Sep. 8, 2000 FAYETTEVILLE, Ark. -- After a few days on the refrigerator shelf, the small, white button mushrooms from the grocery store turn brown and spongy, but when you try to cut them they seem tough as nails. University of Arkansas food science researchers have discovered the reason mushrooms become unusable so quickly and hope the information may one day be used to extend the shelf life of mushrooms in the market.
Graduate student Svetlana Zivanovic and food science professor Ron Buescher will present their findings on Monday, Aug. 21 at the American Chemical Society's 220th national meeting in Washington, D.C.
Americans consume more Agaricus bisporus, or white button mushrooms, than any other kind. Last year people devoured over 8 million pounds of mushrooms, and 85 percent of those consumed were fresh.
Mushroom producers harvest the fruit-bodies at the "close cap" stage, when stems are still short and the caps are closed. They must pluck the mushrooms from the compost pile within a window of a few hours so they can preserve desirable traits. Desirable traits in mushrooms include short stems, closed caps, a white color and firm but tender texture.
Soon after harvesting, however, the mushrooms elongate, producing a flat, thin cap, long stem, a brown color and a spongy and tough texture that is difficult to cut or chew.
Little is known about why mushrooms degrade in quality in this manner. Although mushrooms appear next to carrots and tomatoes in the produce section, they are microorganisms, not vegetables. Most vegetables either soften or toughen over time, but they don't do both. Zivanovic set out to find out why mushrooms exhibit these two seemingly opposite qualities.
Unlike vegetables, mushrooms contain chitin in their cell walls, a substance commonly found in insects and shellfish.
While the mushroom grows, the chitin gets everything it needs from the compost pile, forest floor or tree of its choice. But once picked, the chitin begins to seek those substances from within the cells.
Zivanovic measured the presence of chitin in mushrooms at different stages, and discovered that mushrooms toughen as chitin increases.
At the same time that the caps open, cells stretch and elongate, increasing the spaces between them and leaving a spongy texture in its wake.
Zivanovic hypothesized that mushroom degradation may start when enzymes begin to break down proteins in the cell walls.
She infiltrated discs from mushroom caps with different hydrolyzing enzymes that break down proteins, and analyzed their effects on chemical composition of cell walls and on texture of the tissue. When structural proteins were hydrolyzed, cell wall construction was loosened and resulted in altered texture.
The next step in searching for a way to increase the shelf-life of fresh mushrooms would be to find a way to prevent proteolysis that rises after harvesting .
"If we could prolong their shelf life by even a day, it would be an accomplishment," Zivanovic said.
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