Scientists at the US department of agriculture’s Agricultural Research Service (ARS) have found that proper storage temperatures are vital to minimise bacterial growth and adaptability inside sealed, bagged salads.

Researchers at the ARS produce quality and safety laboratory in Maryland have been studying the safety of new technologies that extend the shelf life of bagged leafy salads.

The work was led by microbiologist Arvind Bhagwat who 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. In his research he discovered that the bacteria present 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 un-refrigerated for extended periods of time.

Bhagwat next tested different temperatures by applying bacteria to fresh-cut lettuce and stored it 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 show the danger in exposing fresh-cut produce to temperature at 15°C or above particularly when low oxygen levels are used to extend the shelf life of fresh bagged lettuce. In comparison, no resistance was induced among bacteria stored under extremely low-oxygen conditions and at temperatures of 10°C. In addition, regular levels of atmospheric oxygen did not induce resistance in the bacteria, even if the sample was left at abusive temperatures.

“The findings underscore a danger involved in abusive storage temperatures, especially when modified-atmosphere packaging (MAP) is used to extend the shelf life of fresh produce,” said Bhagwat. “Abusive temperatures provide an opportunity for pathogens to grow and acquire resistance. Increased resistance gives them the ability to breach the human gastric barrier and cause disease. The findings also highlight the importance of responsible use of MAP: proper storage temperature is important to minimising bacterial adaptability.”