That time of the year again Had an idea flash for a DIY food preservation technique for any kind of food, where the dried food can be stored at ambient temperatures for about 25 years and preserves much of its nutrients, texture and taste. Energy requirements for drying fresh produce are about 0.3 kWh/kg, 2-3 times lower than for otherwise comparable but much less DIY freeze-drying. So with 3 kWh daily excess electricity from a 1000 W photovoltaics off-grid plant, one could dry 10 kg of produce, or 20 kg if using ~40 °C thermal heat for 50% of the energy needs. Processing times are 20 min to 2 hours per batch, also much lower than the 24 h to 36 h for freeze-drying. Materials needed are just a used household microwave, a used household fridge compressor, some tubing and cabling, and an Arduino microcontroller. Probably together still below 100 EUR. With adaptations for running on 12-24 V DC instead, probably below 250 EUR.
For those wanting to try: the gory details are after a little sketch.
The gory details
It’s not freeze drying. The proposal is a DIY microwave vacuum dryer that consumes excess photovoltaics energy and excess heat. This is not freeze drying, because the product is never frozen (thereby also preventing damage through freezing). Instead, the phase change will always be between liquid and vapor, but accelerated greatly by vacuum and microwaves, which makes the processing times so short that the above-ambient temperatures do not damage or microbiologically spoil the products.
Process requirements. The process is commercially developed by (for example) EnWave, calling it the “Radiant Energy Dehydration (REV)” process. According to this sources and an older nutraRev Fact Sheet that vanished from their redesigned website, the process can use an input temperature of 37 °C even for meat products. Which seems to mean, the 37 °C can be applied as thermal energy, and additional energy is applied via microwaves. So the process works at all pressures where water boils at 37 °C or below. Which means about 6000 Pa according to the phase diagram of water.
Parts to use. These low vacuum requirements are good news, as it allows to use a recovered standard fridge compressor as the vacuum pump, which can achieve pressures down to about 1500 Pa (as demonstrated in this video by boiling water at 15 °C). It is also (probably) possible to combine two such compressors in series to achieve a better vacuum. Finally, DIY building of a vacuum chamber from a kitchen dish, plus suction hoses, is explained in another video, “How to Make a Freeze Dryer”. The last part is getting the moisture out of the vacuum – either by just using the vacuum pump (if it does not get damaged that way) or by using a condenser as in freeze dryers: a cold surface to remove the moisture by freezing it, possibly provided with a small Peltier element. In freeze dryers, this surface is usually -50 °C, while in our case (due to the less perfect vacuum), -10 °C to -20 °C would be enough. In addition, we will need a microwave oven; the cheapest solution is a thrown-out AC grid microwave. Solar thermal input (at about 40 °C) is also good, as it reduces the amount of microwave energy needed.
Operation. So one would place a dish of food under a vacuum bowl (normal kitchen glas bowl) into the microwave, extract the air, and apply microwaves. The process duration would depend on available excess photovoltaic energy, but under optimum conditions would be 20 min to 2 hours (as specified for the EnWave nutraRev system).
Energy costs. Energy costs are 0.23 USD per kg of dried berries for nutraRev, which at about 0.12 USD/kWh U.S. electricity costs maps to 2 kWh / kg of dried berries. Now berries (blueberries in this example) have a 85% water content, meaning the energy need is 2 kWh / (1 kg / 0.15) = 0.3 kWh / kg of fresh blueberries.
So for comparison: If you have a 8 m² photovoltaics off-grid plant plant, that’s about 1000 W§, and maps to at least 3 kWh “freely available” excess energy per day in summer, which you just don’t need for normal living. You can dry 10 kg of fruits per day with this. Or probably 20 kg, by also using thermal solar input energy compared to the all-electric energy used in the EnWave process. Seems decent for preserving excess food from dumpster diving quick enough
Sealing and storing the food. For storing the food “with a shelf-life of 24.5 years”, it seems to be enough to just put it into containers and vacuum-seal them (again using the fridge compressor as vacuum pump). One can additionally put oxygen absorbers into them (which is just iron oxide), or rely on vacuum alone. These techniques are explained in the video “5 Year Food Storage: Lisa B on Freeze Dried Storage Methods”. So there is no (strict) need to replace the vacuum with an inert gas and sealing the product in this atmosphere, as usually done in commercial settings.
Usage in the lab
Revisiting this idea for a possible use as a small-scale lab dryer, I’ll add the following thoughts:
There will be a bit of an issue with shielding the microwave radiation as we need to route the vacuum tube through the wall of the microwave oven. However letting it make a 90° bend right after exiting the wall and adding metal shielding so that the tube is sandwiched between the microwave oven and the additional shielding will probably fix this. There are handheld microwave leak sensors that could be used to prove that this shielding works.
A Peltier element inside the vacuum chamber to remove the water vapor through freezing of course won’t work. It contains wires, and metal in a microwave means lots of sparks … . The Peltier element can however be mounted in a second part of the vacuum chamber, located outside of the microwave and connected by a vacuum-proof tube. Water gas molecules travelling through the tube would hit the Peltier element on the other side and freeze stuck to it. With that arrangement of the cold element outside the microwave oven, there is more space and instead of a Peltier element it would be possible to use the inner parts of a commercial deep freezer to create a way more energy efficient cold element that easily reaches -20 °C.
Using fridge compressors as vacuum pumps is highly experimental and would need fixing the lubrication issue that happens when not operating them in a closed loop environment like a fridge where the lubricant is mixed into the pumped medium. Without a closed loop, lubricant has to be continuously added at the input side of the compressor somehow, or it will run dry and break soon. So for a lab device, better use an off-the-shelf vacuum pump.