Researchers said the findings were significant as they demonstrated that foodborne pathogens “can grow significantly on commercially packaged lettuce salads while the product’s visual quality is fully acceptable”. This challenged the widely held view that temperature control of bagged greens was a quality rather than predominantly a food safety issue, said the team.
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.
The findings come in the wake of concerns raised both in Europe and the US over the bacteria levels in packaged RTE salads.
In June, the German Federal Institute for Risk Assessment (BfR) said that high bacteria loads observed in fresh packaged sprouts and RTE salads are likely caused by a combination of factors including poor processing hygiene and humid conditions fostered inside plastic packaging. But it concluded the risk was higher than in some meats and advised consumers to wash products thoroughly before eating. Earlier in the year, the US-based Consumers Union (CU) urged the Food and Drug Administration (FDA) to set performance standards for leafy greens after its latest survey revealed some packaged salads contained high levels of bacteria.
Effect of Storage Temperature and Duration on the Behavior of Escherichia coli O157:H7 on Packaged Fresh-Cut Salad Containing Romaine and Iceberg Lettuce By Yaguang Luo, Qiang He, and James L. McEvoy was published in the Journal of Food Science doi: 10.1111/j.1750-3841.2010.01722.x