As by-products rich in flavonoids and phenolic compounds, onion peels are globally undervalued and often discarded. This study reports the synthesis of carbon quantum dots (CQDs) from onion peels and evaluates their antimicrobial effectiveness against key foodborne pathogens and biofilms on common food contact surfaces, including plastic, stainless steel, and rubber. The CQDs exhibited a quasi-spherical shape with particle sizes ranging from 1.7 to 9.0 nm and contained abundant oxygen- and nitrogen-functional groups, as confirmed by FT-IR and XPS analyses. The CQDs showed significant antimicrobial activity, with minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) against Salmonella typhimurium, Escherichia coli O157: H7, Listeria monocytogenes, and Staphylococcus aureus of 2200/2800 µg/mL, 1400/2000 µg/mL, 1200/1800 µg/mL, and 400/600 µg/mL, respectively. Time-kill assays confirmed these results. In biofilm tests, S. typhimurium formed biofilms more easily than L. monocytogenes. Washing with CQD solution for 5 min reduced biofilm presence by 81.6–91.5% for S. typhimurium and over 74% for L. monocytogenes, with more than 94% reduction after 10 min of treatment (over 94% for S. typhimurium; 95.8–98.8% for L. monocytogenes) across all surfaces, especially on plastic and stainless steel. These findings indicate that onion peel-derived CQDs are promising, eco-friendly agents for disrupting biofilms and turning undervalued waste into valuable products.

Substandard onions were valorized through lactic acid fermentation using Leuconostoc mesenteroides at 36°C for 24, 48, or 72 h to produce a functional antimicrobial marinade. Fermentation markedly increased lactic acid bacteria (LAB) populations (5.51-8.95 log CFU/mL), accompanied by a progressive decrease in pH (5.11-3.27), an increase in titratable acidity (0.14-0.95%), and a slight reduction in soluble solids (7.87-7.37 °Brix), confirming active lactic fermentation. Antimicrobial screening identified the 24 h fermentation as the optimal treatment. When applied as a marinade to ready-to-eat (RTE) chicken breast inoculated with Salmonella Typhimurium and Listeria monocytogenes, 24 h-fermented onion juice significantly reduced the maximum population density (MPD) of both pathogens compared with non-fermented onion juice. In comparative tests with onion-peel-derived carbon quantum dots (CQDs), fermented onion juice exhibited stronger antimicrobial activity under most conditions; however, CQDs showed superior inhibition of L. monocytogenes at 5°C. Texture analysis further revealed that 2 h of marination reduced chicken breast hardness from 328.3 to 282.3 g, indicating improved tenderness, whereas extended marination (4 h) increased hardness to 448.0 g, likely due to reduced water-holding capacity. Overall, fermented onion juice represents a sustainable strategy for valorizing undervalued onions into functional food ingredients while simultaneously improving the microbial safety and textural quality of RTE chicken products.