Dehydrators take the water out of food to prevent rot, mold, bacteria including dangerous organisms like botulism.
Properly dried and stored food can last for years or even decades. Dried food saved ancient cities under siege as they waited out the enemy. Pemmican made of dried meat and tallow was a survival staple for First Nations, frontiersmen, and voyageurs in North America as well as Polar explorers North and South. Dried food was also a staple in the age of sail.
Where weather cooperates, and people have time, food can be dried outdoors in the sun. In the rain forest where we live the sun may work if we get a few days of good weather when food is in season but consistent sun is unreliable so for dehydrating food we generally use other means: dehydrators, ovens, racks behind the wood stove.
Dehydrators: You may be lusting for a freeze drying machine and while there’s nothing wrong with that, a dehydrator is a fraction of the cost and for some foods; like corn, carrots, mushrooms, onions, I think it does as good a job or better than the freeze dryer. They usually take less time to process in the dehydrator and you don’t have to pre-freeze them. I’ve eaten dried corn that’s been sitting in the pantry two years. Toss it in a stew or chowder, let it rehydrate, and it’s hard to tell apart from frozen. On the other hand some foods, like bananas, taste good but stick to the trays and generally don’t seem worth it.
Maybe you can share one with several families: Most of us only use a dehydrator for a week or two per year, then it sits on the shelf. If you pool your resources you can split the cost of a good machine.
What to look for in a dehydrator:
1) Adjustable thermostat: they generally work between 85 F and 165 degrees F with lower temperatures for delicate herbs and higher temperatures for jerky. The one pictured below has a color coded guide next to the thermostat.
2) A timer so it’ll shut itself off if you’re otherwise occupied.
3) A fan to circulate air.
4) Trays. And depending on what you’re drying, the trays may want a mesh screens mat so when your dehydrated food shrinks up it doesn’t fall through slots in the trays all the way down to the bottom. If you’re dehydrating runny stuff like yogurt or fruit leather you’ll want silicone drying sheets line the trays.
5) Size: It’s tempting to get the biggest one you can afford, but is that wise? True, you can dry more food at once, but once it’s done, you have this big thing sitting on the shelf for most of the year.
6) Non-digital: For some reason people these days are in love with digital connections. They plug in their phones to their cars, to their appliances, to their home heating systems, lights, security cameras, door locks, the bank, social media…Jeez Louise! Most people who embrace this stuff think they have more control of their lives but it’s the opposite. Non-digital appliances are usually less expensive and work fine. This is personal preference but really, do you need another digital screen that craps out for no apparent reason and you have to search the internet for a fix, or order new parts, or buy a new machine? Do you need everything connected to everything else? Do you need to be monitored by the developers of digital stuff who sell your information to others? Ick. 
This is an Excalibur we borrow from our kids who got it as a wedding gift. It’s mid-range in the company’s price selection. Does a great job without bells and whistles you don’t need. Note there are screens on the trays so the corn doesn’t fall through.
Shrinkage: Dehydrated foods tend to shrink up. Some, like blue berries and rhubarb shrink up to almost nothing and don’t plump up to their former selves when rehydrated. Others like corn and carrots, plump up nicely.
Blanching: Briefly boiling or steaming fruits or vegetables (2 to 4 minutes usually) then cooling them quickly. This stops enzymes in the food that would affect texture, flavor, and quality. Blanching is for fresh produce. If you’re dehydrating frozen packaged vegetables, they’ve usually been blanched by the processor so you don’t need to blanche them again.
Dehydrating in the oven: if you don’t have a dehydrator you can use the oven. Most ovens run hotter than dehydrators and tend to bake the food even at the lowest settings. Some people leave the oven cracked open and direct a fan towards it. That works in a pinch but confuses the oven which keeps kicking on the heat because its thermostat registers the heat as being too low.
Dehydrating in racks behind the wood stove: I don’t use this method because we’ve got the dehydrator but drying behind the stove has worked for centuries and doesn’t need electricity. Wood stoves give off dry heat. Back in the day we did a dive job in the small Native community of Yakutat, Alaska (raised a sunken fishing boat. Had the whole town for an audience). The couple we stayed with were drying pink salmon behind the wood stove. They’d split the fish down the body leaving the tails intact so they could hang the fish upside down on the racks. Processing fish this way is something you’d want to be shown the ‘ol how-to by someone who knows what they’re doing: how to cut the fish so it dries properly, how to test to make sure it’s done and so on. *
*A concern for everyone in food storage is botulism. With dehydrators, you won’t get rid of botulism spores in food but you can bring water content in foods down below the level where botulism spores can multiply. Here’s a cogent explanation from someone calling themself ‘Zombie Honey-Badger’ on the Survivalist Boards Website:
The clostridium spores are not destroyed by dehydrating, but neither can they grow in a low-moisture environment. Any organism requires a minimum level of moisture to live and reproduce (water activity or aw) , which for Clostridia botulinum is actually pretty high, at about .93 aw.
The water activity scale runs from 0.0 (bone dry) to 1.0 (water). Dry corn is about .14, dry milk is about .2 aw, dried fruit is about .6, aged cheddar is about .85 , bread is about .95.
As a rule of thumb, if a food will support the growth of mold, the aw of that food is above .75. If your food won’t grow mold, then it absolutely won’t grow botulism.
So the answer is, yes your dehydrated foods are safe, whether on a shelf in a dry cave or in a mylar bag with O2 absorbers.
