A news study published in the Journal of Food Science is highlighting the food safety importance of keeping leafy greens, such as packaged lettuce salads, at temperatures below 41 degrees F.  Commonly thought to be only a freshness issue, the research shows that cold temperature maintenance for leafy greens also significantly reduces the ability for dangerous pathogens, such as Shiga-toxin producing E. coli, to proliferate.  For a recap of outbreaks associated with leafy greens, click HERE.

As reported in Food Production Daily by Rory Harrington, the research findings demonstrated that foodborne pathogens “can grow significantly on commercially packaged lettuce salads while the product’s visual quality is fully acceptable”.

The study, published in the Journal of Food Science by Yaguang Luo et al, examined samples of packaged lettuce that had been inoculated with E.coli, resealed and stored at 5°C and 12°C (53°F) until their ‘Best if used by Dates’ had expired. The researchers found that while storage at the lower temperature did not eliminate the bacteria it did limit its growth to within safe levels. However, storage at 12°C for three days allowed the proliferation of E.coli by more than 2.0 log CFU/g. The remainder of the period saw further growth, said the scientists.

The group highlighted that although there was eventually a significant decline in visual quality of lettuce held at 12°C, the quality of this lettuce was still fully acceptable when E. coli O157:H7 growth reached a statistically significant level.

“Temperature control is commonly thought to promote quality of leafy greens, not safety, based at least partially on a theory that product quality deterioration precedes pathogen growth at elevated temperatures,” said the study. “This prevalent attitude results in temperature abuse incidents being frequently overlooked in the supply chain.”

The study noted that packaged fresh-cut salads are marketed as ‘ready-to-eat’ (RTE) while lacking an effective pathogen kill step during their preparation.

The researchers said specific data regarding the effect of temperature of pathogen growth on leafy greens was vital to allow regulators and processors to develop science-based food safety guidelines and practices on the issue.