Drying techniques
Most Western Australian native flowers are dried by simply hanging in an airy darkened room. A large area is required if substantial quantities of flowers are to be dried because the process may take four to six weeks. Fading of colours and browning of foliage are difficult to avoid.
Solar
The greatest advantage of solar drying is that it is cheap. All that is needed is a black painted tin shed or a black plastic tunnel. An efficient solar dryer can reduce drying time to less than three weeks.
Solar drier efficiency can be maximised through by sound design principles. The dryer should be located for maximum exposure to sunlight and drying winds. Design features should allow for good ventilation and air circulation within the structure to eliminate pockets of humid air.
The structure should be reasonably airtight to prevent humid air entering at night or during wet weather. Otherwise, the plant material can easily pick up moisture from the atmosphere which will delay drying.
Hot air
Drying can be effected in two to three days by blowing heated air (60 to 80°C) over the plant material. Because drying is so rapid, a small drying chamber is able to process a large quantity of material. Retention of natural colours is normally better than solar drying.
Practical methods of heating are limited to electricity, gas or oil. Electricity is about 50% more expensive than gas per unit of energy (kilojoule). The price of oil varies but is usually between electricity and gas. One disadvantage of oil is that it is sold on a pay before use system rather than the use, then pay system for gas and electricity.
The combustion gases from both oil and gas contain water vapour, which decreases their drying efficiency relative to that of electrically-heated hot air. Oil and gas burners also constitute a greater fire hazard than electricity.
Once the hot air has passed through the foliage and picked up moisture, it can be either exhausted to waste or dried and then recirculated to conserve energy. Recirculation introduces the complexity of removing water from the hot air. This can be done with either a dehumidifier or by using desiccants.
Popular industrial desiccants can be either solid (calcium chloride) or liquid (glycerol, glycols). The moisture saturated air can be either passed through a bed of solid desiccant particles or bubbled through the liquid desiccant before returning it to the dehydration chamber. To remain effective, the desiccant must be recharged at regular intervals by heating for several hours at 100°C.
Dehumidifiers
Dehumidifiers recirculate air in a closed chamber and remove the water vapour by passing the air through a chilled condenser at one point in the cycle. The dried air can be re-warmed before being recirculated through the plant material. Dehumidifiers dry plant material rapidly (24 to 48 hours drying time) and are suitable for a large throughput.
For maximum heat efficiency, the compressor coils should be located inside the drying room (see below). Complete units are available commercially. Alternatively, most refrigeration companies can design and install a custom-made unit.
Microwave
Microwave energy has the peculiar attribute of being preferentially absorbed by water and hence is a particularly efficient energy source for the process of drying. To date only experimental quantities of flowers have been dried using microwaves.
Pulsed applications of power are needed rather than continuous microwaving to prevent vigorous boiling of the water within the plant with consequent structural damage. The high cost of a commercial microwaving unit is the major barrier to the widespread use of this method. The cost of generating microwaves is also much greater than the cost of generating energy by other methods.
Freeze
Freeze drying relies on the principle of sublimation, whereby ice held under conditions of partial vacuum (less than 4.58 torr) and low temperature (less than 0°C) will evaporate on heating without going through a liquid phase. The absence of liquid water during the dehydration process means that undesirable chemical reactions will not occur. Hence, colour and even fragrance are retained in the dried article. Because the material to be dried is held in the frozen form an important benefit is that no shrinkage or distortion can occur.
A freeze dryer consists of a vacuum chamber, vacuum pump, condenser (to remove water vapour) and a heat source (to supply the energy for sublimation). The most delicate part of the operation lies in transferring the heat to the frozen material at just the right rate to avoid melting.
Conduction plates are most commonly used as heat transfer devices although it is doubtful they would be suitable for whole plant material. Radiant heaters are more likely to be effective for this purpose. Microwaves have been tried but have proved unsuitable.
Rapid freezing is the key to successful freeze drying as this prevents the formation of large ice crystals which cause structural damage. Liquid nitrogen or solid carbon dioxide (dry ice) are popular media.
Drying rate depends on the thickness of the material, but realistic rates of water loss are around 1.6 kilogram per hour per square metre of plant material surface area. This corresponds to a drying time of between 2 and 10 hours. Commercial units usually have a load capacity of between 10 and 20kg of plant material per square metre of dryer space.
Normal operating parameters are vacuum chamber pressure of 0.1 to 0.2 torr and heater input to give a maximum surface temperature on the material to be dried of between 37 and 82°C.
Sand and silica gel
Many delicate flowers irreversibly lose structure during normal air drying. The petals may be held in the desired shape by supporting the flower in fine-grained sand which has been thoroughly dried in an oven. Silica gel, borax and cornmeal or cat litter products may also be used and have the advantage of faster drying because their desiccant activity extracts water from the petals. Drying normally takes only one or two days, with excellent colour retention.
Slight pock marks on the dried petals are normal and it can be difficult to remove some grains of support media if nectar was present on the flower. External heat can also be used during drying to speed up the process but the temperature must be kept below 80°C to avoid scorching. The most common problem with flowers dried in this manner is excessive brittleness caused by overdrying as there is no ready method of monitoring the drying rate.
Silica gel is expensive but can be re-used indefinitely by drying it overnight in an oven set at 110°C.
Acknowledgements
The material for this web page was originally written by Paul Dubois and Daryl Joyce.