Mechanism of freezing treatment to improve the efficiency of hot air impingement drying of steamed Cistanche deserticola and the quality attribute of the dried product

Cistanche deserticola is a famous herb owing to its kidney tonifying and anti-aging value. Freshly harvested Cistanche deserticola has an extremely short shelf life even under low temperature conditions due to chilling. Steaming followed by drying is the main way to extend its shelf life. However, the energy consumption of the drying process of the steamed Cistanche deserticola (SCD) is extremely high due to the low drying efficiency caused by steaming. This study investigated the effect of freezing treatment (FT) on the drying characteristics and quality attributes of SCD from the aspects of cell wall, microstructure, texture properties and moisture state by using X-ray micro-tomography, scanning electron microscopy and low-field pulsed nuclear magnetic resonance analyses. The Results indicated that FT improved the tissue porosity and electrolyte leakage due to ice crystal formation, reduced the tissue hardness due to the microstructural disruption, enhanced the water mobility through the conversion of bound water to free water, as well as altered the cellular structure by reducing the cell wall thickness, which promoted the moisture migration during the drying process and revealed the mechanism of 37.55 % reduction in drying time and 36.64 % reduction in energy consumption, for frozen samples. Regarding the quality attributes of the dried SCD with different FT times, rehydration capacity decreased by increasing the FT time but was still higher than the unfrozen sample. With FT for 7 days, the destruction of the internal structure shortened the drying time and enhanced the extraction process, increasing the bioactive content and antioxidant activity of SCD by 13.90 % and 48.71 %, respectively. The obtained results are expected as a experimental basis for processing and storing Cistanche deserticola on an industrial scale.