Hydrothermal Treatment Effectively Inhibits the Conversion of Rutin to Quercetin in Flos Sophorae Immaturus Compared With Dry Heating: Based on Degrading Kinetic Analysis

Flos sophorae immaturus (FSI) is rich in polyphenols and has been consumed as a health food. This study aimed to explore the impacts of dry heating (DH) and hydrothermal treatment (HT) on the phenolic content of FSI and their degrading kinetics, thereby elucidating the phenolic conversion patterns during processing at different temperatures. Results showed that DH led to a substantial decrease in total flavonoids and total phenols in FSI from 5 to 60 min at a maximum of 210°C. Conversely, FSI subjected to HT exhibited an opposing trend at a maximum of 90°C. Moreover, the highest rutin content (170.2 mg/g) was observed in HT-FSI at 90°C for 120 min, whereas quercetin exhibited the lowest content (5.4 mg/g), indicating a potential termination of conversion. Correspondingly, the rutin-degrading enzyme (RDE) activity of HT-FSI (0.814 U) decreased beginning at 50°C for 120 min and reached the lowest level (0.015 U) when treated at 90°C for 60 min. In contrast, the RDE of DH-FSI (0.463 U) was inactivated under the same processing conditions, yet its lowest level (0.041 U) remained significantly higher than that of HT-FSI. Furthermore, the first-order kinetics revealed that the activation energy of rutin extracted from the HT-FSI (63.15 kJ/mol) was higher than that of the DH-FSI (26.97 kJ/mol), suggesting that rutin conversion was much more difficult during HT. Overall, treating FSI with HT at 90°C for 60 min might be an effective method for rutin retention by inactivating RDE activity.