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This study aimed to investigate the effects of different ultrasound-combined pretreatments on the microwave drying efficiency and dried product quality of star anise，identify the optimal pretreatment method and its underlying mechanism，and thereby provide reference for the green，efficient，and energy-saving drying processing of star anise.
Fresh star anise was subjected to three pretreatments as raw materials：ultrasound （US），ultrasound combined with steam blanching （US-SB），and ultrasound combined with freeze-thaw treatment （US-FT），with samples directly subjected to microwave drying without pretreatment served as the control （CK）. Effects of the three pretreatments on the microwave drying characteristics，physicochemical quality，and antioxidant activity of star anise were systematically analyzed，and the underlying mechanisms were further elucidated through microstructural observation and low-field nuclear magnetic resonance analysis.
All three pretreatments disrupted the cellular structure of star anise and increased the number of water-migration pathways. Compared with the CK group ［drying time of 261.67 min and specific energy consumption of 14.60 （kW·h）/kg］，the drying times of the US，US-SB，and US-FT groups were reduced by 8.92%，46.50%，and 32.48% respectively，while their specific energy consumptions decreased by 13.08%，50.00%，and 33.56% respectively. The US-SB group exhibited the highest drying efficiency. However，polyphenol oxidase （PPO） activity was almost completely inactivated，resulting in no significant difference in the redness value （a*） of dried product compared with the CK group （P>0.05，the same below）. Moreover，its volatile oil content （6.59%），shikimic acid content （8.20%），and 1,1-diphenyl-2-picrylhydrazyl （DPPH） radical scavenging rate （53.95%） were significantly lower than those of the CK group （P<0.05，the same below）. PPO activity in the US and US-FT groups was significantly higher than that in the CK group，and the a* values significantly increased by 15.74% and 15.64% compared with CK group respectively，demonstrating a brighter red color；the DPPH radical scavenging rates increased significantly by 22.28% and 25.53% compared with CK group respectively；no significant differences in volatile oil or shikimic acid contents were observed between these two groups and the CK group. The volume shrinkage rate of US-FT group （41.88%） showed no significant difference with the CK group，whereas those of the US （45.78%） and US-SB （54.18%） groups were significantly increased. Low-field nuclear magnetic resonance analysis showed that the US and US-FT pretreatments increased the free water peak area and promoted water migration，whereas the US-SB pretreatment reduced free water and increased immobilized water. Correlation analysis revealed that a* had an extremely significant positive correlation with the DPPH radical scavenging rate （P<0.01，the same below），while the volume shrinkage rate showed extremely significant negative correlations with volatile oil content，shikimic acid content，and DPPH radical scavenging rate.
Consi-dering both drying efficiency and product quality，ultrasound combined with freeze-thaw pretreatment exhibits the best overall performance，which can shorten drying time，reduce energy consumption，improve star anise color，effectively retain volatile oil，shikimic acid，and antioxidant activity，without increasing volume shrinkage ratio. This technology provides a feasible approach for the green drying and processing of star anise.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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