The work was conducted by microbiologist Arvind Bhagwat with the ARS Produce Quality and Safety Laboratory in Beltsville, Md. He first investigated differences in bacterial growth levels on cut lettuce leaves that had been bagged under very low oxygen levels--an atmosphere known to extend the time that bagged salad appears fresh.

Bhagwat investigated whether the lack of oxygen would make it harder for the bacteria to survive a synthetic gastric juice shock, which mimics the challenge of exposure to human stomach acids. It turns out that the bacteria sitting on vegetables packed in low-oxygen atmospheres were more likely to survive such a shock.

In response to the challenge of being in an air-starved environment, together with the added nutrients provided by the cut leaves, the bacteria became hardier during storage. This increased hardiness only took place when the bagged fresh-cut salad was left at room temperature or unrefrigerated for extended periods of time.

Bhagwat next tested different temperatures. Bacteria were applied to fresh-cut lettuce and stored in sealed bags under different atmospheric conditions for eight days. When stored under very low-oxygen conditions--and warmer temperatures--bacteria showed more resistance towards synthetic gastric juice.

The findings underscore a danger involved in inadvertently leaving fresh-cut produce at temperatures of 59 degrees Fahrenheit or higher, particularly when low oxygen levels are used to extend the shelf life of fresh bagged lettuce, according to Bhagwat. Consumers are advised to keep refrigerator temperatures at 40 degrees F or below, according to experts.

Note: If no author is given, the source is cited instead.

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