Quality Changes in Fresh-Cut Lettuce When Subjected to Ultrasound Combined with Zinc Oxide Nanoparticle (ZnO NP) Treatment

The effects of three preservation methods (ultrasound, ZnO NPs, and ultrasound combined with ZnO NPs) on the odor, microstructure, and edible quality of fresh-cut lettuce were investigated in this study. When stored for 8 days, significant improvements were observed in the following when using ultrasound combined with ZnO NP treatment to better preserve fresh-cut lettuce (and were reduced when compared with the control group): the color (L* value (34.53); a* value (−5.89); b* value (15.00); browning index (40.63); firmness (25.66); propectin (2.12%); chlorophyll (2.75 mg/100 g); cellulose (20.35%); total phenolic content (0.95 mg/100 g); PAL activity (54.91 U·h⁻¹·g⁻¹); CAT activity (41.78 U·min⁻¹·mg⁻¹); ABTS free-radical scavenging ability (137.62 µmol/L); FRAP total reducing ability (94.42 µmol/L) (p < 0.05), PPO activity (0.85 U·min⁻¹·g⁻¹); MDA (1.97 µmol/g); and H₂O₂ (54.26 µmol/g). In addition, the results of the volatile components indicated that the use of ultrasound combined with ZnO NP treatment decreased the production of adverse flavor compounds by inhibiting the generation of aldehydes and ketones, as well as by promoting the generation of olefins, nitriles, and quinolines, and the contents of nitriles and quinolines were 20.07% and 2.07% of the total components, respectively. The resultant microstructure indicated that the microchannels generated by ultrasound allowed for the ZnO NPs to enter the intracellular cavity of the fresh-cut lettuce more efficiently; such a finding could serve as a basis for a hypothesis on the mechanism of ultrasound combined with ZnO NP treatment. The results of fresh-cut lettuce preservation when using ultrasound combined with ZnO NPs were better than those that were obtained when using ultrasound and ZnO NP treatment alone. And, using ultrasound combined with ZnO NP treatment as a new preservation method for fresh-cut lettuce provides a promising preservation idea for other fresh-cut fruits and vegetables.