Case hardening: causes, detection and prevention
It looks finished. It is not.
Case hardening is a sealed, dry outer layer forming while the centre of a piece is still wet, because surface evaporation outran the rate at which moisture could migrate outward. The product passes a surface check, holds moisture inside, and moulds weeks later in a buyer’s warehouse. It is prevented by staging temperature and holding air velocity at 2.5–3.0 m/s.
The mechanism, in one paragraph
Drying is two processes racing each other: water evaporating from the surface, and water migrating from the centre to replace it. While migration keeps up, the piece dries evenly. When evaporation outruns migration — because the air is too hot, too dry or too fast for that product — the surface layer loses its water, its structure collapses inward, and it becomes far less permeable than it was. From that moment the interior moisture has no route out, and the rest of the cycle achieves very little.
Why high-sugar fruit is the worst case
Sugar makes it worse in two ways. It binds water, so migration from the centre is slower to begin with. And as the surface dries, the sugar concentrates further into a dense, glassy layer that is a far better barrier than the original tissue was. Mango, dates, figs and pineapple all sit in this category, which is why the drying schedules for them are staged rather than flat.
Thickness compounds it. The migration distance from centre to surface scales with thickness, while surface evaporation does not, so a thicker slice widens the gradient the whole cycle has to manage. On mango, 5–8 mm is the working range for exactly this reason.
How to detect it before a buyer does
The two cheap checks are cut and weigh. Cut a piece from the middle of a rack and look at the cross-section: a distinct dry rim around a translucent, softer core is case hardening, and it is visible to anyone who looks. And weigh the batch against the expected yield: a batch that has stalled short of its target weight loss while the surface reads dry is a batch with water still inside it.
Surface moisture meters will not catch it, which is precisely the problem — they read the layer that is dry. Water activity measured on a homogenised sample will, because it reflects the whole piece rather than its skin.
Prevention is a schedule, not a setting
| Lever | What it does | Working range |
|---|---|---|
| Staged temperature | Starts gently while the surface is most vulnerable, rises as the interior gradient falls | Within 65–70 °C for mango |
| Air velocity | Carries away surface moisture at the rate it arrives — not faster | 2.5–3.0 m/s across the bed |
| Humidity control | Keeps the air from being so dry that surface evaporation outruns migration early on | Managed through the cycle |
| Slice thickness | Sets the migration distance the cycle has to manage | 5–8 mm for mango |
| Cycle time | Long enough to let migration keep up | 16–20 hours for mango |
The instinct that causes case hardening is raising temperature to shorten a cycle. On high-sugar fruit that reliably makes the cycle longer, because the sealed surface then has to be dried through. The counter-intuitive move — more air at a lower temperature — is usually both faster and better.
Frequently asked questions
Can a case-hardened batch be recovered?
Sometimes, by holding it in a warm, humid environment long enough for moisture to redistribute through the piece, then resuming drying gently. It costs time and it rarely fully restores texture. It is far cheaper to prevent: an even 5 to 8 mm slice and a controlled ramp inside the 40 to 100 °C range do more than any single equipment feature.
Does it happen to products other than fruit?
Yes. Fish is the other common case, with a food-safety consequence attached: a sealed surface traps moisture that supports spoilage inside a product that reads as dry. It is why fish runs at 45–50 °C with humidity control rather than being driven harder, and why final moisture is specified at 15–20 % on a whole-piece basis.
What air velocity is right for my product?
2.0–3.0 m/s across the bed covers most food products, with 2.5–3.0 m/s the usual band for sliced fruit. Below that, surface moisture lingers and cycles lengthen; above it, evaporation can outrun migration on delicate products, and light material starts to move on the tray. It is set per product on the PLC recipe rather than fixed for the chamber.
Related equipment
Have a question this does not answer?
Our engineers answer processing, fuel and control questions directly. Tell us the product, the throughput and the fuel you can source.
Ask an engineer