Comprehensive evaluation on drying efficiency and quality attributes of star anise using different pretreatment and drying methods

To identify the optimal overall drying performance, the effects of various pretreatments, namely hot water blanching (HB), freeze-thawed (FT), and ultraviolet irradiated (UV), followed by different drying methods, including Sun drying (SD), hot air drying (HAD), infrared radiation drying (IRD), microwave drying (MD), and vacuum freeze drying (VFD) on the drying efficiency, quality attributes and comprehensive score of star anise were investigated. The results indicated that FT effectively shortened drying time, reduced shrinkage, and increased the a* value of dried star anise, without significantly affecting the retention of volatile oil (VO) content and shikimic acid (SA) content. UV only enhanced the redness value under certain drying methods and had no significant impact on other efficiency or quality parameters. In contrast, while blanching pretreatment accelerated drying, it significantly reduced the retention rates of volatile oil and shikimic acid. Among the drying methods, VFD maximized the retention of VO content (10.47%) and SA content (10.27%), yielding the highest product quality but with extremely high drying energy consumption. The process combining freeze-thawed pretreatment with microwave drying (FT-MD) performed excellently in terms of drying time, drying energy consumption (EC), appearance quality, and retention of key chemical components. This process achieved a drying time of 5.25 h, EC of 4.04 kW·h, shrinkage ratio of 33.90%, VO content of 7.99% and SA content of 9.77%, leading scores in multiple evaluation models and is therefore recommended as a balanced solution considering production efficiency, product quality, and processing costs. The findings of this study provide valuable methods and foundational data for the further precise processing of fresh star anise.