The surface dried faster than the water could travel out from the middle, and sealed itself. It is called case hardening, and four settings cause it.
What is happening in cross section
Drying works from the outside in. Water has to move from the centre to the surface before it can leave. If the surface loses water faster than the centre can supply it, the outer layer shrinks and hardens into a skin.
That skin is now a barrier. The middle stays wet, the piece feels dry to the touch, and a month later there is a spot of mould in the jar.
Cause 1: too hot at the start
The most common one. Starting fruit at 70 C to save time sets the surface before the middle has moved. Fruit wants 55 to 60 C for the whole run.
Higher temperature does not dry food faster in any useful sense. It dries the outside faster, which is the opposite of what you want.
Cause 2: cut too thick
Every extra millimetre lengthens the distance water has to travel, and the effect is not linear. A 10 mm apple slice takes far more than twice as long as a 5 mm one, and is much more likely to case harden.
Cut 4 to 6 mm, even, and keep pieces the same thickness across the tray.
Cause 3: overloaded trays
Pieces touching each other, or trays packed to the edge, block the airflow that carries moisture away. The result is uneven drying: dry hard edges, wet centres.
Leave a finger of space between pieces, and if you have added extra trays beyond what the maker sells, take them out.
Cause 4: no rest, very dry air
In very dry conditions, high sugar fruit like plums, figs and grapes benefits from switching the machine off for an hour partway through. The moisture inside redistributes towards the surface and the second half of the run goes evenly.
This is the one people find counterintuitive. Stopping the machine finishes the batch faster.
Can you rescue a batch?
| State | What to do |
|---|---|
| Sticky, bends, no mould | Back in at 55 C for 2 to 4 hours, then condition a week |
| Hard skin, soft centre | Slice the pieces open and re-dry, or use them in cooking within days |
| Already jarred, condensation inside | Re-dry the whole batch, do not just wipe the jar |
| Any mould or sour smell | Throw the batch out |
Questions people ask
Can you fix case hardened fruit? Often, by cutting or re-drying at a lower temperature and then conditioning.
Does a lower temperature take much longer? A little, usually an hour or two on a fruit batch, and it is the difference between a jar that keeps and one that does not.
Why is my fruit leather sticky in the middle? Same mechanism. Spread thinner, around 4 mm, and dry at 57 to 60 C.
Safety note. A case hardened batch is a spoilage risk, not just a texture problem. Condition before sealing and discard any batch that shows mould.
What this is based on
- National Center for Home Food Preservation, drying fruits
- University extension publications on drying temperatures and case hardening
- Manufacturer guidance on tray loading