Construction details are given in Post G22-014.
Bottom line: Works just fine if you ventilate it with a computer fan. Leaving this outside on two consecutive chilly, dry, sunny days was adequate to get 1/4″ potato slices dry enough to snap crisply when bent.
It was a little cold yesterday for solar food dehydration, not expected to top 60F. But it was sunny and dry. And that was enough to let me test and refine my revised tote-based food dehydrator (Post G22-014). This is nothing more than an under-bed plastic tote with a bit of radiant barrier insulation outside, some cheap cooling racks inside, and a few holes in the top connected to thin plastic pipe.
I learned quite a bit, despite adverse conditions. Mainly, the setup above has inadequate ventilation. I rapidly got condensation on the inside of the tote, which I fixed by switching to ventilation powered by a 2-watt computer fan. The lesson there is that this either needs power ventilation, or it needs much more ventilation area. That was no surprise, based on my initial use of this setup last fall (Post G21-050).
With that fan running for most of the day, and the dehydrator sitting out in the sunshine from about 9 AM to about 5 PM (EDT), on one chilly, sunny, low-humidity day, using quarter-inch-thick slices of potatoes for the test run, here are the results:
Undoubtedly easier to understand as a graph:
The upshot is, not bad, but not great either. At the end of the day, my conclusions are:
- This removed a lot of water.
- This didn’t remove enough water.
At the end of day one, the potato slices were much smaller, but they were still quite rubbery. They were nowhere near dry enough to store. Ideally, I’d get them down to less than 5 percent water content. And if I planned to eat them, I’d probably put them in the fridge overnight, before putting back out to dry another day.
So, having removed 21 ounces of water on day 1, I still need to remove a further 8 ounces, per the calculation above. Now, on day 2, I have the dehydrator out in the sun, but it’s so cold today (currently 50F) that I’m not expected a lot of drying to take place. That said, I’ll weigh the results at the end of the day and update this post accordingly.
Edit: Final weight, day 2, 8.9 ounces. The potato slices look and feel dry, and can be easily snapped in half. If the original estimate of 20% solids was correct, these are still 19% water by weight. But that number was a rough average for all potatoes.
These were russet potatoes, which are described as high-starch, low-moisture potatoes with 20 to 30 percent solids. That range more than covers the possibility that I’ve now removed all the water from the potato slices. (If they were 25 percent solids to begin with, the solids alone would weigh 9 ounces.)
In short, I think I’m going to say that this worked. Given their appearance and “snap”, I’d have no qualms about bagging and storing these solar-dried potatoes as-is. Two chilly, dry, sunny days, in the tote with fan-driven ventilation, appears to be enough to produce thoroughly dried potato slices.
Whether this will work as well, in mid-to-late summer, with tomatoes, remains an open question. But for now I’m willing to say that this appears to work well enough to be usable. Even in the climate of Virginia.
Some additional detail and observations follow.
Bottom line? Powered ventilation good. Plastic tote bad. The end of the post discusses how easy it should be to scale this up using cheap and readily-available materials.
The Spud Standard for Dehydrator Testing
WARNING: Contains scenes of trespass, theft, vandalism, destruction of private property, food waste, and inappropriate behavior by unsupervised minors.
Of course there’s a standard approach to testing food dehydrators. How could there not be. Near as I can tell, the approved method is to load them with 1/4″ thick slices of potato. Weigh the potato slices before and after, do a bit of math, and you can measure the amount of water lost. The greater the percentage of water lost, the more effective the food dehydrator.
This approach is how food scientists fine-tune the tradeoff between temperature and ventilation in a solar dryer. Too little ventilation raises the relative humidity inside the dryer, and so slows drying. To much ventilation lowers the temperature inside the dryer, and slows drying.
It’s a classic Goldilocks problem. The object is to get the amount of ventilation just right, given the solar energy input and load of produce to be dried, to get the most possible drying. The ideal ventilation has to be found by trial and error, for each drying device.
In addition, different varieties of fruits and vegetables differ in their ease of dehydration. For example, sugars are generally humectants (they attract and hold moisture), making it harder to achieve the same moisture reduction in sweet fruit as in (say) green vegetables. Plausibly, the ideal ventilation rate has to be matched to the produce being dried.
I’m not sure why potatoes were chosen as the standard. I suspect it’s because they are cheap and relatively uniform in composition. The upshot is that good potato performance may or may not guarantee good tomato performance. Where the ultimate goal is to dry tomatoes. But it’s all I have to go on. And it should be enough to allow me to fine-tune this.
