Many consumers assume that ready-to-eat bagged lettuce will start to look nasty by the time any foodborne bacteria build up to unsafe levels. But a recent study published in Journal of Food Science shows that this is not the case.

Scientists bought packaged romaine and iceberg lettuce at stores, then opened the bags and sprayed the lettuce with Escherichia coli (E. coli), a common strain of bacteria that can cause severe diarrhea, abdominal cramps and even organ damage. The bags were then resealed and stored at either 41°F (the FDA-recommended safe storage temperature) or 53°F (what experts call a “mild temperature abuse situation”) until their labeled “best if used by” date. Results: At the lower temperature, E. coli survived but its growth was limited… at the higher temperature, E. coli proliferated to potentially harmful levels within three days—even though the lettuce still looked fine.

Self-defense: Set your refrigerator to no higher than 41°F. This is easy if the control mechanism displays the exact temperature setting. But: The control knobs on some refrigerators just show various warmer or cooler options in the manufacturer’s preset range of about 33°F to 47°F—so you can’t tell exactly how cold the fridge is. Solution: If your refrigerator has no temperature display or you suspect that it isn’t accurate, buy a refrigerator thermometer (about $2 and up at kitchenware stores and online).