Achillea

32 DRYING AND PRESERVING
longer to dry; so do those with high moisture content. Temperatures in the drying
shed vary widely, averaging 70–120F (21–49C). Humidity levels are seldom
controlled by smaller producers and generally reflect the outside humidity. Airdrying
equipment with humidity and temperature control is popular with larger
processors and those whose natural environment is humid. Optimum humidity
levels of 20–60% should be monitored by all processors.
Some materials are best dried in darkness, others in sunlight. Drying sheds
with the ability to adjust the amount of light will allow the grower to dry a range
of materials. Most plant materials, when exposed to sunlight, fade to pale yellow,
which is advantageous when material is to be dyed—a pastel shade is easier to
color than green. This sun bleaching is used by many processors in preparation
for drying; grasses, for example, must be bleached if light color shades are to be
produced. Sun bleaching also provides an autumnal look for grasses, grains,
and thin-stemmed flowers. If the natural plant color is to be retained, drying in
the absence of light is recommended.
Required drying times: Drying times vary considerably, depending on species,
location, drying shed design, and season. Drying times also depend on the
amount of water in the fresh material and the desired water content of the dried
product. This is known as the dry fraction. Dry fractions for all crops are best
obtained by doing simple weighing experiments at the beginning and end of the
drying cycle. This can be done by occasionally weighing individual bunches; to
obtain the dry fraction, divide the dry weight of the plant by the fresh weight. For
example, sinuata statice is approximately 70% water, therefore the dry fraction is
30% (or 0.3). If 100 pounds (46 kg) of fresh sinuata statice is to be dried, drying
is complete when the weight is 30 pounds (13.6 kg). In general, drying times
range from 3 days to 2 weeks in a passive system. In an active system, plant materials
typically dry in 24 hours or less. Failure to adequately dry a plant can lead
to serious mold problems if material is sleeved and boxed too early.
Bunch size and handling: Stems are generally grouped in bunches for resale.
Bunch size is determined by the desired weight of the dried product and the dry
weight fraction of that product. If the final weight of a dried bunch of sinuata
statice (dry weight fraction = 0.3) is to be 4 ounces (114 g), then the initial fresh
bunch weight should be 4/0.3 or 13.3 ounces (379 g). Bunches with too many
stems may reduce air circulation within the bunch, and bunches should not be
placed so close together as to reduce air movement between them. They are normally
hung on strings or wires from the roof, and it is common to date each line
as stems are hung, to ensure proper drying times. In active systems, plants are
often placed on drying racks that can be rolled into the drying chamber.
Mold and insect problems: Poor air circulation, prolonged periods of high
humidity, excessively large bunch size, and overcrowding in the drying facility
are common causes of mold formation. Low humidity and adequate air flow
greatly reduce mold problems. Insects can be treated with chemicals, but these
are highly restricted and require licensing. Heat is an effective way to reduce
insect problems but is usually only possible in active systems or in passive systems
where a heat source is available. Mark Koch (1996a) suggests the following
exposures to control insects: