Tunnel drying is an industrial drying method in which product moves on trays or belts through a series of temperature- and humidity-controlled zones inside an insulated tunnel, allowing staged drying profiles that protect colour, aroma and nutritional quality.
In trade practice
Suppliers set temperature, airflow and belt speed profiles for each product type; buyers should request these parameters in the technical data sheet as they directly affect colour retention, essential oil content in herbs and final moisture. A common mistake is running inlet temperatures too high to speed throughput, which can cause case hardening on the product surface and trap residual moisture inside, leading to mould during storage. QA teams should verify final moisture content and water activity on finished batches, not just at the dryer outlet. Tunnel drying is widely used for sliced vegetables, apple rings and herbs destined for retail and food service.
In a specification
A specification for a tunnel-dried product should state the target moisture content and water activity at point of dispatch, and may require the supplier to confirm the drying profile used and the frequency of in-process moisture checks.