PVD: Passive Vehicular Dehydration

Nothing screams legitimacy like a good initialism, e.g. PVD.

In this instance, PVD stands for Passive Vehicular Dehydration (and is not under any circumstances to be confused with Peripheral Vascular Disease or the international airport in Providence, RI).

Long before we acquired our first dehydrator, and for a long time after we discovered the shelves inside the device were not dishwasher safe, we used passive vehicular technology to dry herbs from the garden in large batches. Our technique is described below.

Be Aware: PVD is a highly technical process, and involves a thorough understanding of radiant energy dynamics and the greenhouse effect - all of which can be learned in ten seconds or less by sitting in a hot car in the summertime. The technique is also dependent on a specific set of technologically advanced hardware: One must have on hand sheets of newspaper (or equivalent), towels, and/or a cardboard box. We find cardboard vegetable crates from the grocer or box lids from cases of copier paper to be ideal.

The most difficult part of the exacting, multiphasic PVD process to master is memorizing the steps, and keeping them in proper order. Step 1: Fill the boxes with greenery. Step 2: Place filled boxes on the car seats or dashboard during the day. 3. Check on the status of the dehydration process periodically, shuffling the herbs from bottom to top to encourage even drying.

DO NOT CONFUSE THE ORDER OF THESE STEPS. Hot, empty boxes do nothing to fill your spice rack or larder, and filled boxes not placed in a warm, dry environment take way too long to desiccate. Further, shuffling herbs that are not yet warm is energy better expended elsewhere.

{{Graduate Level Notes}}: You may consider cracking a window or two to allow for better air circulation, especially on particularly hot days. If you have progressed to this step, also keep an eye out for rain. Great knowledge brings even greater responsibility.

Caution: Your vehicle may smell like pizza for days after any dehydration cycle. Good luck, and good drying.

Ha, I've done this with rosemary and thyme on a hot day, works surprisingly well. The car gets up to proper dehydrating temperatures pretty fast in summer. Only issue is everything smells like herbs for a week after.

I've often mentioned this very idea as a comment to people who are building (or buying!) elaborate solar (or otherwise) dehydrators.  I once spent weeks building one of these, and it successfully dehydrated nothing!  In addition to a vehicle...the attic space of many houses, and any unused greenhouse or cold frame in the summer are viable alternatives.  The old greenhouse at the farm I used to live at, shut up and with a fan running on the screens, is how I know that a whole goat, boned out and sliced into small slivers, will fit into six quart jars when dried down!  Now, I regularly get sliced tomatoes snap dry in my attic...

I love it when my car smells like dehydrating stuff, onions... mmmmm.....

Alas - ecosystem is everything!

Cautionary notes:

1. some places are naturally very humid. PVD can result in large amounts of condensation on the windows when the temp drops at sunset.

2. Being in one of those humid spots, I empty my dehydrator onto metal trays and tuck them in the fridge to cool quickly in as low humidity as I have easy access to. After 15 minutes of cooling, I jar the dried food up quickly in glass and use a metal lid. Plastic allows humidity in. Some people have secure metal containers which also work so long as they seal thoroughly.

3. generally, driers suggest that food is safer dehydrated *not* in direct sunlight - thus seats maybe better than dashboard when possible.

When I taught energy efficient food preservation methods for many years at the Midwest Renewable Energy Fair I used to bring this method up first and foremost, telling folks that most of them probably already owned a food dryer without even knowing it. My more advanced method ;>) has the car parked with the largest window facing south, usually front but sometimes back window, rolling windows down about 1", and covering foods with a dark colored cotton cloth to keep UV light off foods to preserve color and vitamins. The windows should be closed at night if stuff isn't dry enough after the first day. I had a couple of neighbors who used this method almost exclusively.  One had an old 1970's clunker with lots of space and really big windows and he even dried blanched sweet corn on cookie sheets. Another used to work at Seed Savers in Decorah, Iowa and she "rescued" produce after harvesting seeds, especially sweet peppers, and had her car parked at work and would tend to stuff on lunch break.

Alder Burns wrote:I've often mentioned this very idea as a comment to people who are building (or buying!) elaborate solar (or otherwise) dehydrators.  I once spent weeks building one of these, and it successfully dehydrated nothing!  In addition to a vehicle...the attic space of many houses, and any unused greenhouse or cold frame in the summer are viable alternatives.  The old greenhouse at the farm I used to live at, shut up and with a fan running on the screens, is how I know that a whole goat, boned out and sliced into small slivers, will fit into six quart jars when dried down!  Now, I regularly get sliced tomatoes snap dry in my attic...

