1. Select the fullest ripest produce, free of blemishes and insect damage.
2. Wash them, slice them into manageable pieces be it halves, quarters or even smaller for very large fruits and vegetables. Remove large pits and seed, particularly from stone fruits such as plums.
3. Load your produce onto dehydrator trays, or baking sheets. Be sure to leave a small separation between each slice - do not allow them to touch or overlap.
4. 130 - 140 degrees F is best for most dehydrators. If using an oven subtract about 5 degrees. Always allow for some air circulation, with dehydrators this is already done for you, with an oven - leave the door open.
5. Most dehydrators will take a full day - 24 hours. Check on them periodically as your schedule allows, they should be leathery but flexible, slightly moist and a tad tacky. Re-position them periodically as some may stick to the mesh or sheets
6. Over-Drying - You never want any dehydrated fruit or vegetable to be 100% moisture free - if that were the case you would end up with some facsimile of rock candy.
If by chance you do over dry them, they can generally be re-hydrated slightly by immersing them in cool water briefly and leaving them out overnight [after taking them out of the water]. Tomatoes are sometimes re-hydrated in olive oil.
7. Oxidation - Light colored fruits will frequently oxidize, turn brown. This is most noticeable in apples and pears. You can pre-treat them by dipping them in ascorbic acid for a few minutes prior to initiating the drying process. This is not necessary with darker fruits such as plums.
Sun drying is the oldest method, it dates back to our primitive ancestors. Warm sunny days of summer and early Autumn are ideal , temperatures of 85 to 100 F for sun drying are optimal.
Techniques differ very little, be it tomatoes or plums, peaches or onions. The fruit is generally sliced in halves, or quarters for larger produce and set out in direct sunlight, either on screens or drying trays. Insects can at times be bothersome, placing plastic over the produce will keep the bugs away, but hamper air flow. Placing a screen over the produce will also work so long as it has a fairly high mesh count that allows for air flow and does not inadvertently shade the produce. Cheesecloth or fine netting will also suffice.
Dew can be a problem. If you anticipate morning dew, sheltering the trays indoors overnight is advisable, a porch, greenhouse or garage. Needless to say you'll need to bring them inside should rain be looming as well.
In the case of tomatoes and some vegetables Salt can be lightly sprinkled on the cut surface to help draw moisture from the produce. Use your judgement on this, naturally you don't want a salty prune but a pinch of saltiness to sun dried tomatoes can't hurt.
Rotating dryer trays and turning the produce on a regular basis will help to ensure they dry uniformly. One overly moist spot on a dried fruit or vegetable is an open invitation to mold - fungus and can spoil an entire batch.
Do not attempt to dehydrate your produce in a microwave oven, it just doesn't work. An electric dehydrator will maintain a steady reliable heat, use relatively little energy and the end result is palatable.
I generally set the dehydrator temperature at 135F to 140F. If the dehydrator doesn't have a thermostat, a reliable thermometer on the bottom tray will suffice. Place the produce on the dehydrators trays , leaving an inchh or two between trays. It may be necessary to turn the tomatoes, plums, or what have you and rotate the racks while drying. Towards the end of the drying cycle, your produce can easily scorch, so be sure to examine them occasionally and remove any finished product.Over-dried fruits are tough, at times like rawhide and can be difficult to cook with and even harder to chew. If under dried enough, they can easily mold.
If you don't have a dehydrator and don't want to buy one at this time Oven Drying will also work for small batches.
A reliable oven thermometer is advisably placed on the top tray in order to monitor the temperature. Keep the door open, or you can't end up with mush. Rotate the racks for even drying. A temperature 125 - 135 F is best.
How to Make Prunes
Sun Dried Tomatoes
Sun Dried Goji Berries