Wash and dry the apples. Using a mandolin slicer or a sharp knife, slice the apples thinly and evenly between ⅛ inch and ¼ inch thick, so they can dry properly.
Make a pretreatment solution (acidulated water) by combining the lemon juice or white vinegar with one quart (approximately one liter of water) and soak the apple slices in it for 10 minutes. Drain off the pretreatment solution, place the apples on a baking tray, and pat dry using a kitchen towel. Make sure to dry them properly before dehydrating them to enable them to dry faster.
Place the apple slices on top of the dehydrator tray or rack in a single layer without overlapping and sprinkle some cinnamon on top. You can also sprinkle the cinnamon once they have dehydrated if you prefer a less intense flavor. Make sure there is some space between the slices to enable air to circulate properly. Carefully place the rack inside your Ninja Foodi.
Set your Ninja Foodi to the dehydrator function at 135°F (57.2°C), close the lid, and set the time to 8 hours (see notes for how to air fry if your air fryer does not have the dehydrator function).
Dehydrate the apples, quickly checking on them at every 3-hour interval until they develop a crispy texture and feel dry to the touch. Dehydrate them for a shorter amount of time, if you prefer your apple chips leathery.
Once the apples have dried, switch off the dehydrator and remove the rack. Allow the apple slices to cool to room temperature.
Once cooled, transfer the dehydrated apple chips to a bowl. Sprinkle with some cinnamon on top (optional) then transfer them to airtight jars or containers, properly labeled with a best-by date. Store in a cool, dry, and dark place.