A large number of local herbs in India and Southeast‑Asia belong to food‑medicine‑homologous raw‑materials. More and more processors replace traditional sun‑drying and hot‑air‑drying with vacuum freeze‑drying to preserve plant polyphenols, volatile oils and other active substances to maximum extent. Nevertheless, herbal materials feature complex fiber structure. Some herbs contain high volatile‑oil content and suffer activity loss, volatile‑oil escape and fragment‑curling defects during freeze‑drying.
Pretreatment plays vital role. Sort herbs and remove rotten impurities; choose chopping or whole‑plant processing according to plant parts (leaves or rhizomes). Complete washing quickly and avoid long‑time soaking to prevent massive dissolution of water‑soluble active components. Fully drain surface water before tray‑loading. Some rhizome herbs shall be sliced into uniform thickness for consistent freeze‑drying performance.
Pre‑freezing requirements differ for leaf‑type and rhizome‑type herbs with distinct eutectic points. Thin leaf‑materials need shorter pre‑freezing duration. Thick rhizomes require lower temperature and longer holding‑time for complete core‑freezing. Incomplete pre‑freezing causes partial melting in sublimation phase and severe destruction of active substances.
Protect volatile oils in sublimation phase. Volatile‑oils easily escape under high‑temperature conditions, so shelf temperature shall not be set too high. Do not blindly pursue fast cycle; over‑rapid sublimation aggravates loss of volatile effective constituents. Adopt moderate‑vacuum to balance sublimation efficiency and activity retention.
For discharging: finished herbal goods have well‑developed pores and absorb moisture very fast. Inert‑gas back‑filling is recommended during pressure‑release. Start packaging immediately after discharging and minimize air‑exposure duration. Moisture‑absorbed herbal products suffer quality degradation and are vulnerable to mildew.
Besides, pay attention to change‑over cleaning. Many herbs release special volatile odor. Thorough cleaning and purging are mandatory before switching to other food‑material batches to prevent cross‑flavor. Full‑chain control including sorting‑washing, differentiated pre‑freezing, low‑temperature sublimation and fast discharging‑packaging enables high‑activity freeze‑dried herbal output.
