Poria cocos is characterized by the dried sclerotium of a fungus in the Polyporaceae family. It can often parasitize on the roots of Pinaceae plants, thereby serving as a commonly used conventional Chinese medicinal material in China. Post-harvest Poria cocos is required for the appropriate primary processing at the origin, before it is used as Chinese herbal decoction pieces. Some primary processing has been widely selected in the main producing areas of Poria cocos, including the static sweating, steam sweating, and pharmacy sweating. Among them, static sweating can share the long processing cycle and high labor intensity. The content of water-soluble polysaccharides can significantly improve their solubility, whereas the content of triterpenoid components can decrease significantly. The steam sweating can be expected for the high processing efficiency and yield. There is a significant decrease in the content of water-soluble polysaccharides. Therefore, the pretreatment can be utilized to retain the medicinal components of Poria cocos. This study aims to clarify the primary processing of Poria cocos at the origin for the high drying efficiency and product quality. The optimal sweating was obtained for the Poria cocos pretreatment. A systematic investigation was made to explore the vacuum-pulsation infrared drying and the quality of Poria cocos cubes. Poria cocos was pretreated by static sweating, steam sweating, and pharmacy sweating. An analysis was carried out on the moisture content and water-soluble polysaccharide content after pretreatment. Furthermore, the Poria cocos was cut into cubes of 8, 12 and 16 mm after sweating treatment under different vacuum times (2, 5, 10, 15 and 25 min) and atmospheric pressure times (2, 4, 8 and 12 min). Finally, the drying performance was evaluated on the crushing rate, shrinkage rate, color, and water-soluble polysaccharide content. The pretreatment results indicated that the moisture content of Poria cocos peels remained unchanged during steam sweating, while the red Poria cocos and Poria cocos varied from 49.87% and 51.95% to 52.57% and 53.84%, respectively. However, the steam sweating led to the inactivation of the mycelium of Poria cocos, thus preventing the secondary growth. In static sweating, there was a decrease in the moisture content of Poria cocos peels, red Poria cocos, and Poria cocos. In pharmacy sweating, there was a gradual decrease in the moisture content of Poria cocos and Poria cocos peels. The moisture content of red Poria cocos at the end of sweating was higher than that of static sweating. Both static sweating and pharmacy sweating were involved in the secondary growth. The water-soluble sugar content was the highest after 3 days of pharmacy sweating, leading to optimal sweating. The vacuum pulsed infrared drying test was performed on the Poria cocos cubes. The results showed that the drying time increased first and then decreased with the increase of vacuum time and normal time. The smaller the size of Poria cocos cubes were, the shorter the drying time was. The smaller the size of Poria cocos cubes, the higher the breakage rate and the lower the shrinkage rate were; The larger the size, the lower the breakage rate was, and the higher the shrinkage rate was. The brightness (L*) decreased first and then increased with the extension of vacuum time and normal time. And the content of water-soluble polysaccharides increased first and then decreased with the extension of vacuum time. When the size of Poria cocos cubes was 12 mm, the vacuum time was 10 min, and the normal time was 4 min, the high drying efficiency and dry product quality were achieved with a breakage rate and shrinkage rate of 22.45% and 25.42%, respectively, L* of 53.25, and a water-soluble polysaccharide content of 35.21 mg/g. The optimal sweating was identified for the Poria cocos pretreatment. The optimal vacuum pulsed infrared drying was determined for high efficiency and quality control during drying. The finding can provide a specific and feasible technical solution to the modern and standardized production of Poria cocos.

