1Before you dehydrate your tomatoes, take out seeds and juices from tomatoes as best you can.
Dehydrate tomato slices cut thinly until they are crunchy and not soft. They will dehydrate quicker if cut very thin. I have also found that they could stick to the dehydrator racks so I brush a bit of olive oil (do not make areas wet). It really helps release the slices.
2You can use your convection oven turned down to 170F degrees and leave door ajar, about an inch. If you have to pry oven open, do so just a little, to allow air flow. Let them dehydrate until they are not soft. It could take a bit..maybe 5 hours or so...depending on your oven and thickness of your tomato slices. Remove the ones that seem done to you and allow the others to continue dehydrating. You can also use a dehydrator or just your regular oven. Do the same as you would do for the convection oven, put on lowest temperature around 170F degrees and pry open just a little (maybe an inch. It may take a little bit longer in a regular oven.
3After tomatoes are dehydrated without any soft spots (if so, they will milddew and spoil). Use a blender or if you have a Magic Bullet, they work well for powdering. Make sure no pieces left in mix.
Put in either jars with perhaps 'Press and Seal' wrap over top before covering or vacuum seal. Vacuum sealing is best and store on your pantry shelf! Don't forget to label them. Bravo, you just made Powdered Tomatoes for Prepardedness or use in casseroles, spaghetti sauce, meats etc. Anything you would need tomato in and add spices accordingly. They will last a very long time-6-12 months, depending on how and where you store them.