Shielded greenhouses: useful if drying on racks is not suitable.
They are PVC or Plexiglas greenhouses of various sizes, closed, where the racks are arranged inside or the drugs are collected and hung on bunches, as in the case of tobacco. The screened greenhouse is therefore a greenhouse that has the possibility of shielding the roof, which is necessary to avoid excessive temperature rise inside the room.
On racks the drying temperature reflects the environmental temperature; in the case of shielded greenhouses, on the other hand, the temperature can become a problem, because it can reach much higher values than outside. However, when the mercury column rises too high, the shield goes to cover the roof and the operators open and ventilate the environment. This procedure therefore involves intervention and human control, so that the temperature does not exceed certain values; values that are closely related to the thermal sensitivity of the active ingredients that make up the phytocomplex. In fact, thermolability is a common element in many classes of secondary metabolites.
The drying temperature of certain drugs cannot exceed 40-45 ° C, since at higher values the thermal sensitivity of some active principles leads to a qualitative deterioration and prevents their medicinal use.
If for certain drugs the temperature is a binding element the drying method, for others, however, it does not constitute a problem, because the active principles do not degrade at the temperatures normally used. Even if some drugs are dried at 100 ° C, for example in a stove, beyond this threshold, the morphological aspect of the drug is always altered.
Most drugs are dried at temperatures ranging from 40 to 60-65 ° C. In the range between 60 and 80 °, coriaceous and woody drugs are generally dried, while at temperatures ranging from 35 to 60 ° the herbaceous drugs are dried.
In screened greenhouses the temperature is controlled by an operator by hand, so that it remains in the most suitable ranges. However, there are shielded greenhouses with a self-heating system, therefore equipped with an automated temperature control. These are greenhouses that adopt a particular heating strategy, which can be a boiler, or a system of cavities (two parallel roofs) between which the air is heated by solar radiation; when the temperature reaches too high values, automatically open ventilation structures that bring it back to the required parameters.
