Flat dual-frequency sweeping ultrasound enhances the inactivation of polyphenol oxidase in strawberry juice

In the current study, the effects of flat sweeping frequency and pulsed ultrasound (FSFPU) application with different frequency modes on the inactivation of polyphenol oxidase (PPO) in strawberry juice were investigated in the temperature range of 40–60 °C. The sweeping frequency modes employed includes single-frequency modes of 22 ± 1, 40 ± 1 kHz, and dual-frequency of 22 ± 1 and 40 ± 1 kHz in simultaneous modes. Results suggested that the inactivation patterns of PPO in strawberry juice all followed the first-order kinetic model (R2 = 0.965–0.997). The D values of PPO during thermal inactivation at 40–60 °C were in the range of 126.5–22.8 min, respectively. Remarkable decreases of the D values of PPO can be observed under the different ultrasound frequency modes. They were 100.1–20.3 min (22 ± 1 kHz), 73.3–18.6 min (40 ± 1 kHz), and 56.9–14.5 min (22/40 ± 1 kHz). The activation energy (Ea) during thermal inactivation was 77.25 kJ/mol, which decreased to 73.65 kJ/mol (22 ± 1 kHz), 62.86 kJ/mol (40 ± 1 kHz), and 60.17 kJ/mol (22/40 ± 1 kHz) during thermosonic inactivation, respectively. The results showed that both thermal and thermosonic treatment could effectively reduce the PPO activity in strawberry juice. Moreover, the effect of PPO inactivation was remarkably enhanced by the FSFPU treatment, and dual-frequency mode ultrasound had the highest effect of PPO inactivation. It can be concluded that flat sweeping frequency and pulsed ultrasound can be used as an efficient and low-cost method to facilitate the inactivation of PPO in strawberry juice.