Antimicrobial capacity of rare earth-doped ZnO composite coating and its influence on fresh orange shelf life quality

Antimicrobial capacity of food grade ZnO particles can be enhanced by doping rare earth (RE) metal or metal oxides. Studies have demonstrated the effectiveness of RE metals to improve antimicrobial actions through ROS generation and photo-catalysis, however, few investigations have been attempted on its antimicrobial applications, such as in food preservation. In this work, we developed a La-Ce-ZnO containing composite coating based on carnauba wax, Tween-80 and oleic acid, and examined its effectiveness against Escherichia coli and Penicillium expansum. The La-Ce-ZnO powder was characterized to be uniform micro-size particles with a red shift in UV-Vis spectra showing the imbedding of RE within the matrix. Results showed that developed coatings can reduce > 2.5 log E. coli attached in vitro. Using fresh Aiyuan 38 orange as model food, significant inhibition against P. expansum and natural microorganisms were also observed, particularly for 2 % inclusion of the RE doped ZnO. Quality evaluation during 25 days storage revealed that coatings did not affect orange weight losses and firmness, whereas caused faster deterioration in color. Additionally, decreased antimicrobial activities and impact on orange quality were more severe when used 4 % La-Ce-ZnO in the composite coating. Findings in this study can be useful to further understand the antimicrobial mechanisms of La, Ce or other RE elements, and expand their potentials in food applications.