves and corn husks are used this way elsewhere.  Bet you can find a nice, non-toxic, waxy, local leaf if you care to experiment.

In most cases it seems airflow is a very important factor.
Air needs to be heated then forced through the food
either with a solar chimney effect or a fan running off a solar panel.
I have a small 12w solar panel and run a "big" computer cooling fan from it.
The amount of air it moves is quite surprising and it runs in pretty
cloudy conditions too albeit slower.
I like the soda can designs and my next will be built with them.
In our bright sunlight monotoring it is important as food can get cooked.

Do you think that people are using solar panel for these job, this is the first time I am seeing when a person use solar energy for dry fruits. I will also give it a try of burning dry nuts.

It seems that the design where the air goes up, then down through the food, then up again in the back, is the big winner. The designs where the hot air enters the bottom is not doing as well.

I suppose it might be that there is more air travel, thus more pump?

I thought it worked well because the air becomes heavy with water. But I'm told that air with water is actually lighter than air without water - hence clouds.

??

Mark Vander Meer wrote:This is a food dehydrator that works even in winter in Missoula

It would be lovely if there were more images or at least a rough drawing for this dryer from Wheaton, or VanderMeer or Lucas McKeever? McGeever?

an intense internet search hasn't given up the info from what I have attempted to find.




Below is an attached PDF I found online about a Solar Dehydrator done at Appalachian State Univ in the late 90s with interesting info in it.. about the heating of the air.

I went and built my version of one of those dryers as seen on Youtube. The biggest "problem" was I had no clear cut guidelines as to how much and where insulation should be used. Secondly, the thermal siphoning using a popcan heater and my box was rather minimal in NE Ohio. Without photos it would be hard to critique my mistakes. I started to dry tomatoes but by the third day they went moldy. I used some used lumber for the window screen trays. Some tomatoes had dried but the majority went bad before drying was complete. My exhaust area may have been too much-- that could have been easily remedied. The heater was connected by a dryer hose(non-insulated). That in itself maybe an issue. The heater has 2500 sq in surface and the dryer has 1000 cu in of space +. I thought that should have handled the needs. I am determined to make it work but need some helpful ideas from people who are more experienced than I.

We have such a dry sunny climate we only have to make sure there's a cross-breeze in this excessively glazed room.

Planning to build one of the solar food dehydrators this winter and was wondering if it matters what color I paint the outside ... meaning would it raise the temp inside too much if it was all painted black or does it matter? Thoughts?

Going with the fact that black absorbs light and white reflects, black paint would cause the temperature of the wood to increase thus slightly increasing the inside temp.

Hey Spencer Davis (Group)

I certainly understand the principle of increased temperature collection the darker the color. But, my actual question was, would it raise the temperature TOO much if I paint it black.

Dan