Dehydrating basics: thickness, airflow, and how to actually tell when food is done

It depends mainly on slice thickness, and the relationship is close to linear: a slice cut twice as thick takes roughly twice as long to dry, all else equal. This is why a recipe's stated time range, whether it comes from a machine like the Excalibur Select Series or any other dehydrator, is only useful if your slices match the thickness it was written for. A manufacturer's baseline range assumes a specific slice thickness in a dry kitchen; thinner slices finish faster, thicker ones take proportionally longer, and everything scales from there.

Yes, by a meaningful amount. A humid kitchen adds around 25 percent to a drying run, because the air already carries more moisture and has less capacity to pick up additional moisture from the food on each pass through the machine. This is true regardless of how good the dehydrator is: airflow moves moisture away from food by absorbing it into the surrounding air, and air that starts closer to saturated simply cannot absorb as much per pass. If a kitchen runs humid, plan for the longer end of any time range from the start rather than checking at the shorter end and being surprised.

Not on a machine with a rear or horizontal fan. That airflow layout pushes air across every tray at close to the same rate, which is exactly why models like the Cosori 5 tray, the 10 tray version, and the Magic Mill Pro never ask you to swap trays top to bottom during a run. Older stacked round dehydrators pull air vertically through a column of trays, and the tray closest to the fan dries measurably faster than the one furthest away, which is the specific problem tray rotation exists to fix. If a dehydrator has a rear or horizontal fan, that problem simply does not apply to it.

Cool a test piece completely before judging it. Warm food always feels softer and less dry than it actually is, because heat keeps the surface moisture mobile and the texture flexible; the same piece cooled to room temperature will feel noticeably firmer and drier within a few minutes. Once it is cool, judge by texture: properly dried fruit is leathery and bends without snapping, while vegetables should be brittle enough to snap cleanly rather than bend. For anything you plan to store for more than a few days, seal a small amount in a jar and leave it a full day: condensation on the inside of the glass means there is still too much moisture left, and the batch needs more time before it goes into long term storage.

Case hardening happens when the drying temperature is set too high for what is being dried, and the outer surface dries into a hard shell before the moisture inside has had a chance to migrate out. That shell then traps the remaining moisture underneath it, so the food looks and feels dry on the outside while staying wet in the center, which is exactly the kind of hidden moisture that leads to spoilage in storage. The fix is temperature discipline rather than more time: running a lower temperature for longer gives moisture time to move from the center to the surface before the surface seals over, while cranking the heat to finish faster is the single most common way to cause it.

Herbs want a notably lower temperature than fruit or vegetables, around 95 F, because their flavor comes from volatile oils that break down and evaporate at higher heat, and drying them too hot leaves a dull, flat herb with much of its aroma gone. Fruit and vegetables tolerate more heat than herbs without the same loss, which is part of why a single dehydrator setting is rarely right for everything you might dry in it. Whatever the setting, silicone sheets with raised edges are worth adding to the tray any time the food being dried is pureed or very wet, since raised edges are what keep a fruit puree from running off the tray before it firms up into leather.