Inactivation and structural changes of polyphenol oxidase in quince (Cydonia oblonga Miller) juice subjected to ultrasonic treatment.

BACKGROUND: Polyphenol oxidase (PPO) is considered a problem in the food industry because it starts browning reactions during fruit and vegetable processing. Ultrasonic treatment is a technology used to inactivate the enzyme; however, the mechanism behind PPO inactivation is still unclear. For this reason, the inactivation, aggregation, and structural changes in PPO from quince juice subjected to ultrasonic treatments were investigated. Different intensities and times of ultrasonic treatment were used. Changes in the activity, aggregation, conformation, and structure of PPO were investigated through different structural analyses. RESULTS: Compared to untreated juice, the PPO activity in treated juice was reduced to 35% at a high ultrasonic intensity of 400 W for 20 min. The structure of PPO determined from particle size distribution (PSD) analysis showed that ultrasound treatment caused initial dissociation and subsequent aggregation leading to structural modification. The spectra of circular dichroism (CD) analysis of ultrasonic treated PPO protein showed a significant loss of α-helix, and reorganization of secondary structure. Fluorescence analysis showed a significant increase in fluorescence intensity of PPO after ultrasound treatment with evident blue shift, revealing disruption in the tertiary structure. CONCLUSION: In summary, ultrasonic treatment triggered protein aggregation, distortion of tertiary structure, and loss of α-helix conformation of secondary structure causing inactivation of the PPO enzyme. Hence, ultrasound processing at high intensity and duration could cause the inactivation of the PPO enzyme by inducing aggregation and structural modifications. © 2019 Society of Chemical Industry.

Highly sensitive fluorescence sensing of zearalenone using a novel aptasensor based on upconverting nanoparticles.

A facile strategy was successfully developed for the detection of zearalenone (ZEN). In this assay, highly fluorescent upconversion nanoparticles were synthesized and conjugated with the complementary oligonucleotide of ZEN aptamer for use as signal probes. Magnetic nanoparticles immobilized with the ZEN aptamer were assigned as capture probes. The results exhibited that the linear correlation between the decreased luminescence intensity of the signal and the concentration of ZEN was very strong (R2=0.9957) in the range of 0.05-100µg/L. In addition, the limit of detection of the proposed method (0.126µg/kg for corn and 0.007µg/L for beer) was significantly lower than the existing methods. Furthermore, the reliability of the competitive immunoassay format was validated by comparing the results determined in real food samples to those obtained from a commercially available method. Overall, the novel aptasensor have showed great potential for rapid and accurate ZEN determination.