Degradation behavior of polyphenols in model aqueous extraction system based on mechanical and sonochemical effects induced by ultrasound

In this study, the temporal and spatial variation of mechanical effects (ME) and sonochemical effects (SE), the degradation behavior of polyphenols as well as the correlation coefficients were comprehensively analyzed. The ME and SE showed inverse dependence along with the axial direction of transducer. Higher irradiation distance (50 mm) enhanced the ME but inhibited the generation of hydroxyl radicals (8.29 μmol/L). Rutin and cyanidin-3-glucoside, compared with thermal degradation (70°C, 6.41%, and 11.45%), were more susceptible to free radical degradation due to the boost of SE at low temperature (10°C, 20.23%, and 34.41%). The concentration of hydroxyl radicals at 70°C (18.38 μmol/L) was about 5.77 times lower than that at 10°C (3.22 μmol/L). No significant differences in the ME were observed for a wide range of temperatures (10–70°C). The impacts of increasing duty cycle on further promoting the ME and SE were limited. The concentration of hydroxyl radicals (0–23.37 μmol/L) increased linearly with sonication time (0–45 min) and the degradation of caffeic acid only occurred on a time scale (25 min). The free radical scavenger can minimize the degradation of polyphenols by inhibiting the hydroxyl radicals (23.37 μmol/L without t-BuOH; 12.17 μmol/L with t-BuOH). The above results showed that the degradation of polyphenols attacked by hydroxyl radicals presented a dose-response relationship. The correlation between the hydroxyl radicals and polyphenols as well as the mechanical effects and extraction yield indicated that the stability of polyphenols exposed to ultrasonic field depended on the structure-radical scavenging activity relationship and the extraction may be accompanied by the degradation.