I'm curious about the dried goat meat, since your climate shouldn't be too terribly different from ours in south-central KY - does dried meat keep well? I've worried about it drawing moisture and getting moldy. (I have goats - and also two chest freezers, but I'd like to keep some meat in ways that don't require electricity, in case the power goes out. I could can some of the meat, and have done, but wondered if drying it would work here.)

Kathleen Sanderson wrote:

Alder Burns wrote:The old greenhouse at the farm I used to live at, shut up and with a fan running on the screens, is how I know that a whole goat, boned out and sliced into small slivers, will fit into six quart jars when dried down!

I'm curious about the dried goat meat, since your climate shouldn't be too terribly different from ours in south-central KY - does dried meat keep well? I've worried about it drawing moisture and getting moldy. (I have goats - and also two chest freezers, but I'd like to keep some meat in ways that don't require electricity, in case the power goes out. I could can some of the meat, and have done, but wondered if drying it would work here.)

What is jerky if not dried meat?  I know the process for making jerky is slightly more complicated than that, but not much.

Given the climate and the dehydrating technology, I'd worry about the total drying time required.  Raw meat is something you'd want to dry quickly, not over several days.  I'd want to keep the windows of my PVD closed to keep flies off of the meat, but that also restricts air flow.  Hence Alder Burns' mention of using a fan, I'm sure.  I'd also be very curious to read about Alder Burns' experience with this.

If one is already cutting meat into small enough pieces to be dehydrated - i.e. jerky slices - vacuum sealing the dried product in glass mason jars would be an excellent solution for long term storage without absorbing moisture out of the atmosphere.  A Food Saver with an accessory attachment + a jar sealing kit + some large mason jars is a modest investment that would do very well for preserving jerky... and any and all other dry goods!  I love mine, and I use the jar sealer far more often than the main plastic bag sealing function.  Compared to the plastic bags, mason jars for vacuum sealing dry goods are infinitely reusable and work more reliably in my experience.  Fairly large mason jars exist, and I have found that acquiring free, used mason jars is definitely possible if you scan the interwebs often.

Salt curing and smoking meats are also tried-&-true refrigeration-free methods for preserving meat.  There will be copious information available online for both techniques.  Plus, you can salt and smoke larger chunks of meat, up to and including whole joints; you don't have to process everything down to jerky-sized slices.

Matthew Nistico wrote:

Kathleen Sanderson wrote:

Alder Burns wrote:The old greenhouse at the farm I used to live at, shut up and with a fan running on the screens, is how I know that a whole goat, boned out and sliced into small slivers, will fit into six quart jars when dried down!

I'm curious about the dried goat meat, since your climate shouldn't be too terribly different from ours in south-central KY - does dried meat keep well? I've worried about it drawing moisture and getting moldy. (I have goats - and also two chest freezers, but I'd like to keep some meat in ways that don't require electricity, in case the power goes out. I could can some of the meat, and have done, but wondered if drying it would work here.)

What is jerky if not dried meat?  I know the process for making jerky is slightly more complicated than that, but not much.

Given the climate and the dehydrating technology, I'd worry about the total drying time required.  Raw meat is something you'd want to dry quickly, not over several days.  I'd want to keep the windows of my PVD closed to keep flies off of the meat, but that also restricts air flow.  Hence Alder Burns' mention of using a fan, I'm sure.  I'd also be very curious to read about Alder Burns' experience with this.

If one is already cutting meat into small enough pieces to be dehydrated - i.e. jerky slices - vacuum sealing the dried product in glass mason jars would be an excellent solution for long term storage without absorbing moisture out of the atmosphere.  A Food Saver with an accessory attachment + a jar sealing kit + some large mason jars is a modest investment that would do very well for preserving jerky... and any and all other dry goods!  I love mine, and I use the jar sealer far more often than the main plastic bag sealing function.  Compared to the plastic bags, mason jars for vacuum sealing dry goods are infinitely reusable and work more reliably in my experience.  Fairly large mason jars exist, and I have found that acquiring free, used mason jars is definitely possible if you scan the interwebs often.

Salt curing and smoking meats are also tried-&-true refrigeration-free methods for preserving meat.  There will be copious information available online for both techniques.  Plus, you can salt and smoke larger chunks of meat, up to and including whole joints; you don't have to process everything down to jerky-sized slices.

I’ve done something similar to this with my car except I used already cooked meat instead. I call it dashboard jerky. I Never could decide if doing it raw would work as you said it’s got to dry quickly.

During the summer If I get a good thin steak at a restaurant or one that I’ve cooked at home. I will wrap it in bunch of clean napkins and let it dehydrate on the dashboard. Takes a day or two in the summer heat here in the high desert. I don’t ever store it for long term but it gives the steak a fantastic jerky consistency. Raw meat just doesn’t sound like it would cure properly even in a hot car.

