4. Conclusions
Chopped peppermint plants (Mentha piperita L.) including
leaves and stems were dried in a rotary drum dryer (a prototype
dryer with the capacity of 15 kg fresh material) to preserve product
quality and conserve energy. The rotary drum dryer removed
moisture from the drying materials using heated air at a constant
temperature profile (∼36 ◦C) or a rectangular wave-shaped temperature
profile (∼36–37 ◦C in 45min and ∼52–58 ◦C in 15min in a
cyclic fashion). The material was dried until the moisture content in
the leaves reached to 9–11% as determined by the rapid microwave
oven method. The leaves and stems of peppermint plants had different
drying curves, with leaves drying faster than the stems. The
first cutting from the peppermint plants dried more slowly than
the second cutting of peppermint plants since the leaves and stems
of the first-cut samples contained more biomass. The cutting order
of harvested peppermint plants affected their quality parameters
such as color, essential oil content and essential oil composition.
The rectangular wave-shaped air temperature profile was more
effective in drying peppermint than the relatively low and constant
air temperature. Variations in ambient air conditions drastically
changed the specific energy consumption of dryer. Specific energy
consumption values ranged 0.468–0.864 MJ kg−1 water ◦C−1. Hot
air drying of peppermint plants in the rotary drum dryer caused
considerable change in color but conserved the main essential oil
content. In addition, forced hot air drying favored menthol content
of dried peppermint leaves. Natural variation between the
properties of peppermint plants and climatic conditions during
drying was the major obstacle to obtaining reproducible results
for product quality, energy consumption and required drying
time.