What I learned from this first trial run
1: The unexpected math of success under the spud standard.
Say I have 100 ounces of potato slices. Potatoes are about 80 percent water. I want to dry them to the point where they are about 5 percent water. (Typical for sun-dried tomatoes).
Question: How much water do I need to remove?
That seems straightforward enough, doesn’t it? Naively, I’d figure, well, I start at 80%, or 80 ounces of water in that 100 ounces of potatoes. I want to end up at 5%, so … I have to remove 75 ounces of water, right? Ergo, the spuds are done when they weigh 25 ounces?
Wrong. But I kept fuzzy-thinking my way through it, to get that answer, until I actually did the math.
The answer is that you have to remove almost all the water, to get the residue down to 5%. In the case of potatoes, of the original 80 ounces of water, you have to remove 79 ounces, to get the residual down to ~5 percent water. You end up with a total end product weighing 21 ounces. That’s the original 20 ounces of solids (1 – 80%), and one ounce of water, or roughly 5% water in the final product.
In the case of 100 ounces of tomatoes, at 94% water, I would need to remove 93.7 ounces of water, to dry them down to (roughly) 5% water content. That would leave me with the original six ounces of solids (1 – 94%), and 0.3 ounces of water.
I have read repeatedly that the process of drying slows, as the drying progresses. And now, I think I get it. By the end of this, I’ll be trying to wring the last quarter-ounce of water out of some already very dry produce.
No wonder this took 24 hours in an electric dehydrator, last year.
1A: Corollary, don’t toss that electric dehydrator quite yet*
To me, that suggests that no matter what, given the relatively humid summer climate here, I’m probably going to end up finishing these off in an electric food dehydrator. I doubt that any direct solar device can take (say) air at 90F and 75% relative humidity, and use that to dry produce down to a 5% water content. Not while keeping the peak temperature below 140F.
Plausibly, there is some on-line reference that will let me check that, but if so, I have yet to find it. Without finishing these off in an electric dehydrator, I might have to ruin a few batches of tomatoes to conclude that solar drying alone won’t hack it in my climate.
* My wife just reminded me that it’s her dehydrator, because she bought it. So I had better not throw it away. He who chucks what isn’t his’n must buy it back or go to prison.
2: Power ventilation required for this setup.
As noted above, trying to vent this with just the natural thermal siphon, and a long plastic “chimney”, was a total failure. I either need to keep that small fan running, or I need to cut some much larger holes. More on that in the conclusions section.
3: Chuck the drying racks.
This one isn’t rocket science. Stuff falls through the cracks. The smaller the stuff, the more falls through. When that happens, you have to remove a rack to get at the fallen pieces.
To the point where, when I tried to re-load the solar dryer this morning, I tossed out the racks and just put the potato slices directly onto the bottom of the tote. Trying to get them to sit on the racks was just too tedious.
One option would be to add some mesh silicone food-dryer sheets, on top of the racks. That doesn’t cost much and would solve the drop-through problem.
But in the end, given that this is destined for drying tomatoes, I think I’ll just skip the racks and put the produce directly on the bottom of the dryer. This is not as unreasonable as it sounds, because the approved method for drying paste tomatoes is to cut them in half lengthwise, and dry the halves skin-side down. That means there won’t be much water leaving through the bottom of the tomato in any case. So sitting it directly on the plastic of the tote shouldn’t matter.
A second reason to do this is that it certainly feels like the warmest part of the interior of the tote is the plastic floor. That’s where the sunlight hits, and that has black plastic and radiant-barrier insulation behind it. Arguably, I might get faster drying by placing the produce on the floor of the tote than by suspending it in the middle of the tote with drying racks.
Edit: Confirmed. I had a temperature data logger inside the tote on both days. The air temperature around the racks seldom exceeded 100F — which isn’t bad, really, considering that it was about 60F outside the tote. But the temperature on the black floor of the tote approached 150F at noon on the second day. I’m guessing this explains why these chips got all the way to “snap” dry on day two. They were subjected to some fairly high temperatures.
I think the moral of this story is pretty clear. With the high rate of ventilation, the air temperature stayed relatively low. But all the heating is going on from sunlight hitting the bottom surface of the tote. Placing the food directly on that hot surface gets it warmer, and almost certainly leads to faster and more complete drying, compared to suspending it mid-way-up the tote on cooling racks.