I've been doing the car-window dehydration (no fancy acronyms for me!) for years, with fruit. I just crack the windows a bit to let condensation out. Works on hot days. See pictures, including cracked door window. We don't use our cars as much as most families, so it's not too inconvenient. I've even done it in the back window while driving around town. I'll roll up the windows in the evening so bugs don't get in, or else bring the trays inside. Takes a couple days, maybe three, for prune plums. I use folded or rolled towels to angle the trays toward the sun better.

I also have done simple rooftop dehydration with an old glass shower door (with safety glass). See other picture and this youtube: https://www.youtube.com/watch?v=BJf-6XfN4uY . I later made a lightweight wood rectangular frame to fit under the glass shower door to keep the fruit a bit toastier and elevated off roof surface for more ventilation. But I'm not sure making that frame was really much improvement unless to keep the fruit safe from night critters.

For years I've done simply canning of dried fruit in mason jars with no sort of pasteurization. Just pack 'em nice-and-dry and don't fill the jar too high, and they keep without refrigeration for years. I've only noticed spoilage (a bit of mold) in maybe 2 of the probably 100 such fruit jars I've canned in past dozen years, likely because the fruit wasn't quite dry enough in those 2.

I'm skeptical about sunlight UV being bad for fruit. Can't get much more natural than sunlight. Would like to see actual science on those claims. Plus, it would just be MUCH more difficult to dehydrate passively without direct sun, so if there was a small tradeoff in some vitamins, it would still be worth it. KISS!

Yes! I love this method! I’ve dried apples from our trees in my car. No bugs. No mess!
Y’all have inspired me to dehydrate other garden goodies!

Kit Collins wrote:I'm skeptical about sunlight UV being bad for fruit. Can't get much more natural than sunlight. Would like to see actual science on those claims. Plus, it would just be MUCH more difficult to dehydrate passively without direct sun, so if there was a small tradeoff in some vitamins, it would still be worth it. KISS!

First of all, I love your photos illustrating PVD of plums into prunes.  Thanks for sharing!

But as for the quote above, I am baffled by your logic.  True, you can't get much more natural than sunlight.  But there are many 100% natural phenomena that stimulate the breakdown of complex organic molecules as part of the processes we lump under the term "decay."  That's great for nature as a wholistic system, but not so great for humans attempting to preserve edible foodstuffs.  I have made the mistake in the past of passively air drying herbs in direct sunlight, and they browned far faster than when I switched to doing so in the shade.

Though you make a good point that some nutrient loss may be an acceptable tradeoff to enable the entire passive solar dehydration system to work effectively before the food spoils.

Though my hands on experience is limited, my understanding of dehydrators is that you don't need heat in order to dry food; you need airflow.  Heat helps, since hot air can absorb more moisture, but it isn't strictly necessary given adequate airflow.  Hence some solar dehydrator designs collect solar energy upstream from the food, thus driving airflow without directing heat to the food itself.  But of course I'm now discussing more complex designs than the simple passive arrangements that are the subject of this thread.

Back to the issue of nutrient degradation.  I'm no nutritionist, so I'm not familiar with any original science to share.  Here is Google's AI summary, which both supports my assertion but also lists actual benefits of "controlled" and "moderate" exposure, neither of which adjective I suspect can be assigned to a solar food dehydrator:

Nutrients Most Affected UV light is particularly harsh on light-sensitive compounds. The most vulnerable nutrients include:
- Vitamins C & E: These are the most sensitive to light and will degrade rapidly upon UV exposure.
- Vitamins A & B2 (Riboflavin): Will quickly break down; for example, milk left out under UV light loses Vitamin A in just a few hours.
- Antioxidants: Can undergo oxidative degradation as they are used up neutralizing the reactive oxygen species (ROS) created by UV stress.
Unwanted Side Effects Beyond losing nutritional value, UV light accelerates fat oxidation, which causes fats and oils in foods to go rancid, leading to foul odors and off-tastes.
Beneficial Effects (Specific Cases) In some controlled agricultural and food science settings, low-dose UV treatment is intentionally used to boost specific nutrients:
- Vitamin D: Moderate UV exposure is famously used to boost Vitamin D levels in edible mushrooms and bread.
- Bioactive Compounds: Controlled UV-A exposure has been shown to temporarily increase lycopene and phenolic compounds in some vegetables like tomatoes.

But as for the quote above, I am baffled by your logic.  

Matthew, basically for me it's a matter of keeping things simple. I guess I've made things too complicated in the past and I'm reformed now! Natural and simple is generally better; complicated and artificial is generally not as good. That's become a matter of faith for me, but I'm still willing and quite able to read actual science. It's just that I haven't actually seen a study on the "science of nutrition in fruit dehydration" yet.  From my experience doing it, it occurs to me that it is nearly impossible to do dehydration in a low-tech manner without sun/UV exposure. It would only be possible in the hottest, most arid locations. So logically, it is very likely that the "ancients" would have typically dried fruit (and other food) with direct sun. Seems like they did OK with it too. So I'm not worried at all about it.