Drying temperature, air velocity and humidity of drying medium are the three important parameters during hot air drying of fruits and vegetables. Among them, drying temperature and air velocity are positively related to drying efficiency. Relative humidity (RH) is usually used to reflect the size of the humidity or humidity content of the medium at the constant drying temperature and total pressure. However, it is still unclear on the influence mechanism of RH on drying. The manual regulation of relative humidity cannot fully meet the large-scale production, due to the low quality, drying efficiency, and high energy consumption. Therefore, this study aims to clarify the influence and control mechanism of RH on the heat and mass transfer during hot air drying of fruits and vegetables, in order to reduce the emission and loss for energy saving with high efficiency. The drying quality was optimized in the control of RH in four aspects. The low RH was used to increase the mass transfer coefficient and the evaporation of water on the surface of the material. While the high RH was to increase the convective heat transfer coefficient and the heating rate of the material. In the high RH, there was the increase in the temperature rise rate of the material, and the internal water migration, but there was the low evaporation of surface water. In the low RH, there was a low heating rate of the material and the internal water migration, but there was a high evaporation of surface water. As such, there was the coupled influence of RH on the heat and mass transfer. High RH was mainly reflected in the heat transfer, whereas, low RH was in the mass transfer. High RH was used to avoid crust formation on the surface of the material, in order to improve the rehydration for the less shrinkage rate. The honeycomb porous structure was formed and maintained on the surface of the material under the step-down dehumidification and multi-stage dehumidification drying, leading to high drying efficiency and quality. An optimal RH control was achieved using material temperature. The stage dehumidification and drying were attributed to: the relative magnitude of thermal resistance to heat transfer and internal heat conduction, and the relative magnitude of mass transfer resistance and internal mass transfer resistance during drying. The heat and mass transfer resistance depended mainly on the drying conditions, types and thicknesses of materials. The stage dehumidification and drying were suitable for the drying of this material at Bi_h>1 and Bi_m>0.1. The influence mechanism was given on the relative humidity during hot air drying of fruits and vegetables. The finding can also provide the theoretical basis and technical support to the influence and control mode of RH in the hot air drying of fruits and vegetables.

Step-down relative humidity (RH) means that the RH is gradually reduced to improve the drying efficiency and quality in hot air drying. The hot air drying with step-down RH has been successfully applied to the yam slices, Dahongpao pepper, and papaya slices drying. This study aims to regulate the step-down RH in the period of dehumidification, in order to improve the efficiency and quality. Three stages of dehumidification were two-, multi-stage dehumidification drying and RH control, according to the material temperature. The internal moisture migration and surface moisture were also selected to represent the changes in RH. Specifically, there was an increase in the surface moisture evaporation, when the RH shared an upward trend. Once the RH showed a downward trend, the internal water moisture increased much more than the surface water evaporation. As such, the RH control regime was as follows, according to the RH changes. In the early drying, the evaporation of the material itself caused the RH rise. The middle drying then reached, after the RH was stable. If the RH shared a downward trend, the dehumidification was closed to improve the RH, in order to reduce the surface water evaporation (E) and improve the internal water migration (D). If the RH was rising, the dehumidification was opened to reduce the RH for the high E. If the moisture evaporation on the surface was not enough to increase the RH, or the temperature of the material approached the drying medium temperature, the drying was transferred to the later stage. At the same time, the dehumidification was also opened to reduce the RH for the high E value. The drying end point was reached when the change rate of RH was less than the critical range. Under a drying temperature of 60 ℃ and air velocity of 3.0 m/s, the drying test of the carrot showed that 0-8 min was the early drying, and the RH change rate was less than 0.5% at 8 min. Afterward, the carrot entered the middle drying. The duration of the RH decreasing trend was gradually shortened in the drying period from 8 to 131 min. Whereas, there was the extended in the duration of RH rising trend. Within 137-143 min, there was no rising trend of RH in the period of late drying. At 370 min, the RH change rate was less than 1%/min, indicating that the drying process ended. The RH control mode was used to increase the D and decrease the E value. Consequently, the D and E values remained basically equal within 0.2-2.3 h. Meanwhile, the material temperature presented a step-rising trend correspondingly. Within 0-0.2 h, the accumulation of water (Q) raised rapidly in the material, where the layer of water film was formed to wrap, particularly without the outstanding crust. Within 0.2-2.3 h, the Q value fluctuated up and down at the zero point. The drying rate decreased gradually to produce three zero points in total. The water migrating to the surface was immediately evaporated on the surface without accumulation, indicating the delaying time of crust occurrence to remove a large amount of water. Furthermore, the Q value was gradually less than 0, where the surface of the material produced the outstanding crust to gradually thicken after 2.3 h. After that, the rehydration ratio and shrinkage rate were (4.41±0.02) g/g and (27.32±1.51) %, respectively, at the drying time of 6.1 h. The drying time was shortened by 24.6%, compared with the constant 20%RH. More water migration channels were retained to realize the automatic control of RH. The finding can also provide the theoretical basis and technical support for the RH control during hot air drying of fruits and vegetables.