Effects of different drying methods on the quality and volatile components of Eucommia ulmoides male flowers

Eucommia ulmoides male flowers contain various biologically active substances, such as phenylpropane compounds, cyclic ether terpenes, lignans, and flavonoids. Eucommia ulmoides male flowers also share the antibacterial, anti-inflammatory, analgesic, anti-aging, anti-fatigue, antiviral, anti-mutagenic, anti-tumor, immune-enhancing, sedative, weight-reducing, blood pressure-lowering, and central nervous system-stimulating functions. However, the content of high moisture in flowers has made them susceptible to browning and rot, thus limiting their economic and medicinal application. Therefore, Eucommia ulmoides male flowers need to be dehydrated promptly after harvesting, in order to better preserve their nutritional value. However, there are no studies on the most suitable drying for Eucommia ulmoides male flowers as raw materials. The current research aims to fill this research gap. Five drying methods were also selected as vacuum freeze drying, microwave vacuum freeze drying, hot air drying, far infrared radiation drying, and heat pump drying. Fresh Eucommia ulmoides male flowers were used as the study object. A systematic investigation was made to explore the effects of different treatment methods on the quality characteristics, volatile components, and energy consumption. The results showed that microwave vacuum freeze drying was the most efficient, requiring only 6 hours, 57% less time than vacuum freeze drying. Color analysis revealed that the vacuum and microwave vacuum freeze drying shared the larger L* values (49.20 and 47.99, respectively) and smaller total color differences (ΔE), 5.34 and 5.74 respectively. There was also very similar to the fresh Eucommia male flowers. Scanning electron microscopy revealed that smoother surface morphologies were observed in the samples that dried by vacuum and microwave vacuum freeze drying. But there were the outstanding wrinkles in the rest. In terms of antioxidant capacity, the samples after microwave vacuum freeze drying presented the strongest DPPH free radical scavenging ability (94.74 mg/g) and ferric-reducing antioxidant power (39.63 mg/g). The ABTS free radical scavenging of the samples treated under microwave and vacuum freeze drying was significantly higher than that of the rest. Additionally, 27 compounds were detected in the volatile components. Among them, microwave vacuum freeze drying produced the most volatile components with 18 kinds, the number of 1.8 times in the fresh Eucommia ulmoides male flowers. Correlation analysis was also performed on the types, content, and antioxidant capacity of volatile components in Eucommia ulmoides male flowers before and after drying. There were varying degrees of correlation between L* and the content of geniposidic acid, chlorogenic acid, and aucubin, as well as their free radical scavenging. While there was no significant correlation with the types of volatile components. Lastly, a comparison was conducted on the energy consumption required for different drying methods. The vacuum freeze drying required the least energy consumption, whereas, the microwave vacuum freeze drying required the least. In conclusion, the drying dominated the significant impact on the quality characteristics and volatile components of Eucommia ulmoides male flowers. Therefore, the most suitable drying was set as the microwave vacuum freeze drying. The reason was that the drying time and energy consumption were reduced to effectively maintain the quality and volatile components of Eucommia ulmoides male flowers. This finding can provide valuable insights for the application of microwave vacuum freeze drying, particularly for the drying processing, innovation, and utilization of Eucommia ulmoides male flower products in food and medicine industries.