Kit Collins wrote:

But as for the quote above, I am baffled by your logic.  

Matthew, basically for me it's a matter of keeping things simple. I guess I've made things too complicated in the past and I'm reformed now! Natural and simple is generally better; complicated and artificial is generally not as good. That's become a matter of faith for me, but I'm still willing and quite able to read actual science. It's just that I haven't actually seen a study on the "science of nutrition in fruit dehydration" yet.  From my experience doing it, it occurs to me that it is nearly impossible to do dehydration in a low-tech manner without sun/UV exposure. It would only be possible in the hottest, most arid locations. So logically, it is very likely that the "ancients" would have typically dried fruit (and other food) with direct sun. Seems like they did OK with it too. So I'm not worried at all about it.

Fair enough.  As a general rule of thumb, I would agree: simple and natural are often preferable ways to design most systems.  Nor would I be overly worried about loss of nutrients if I were operating a passive solar dehydrator, such as a PVD.  I would say the results (slightly nutrient-deprived dried fruits and veggies) are acceptable for the amount of effort expended (minimal).  And for sure, people have dried foods under direct sunlight for millennia and similarly found the results acceptable.

Still, that doesn't make it optimal.  I for one would be willing to expend the minor expense of running an electric dehydrator (fast drying with minimal nutrient loss) or to expend the effort to build a more complex solar dehydrator along the lines of those designs Paul has published here on Permies (significant time to build, but free to operate with a large capacity).  But I'm not trying to turn this thread into a debate between different dehydrator designs.  Yes, as you said, a simple, passive, tray-of-fruit-in-the-window-of-your-car type system will almost invariably involve direct sun/UV exposure to your food.  So be it.

It was mostly just the way you phrased your statement - "Can't get much more natural than sunlight" - which struck me as odd, as it seems to imply that anything in its natural state or exposed to natural elements must be benign.  Apple trees and castor plants are lovely and very useful parts of nature.  But chew on the seeds of either and you will quickly discover how benign they are, since both are 100% naturally poisonous.

My opinion--it's fine to dry sliced fruit in direct sunlight, such as a dashboard--which I've done, in conjunction with using a dehydrator--but herbs should be dried out of direct sunlight. Here's how I do herbs: I have nails up and down the rafters in my kitchen--I also use these to hang onion braids on, and use the rafters to string a rope from which I hang ears of dent corn to dry enough for shelling and storing.  Most herbs I can clip a forked piece which can then just hang from the nail--if using finer ones, I suspend a clump of them from a string. After a few days, depending on the weather, the herbs are dry enough to take down and put in cake pans. If it has been unusually dry, low humidity, I may be able to strip the leaves off the stems now. Then the next time I use the oven, after turning it off and removing whatever I was baking, I put my cake pans in, at first with the door open and only a minute at a time. Usually they need a turn in the oven after being stripped off the stems to get to the crisp, crumbly stage from where I can put them in long term storage in glass jars.

Love this presentation :)   I've been doing PVD for years, although I add a small fan.   My ancient VW station wagon doesn't leave the driveway very often (I'm an old hermit)  so I clip a small fan to the back of the driver seat facing the rear (plugged into the garage by extension cord).   Then I place 4 bricks in the cargo area for spacers and top with a recycled 30" square window screen.   Spread one layer of herbs on the screen.  Crack a couple windows on the car and check a couple times a day to flip and stir all sides of the herbs.   It's remarkably fast on a hot summer day with low humidity :)  The smaller herbs dry in one day, comfrey and mullein dry in a couple days.  I just drive it around on short errands with the windows closed when I need to.  

Considering UV light damage to dehydrated foods:

Car windshields (front) and sunroofs block around 98% of all UV light (UVA and UVB). Side and rear windows vary, blocking most UVB, and 40 to 70% of UVA.
The roof tends to block 100% of all solar radiation, but does a good job of holding in the heat through thermal transfer and convection.

So, concerns about losing vitamins through vehicular dehydration are probably misplaced. My personal goal is to produce and preserve more food than we need. If we lose a little bit of the vitamin content from each slice of fruit, my answer will be to eat two slices.

Yeardly Arthur wrote:Car windshields (front) and sunroofs block around 98% of all UV light (UVA and UVB). Side and rear windows vary, blocking most UVB, and 40 to 70% of UVA.
The roof tends to block 100% of all solar radiation, but does a good job of holding in the heat through thermal transfer and convection.

Good point!