The dehydration process
Dehydration is a centuries-old process for preserving food products. The reduction in water content inhibits both microbiological and enzyme activity.
By reducing the weight of the food, transport and storage is also simplified.
Effective dehydration requires heat, adequate air velocity over the product, low humidity and minimising the product thickness. The time taken for a batch can vary from a few hours for some herbs, to around 20 hours for many fruits, and a week or more for macadamia nuts.
At the commencement of the drying cycle, the maximum amount of heat is introduced. The surface of the product is wet and the evaporation rapidly consumes heat. The humidity is at it’s maximum (typically around 70%) and the temperature slowly climbs towards the set point.
As the surface becomes dryer, the temperature increases and the humidity falls. With some products there is a tendency for a semi-impervious skin to form and it may be beneficial at this stage to continue for a period with heat only, allowing the humidity to rise temporarily while the moisture diffusion within the product re-softens the skin and improves the progression of drying. This action can be pre-set in the controller, to suit the product.
Although higher air temperature promotes faster dehydration, any benefit may be offset by product degradation due to loss of nutrients and/or flavour due to the evaporation of volatile substances. Typical drying temperatures using lower humidity are 45 – 50C.
Batches of most fruit products can usually be dehydrated in 18 – 20 hours, depending on the initial thickness of slices. Nuts and similar products within a shell, may take a number of days.