Effect of ultrasonic treatment on enzyme activity and bioactives of strawberry puree

The aim of this study was to evaluate the impact of ultrasound (US) at different frequencies (20, 370, and 583 kHz) and power levels (35 and 48 W) on the residual activity (RA) of peroxidase (POD) and polyphenol oxidase (PPO) in strawberry puree. Total anthocyanin content (TAC), total phenolic content (TPC), ferric ion reducing antioxidant power (FRAP) and trolox equivalent antioxidant capacity (TEAC) were also assessed. Results were compared with untreated, thermally treated at 40 °C (control) and pasteurized (90 °C) strawberry puree. POD and PPO RA were significantly (P < 0.05) reduced, whilst there was a significant (P < 0.05) increase in TAC (5%–19%) in all US-treated samples in comparison with the untreated samples and the controls. US at 20 kHz (35 W) increased significantly (P < 0.05) TPC (9%) and FRAP (6%) in strawberry puree, whereas the effect of 583 kHz (48 W) on these parameters was insignificant (P > 0.05). Pasteurization inactivated POD and PPO, however, decreased dramatically TAC (14%), TPC (17%) and FRAP (9.5%) in strawberry puree. These findings suggest that US is a promising novel non-thermal food technology that can be tailored to improve the quality of strawberry puree by inactivating enzymes responsible for food deterioration whilst maintaining the content of bioactive compounds.