Effect of carbon dioxide micro bubbles in combination with chlorine dioxide solution to reduce microbial contamination and browning of fresh-cut cos lettuce (Lactuca sativa L.)

In this research, the effect of carbon dioxide micro bubbles (CO₂-MBs) in combination with chlorine dioxide solution (ClO₂) to reduce microbial contamination and maintain the quality of fresh-cut cos lettuce was investigated. Fresh-cut cos lettuce was washed in 5 ppm ClO₂ for 5 min and compared with CO₂-MBs + 5 mg L^‑1 of ClO₂. Fresh-cut cos lettuce washed in water (5 min) were used as control. Treated lettuce then was packaged in modified atmosphere packaging (MAP) and stored for 10 days at 4°C. Initially, ClO₂ water and CO₂-MBs combined with ClO₂ reduced the total mesophilic population by 1 log reduction. However, treated lettuce washed in CO₂-MBs + ClO₂ showed lower amount of total bacteria count after 2-10 days of storage. Washed with CO₂-MBs + ClO₂ also inhibited the growth of yeast and mold and coliform bacteria during storage. ClO₂ solution treatment activated browning symptom as compared with control. However, CO₂-MBs + ClO₂ reduced browning symptom of fresh cut lettuce compared with ClO₂ water alone. The amount of phenolic compounds of the fresh cut lettuce was reduced by CO₂-MBs + ClO₂ treatments. Washing with CO₂-MBs + ClO₂ also reduced PPO activity as compared with washing in ClO₂ alone which is correlated with a high value of L* and a lower browning score during storage. An inhibitory effect of browning enzyme and substrate resulted in a lower browning symptom of fresh-cut lettuce during storage. This result indicated that CO₂-MBs + ClO₂ could be an alternative method to ClO₂ for fresh-cut cos lettuce due to a bacterial inhibitory effect and reduction of the adverse effect of ClO₂ to browning.