Washing mushrooms stalls germs, extends shelf life

University Park, Pa. -- A two-step process of washing harvested mushrooms greatly inhibits the growth of food-borne pathogens and extends their shelf life, according to Penn State food scientists.

"We have found that the washing process limits growth of pathogens such as Listeria and Samonella in whole mushrooms," said Robert Beelman, professor of food science at Penn State. "The protective effect of washing was more pronounced in sliced mushrooms, which are more prone to bacterial growth than unsliced ones."

The mushroom business in the U.S. has boomed in recent years, with total annual sales touching nearly $1 billion. But the industry has also been plagued by recalls, following reports of contaminated products.

Beelman and Naveen Chikthimmah, instructor of food science, and Luke LaBorde, professor of food science, studied how washing affected the survival of food-borne pathogens such as Listeria and Samonella in whole and sliced mushrooms.

In their study, the researchers divided a 15-kilogram batch of Agaricus bisporus mushrooms -- the button variety, which is the predominant edible species worldwide -- into two equal groups, one of which was washed. Using a process patented by Beelman, the researchers first washed the mushrooms with an antibacterial solution and then washed the mushrooms again with a neutralizing solution containing preservatives that inhibit browning.

Next, the mushrooms in both groups -- washed and unwashed -- were inoculated with food-borne pathogens.

The researchers found that washing mushrooms before inoculating them with pathogens significantly reduced the growth of both Listeria and Samonella. The pathogens grew more rapidly in sliced mushrooms but to a significantly lesser degree in the washed ones.

Researchers think that the slicing causes the release of nutrients in fluids within the mushrooms, which provides a nourishing environment for the growth of microbes.

"The two-step process helps remove soil residue off the mushrooms and prevents them from getting spoiled by non-pathogenic bacteria," said Beelman. The team's findings appeared in a recent Science and Technology section of Mushroom News.

Beelman, whose work was funded by the Mushroom Council, says the findings have important implications for not only preventing mushrooms from going bad, but extending their shelf life as well.