Hypoxic atmospheres can induce subsequent increases in cutting-induced hydrogen peroxide production by fresh-cut Romaine lettuce.

The potential of hypoxic treatments to induce increased hydrogen peroxide at cut edges in order to improve food safety of fresh-cut lettuce was studied. Hypoxic atmospheres (0.5 kPa O2) were applied at 4°C for several days before fresh-cut processing in Romaine lettuce grown in both growth chambers and in the field. When the hypoxic atmosphere was applied to young growth chamber-grown lettuce, the production of hydrogen peroxide at the cut was increased by 30 to 50%. This resulted in lower survival of epiphytic bacteria and E. coli O157:H7 inoculated onto the lettuce prior to cutting. Older lettuce grown in the chambers did not show such a dramatic response. Response to hypoxic treatment increased up to seven days, after which it began to decline. Chamber grown lettuce infected with powdery mildew showed a reduced response to hypoxic treatment. Romaine lettuce grown in the field and harvested at the same stage as the young chamber grown lettuce showed varied response to hypoxic treatments, showing large increases in H2O2 production or no response to the treatment. Lettuce tested immediately after a severe heat stress event in the field showed no response to hypoxic treatments. Exposure to severe stress or infection by powdery mildew appears to reduce response to hypoxia. These results suggest that treatments to enhance wound-generated oxygen radical production are only effective in cases where the plant tissue has not been exposed to other stresses. This work helps to highlight the importance and limitation for stress treatment cross tolerance adaptation approaches to manage fresh-cut produce quality and safety.