4: Variable ventilation needed.
In this run, I replaced the original under-ventilation via a chimney, with what is arguably over-ventilation with an electric fan. I think the right answer is that any rigidly fixed amount of ventilation is a mistake. And that I therefore need to rig up some way to provide a variable amount of ventilation, depending on the circumstances. Either large and variable vent openings, or a variable-speed fan.
5: The second day of drying is a big waste of space
As I write this, those slices are back out in the sunshine. But the slices that more-or-less filled the cross-sectional area of the tote on day one now cover maybe one-quarter of the bottom of the tote on day two.
The obvious solution is to do this as a continuous process, not a batch process. Each day, I should move yesterday’s mostly-dried produce to the last one-quarter of the tote bottom area, and fill the rest with new, wet produce. Assuming a two-day total drying time, each day I’ll bag up the two-day-dry produce, shift the one-day-dry produce to the end of the tote, and fill the rest with fresh produce.
Assuming this will dry tomatoes in two days, I should be able to feed it maybe a pound-and-a-half to two pounds of fresh paste tomatoes per day. And extract about two ounces of dried tomatoes every day.
6: Scale it up cheaply.
WARNING: Contains depictions of fraud, stereotypes of the elderly as cognitively impaired, and food adulteration.
A pound and a half of tomatoes? All this fuss and bother, and that’s all I can put through this in a day? You’ve got to be kidding me. Last year, I wasn’t even trying, and I got way more than that. This year, I’m really hoping to ramp it up.
I started this last year, from the idea that it was easier to buy a plastic tote than to construct a clear-topped box. To which I added vent holes, vent tubes, insulation, a chimney, then a fan, and some drying racks.
But when I boil this down, all I need is a cheap clear-topped box and a vent fan. And something clean to put the tomatoes on. With that power ventilation, I really don’t have to worry about the size or shape of the box. The plastic tote is nicely sanitary, but that’s about all it adds to the equation.
If I wanted to dry (say) 15 pounds of tomatoes per day, I’d need roughly 10 times the area of this tote, or about 24 square feet. I could exceed that with:
- A 4′ x 8′ sheet of foam board for the bottom.
- A sanitary drying surface (cookie sheets, parchment paper, any food-safe plastic sheet, …)
- Cardboard and tape for the sides.
- A bit of screening or tulle to cover the vent holes.
- A sheet of clear mylar or similar for the top.
- One or more computer fans.
Place the sanitary drying surface on the foam, in the sun. Cover with tomatoes. Make the top and sides (with vent holes and fan) as one unit. Place that over the drying surface. Weight it down, turn the fan on, and come back at the end of the day.
“When the soup is made the stone may be thrown away.” The obvious solution to scaling this up is to get rid of the tote.
Counting tomatoes before they are ripe
I think I’ve learned about all I can learn, at this point. Let me recap.
Short of turning this thing into a solar oven (by adding reflectors on the outside, to direct more sunlight into the box), it looks like it’s going take (at least) two days to dry a batch of tomatoes. Even then, I may want to finish these in an electric dehydrator.
The through-put of the tote is nowhere near large enough. If all goes well, I would need dozens of these to handle the paste tomatoes that I am in the process of raising. (First batch of 18 seedlings will go in the ground later this week.)
But as long as I use powered ventilation, and place the tomatoes directly onto the drying surface, I think I can make this out of almost anything, and make it any size or shape I desire. All I need is a big, flat box with a clear lid and something clean to place the tomatoes on. I can make one ten times the size of that tote from stuff I have laying around in the garage and kitchen: Foam board, cardboard, tape, screening, clear plastic, cookie sheet/parchment paper. And a $10 computer fan.
At the end of the day, that’s pretty obvious. I really don’t need much more than a scaled-up version of a pizza-box solar oven.
So I think this is the end of the line for the plastic tote food dehydrator. It works well enough that I’ll keep it around, in case I have a small batch of something to dry. Certainly, having that nice clean plastic interior makes this really easy to use.
But for later this summer, when I hope to have a ton of tomatoes coming ripe, I clearly will have to step up my game. But now I have a plan. The plan is to cobble up a great big version of this plastic-tote food dehydrator. The key, I think, is to be willing to tether this to the electrical grid, to get that 2-watts-worth of powered ventilation. Beyond that, all I need is a clear-topped box with a clean surface for drying the food. And I can come up with that, of almost any reasonable size, once the tomatoes start ripening.
