Pennsylvania grew 355 million pounds of button mushrooms—$274 million worth—during 1994-95, according to the Pennsylvania Department of Agriculture.
But many mushrooms became brown and spotted before making it to the soup pots and salad bowls of American homes. Those unsightly mushrooms headed to the cannery—resulting in lost profit for growers and less produce for consumers.
The white button mushroom, Agaricus bisporus, the one most widely used by Americans, stays fresh only three to five days after harvest, according to Ramaswamy Anantheswaran, associate professor of food science, and one of those days may be taken up in travel to market. To extend that short shelf life, Anantheswaran is working on developing packages that maintain the oxygen concentration and humidity at an optimum level. The technique is called modified atmosphere storage and uses plastic films that allow only certain amounts or kinds of gases to diffuse through. "Obviously you cannot precisely control the atmosphere within a package. But if we can maintain the atmosphere within a range, we can improve the quality of the produce," Anantheswaran says.
Fresh mushrooms respire: they take up oxygen and produce carbon dioxide. But in a micro-environment that contains less oxygen than normal air, spoilage is slowed. This preservation process appears to have been used centuries ago by the ancient Chinese. Chinese documents show reports of litchi fruits being sealed in clay pots along with leaves and grasses before transport to other areas of China. During the two week journey, the respiration of the fruits, leaves, and grasses caused a high carbon-dioxide, low oxygen environment in the pots and slowed the ripening of the litchis. Unfortunately, dangerous microorganisms, such as Clostridium botulinum, can also flourish in low-oxygen micro-environments. These pathogens could be dangerous even before the telltale signs of spoilage—brown slimy spots on the surface of the mushrooms—would prevent consumers from eating them, and botulism can be fatal. As a result, Anantheswaran says, modified atmosphere storage "has not caught on because of the problems in temperature control in the transportation chain."
Anantheswaran began to look at the problem from another perspective. Water makes up 90 percent of the weight of freshly harvested mushrooms. With their porous skins, they give off large amounts of water vapor. The products currently being used for packaging allow gases to pass through a plastic film but do not allow enough water vapor from the respiring mushrooms to flow out. Too much moisture builds up in the package, allowing microorganisms to grow on the mushroom tissue, causing unsightly yellow and brown blemishes.
To alleviate the humidity problem, Anantheswaran has been testing commercially available water vapor absorbers such as montmorillonite clay and silica gel—the crystals packed with electronic equipment to eliminate moisture in packaging. In controlled experiments, mushrooms packaged with clay and silica had a higher weight loss due to moisture loss, but their color improved and fewer microorganisms were found growing on their surfaces.
"Modifying the humidity in mushroom packages is particularly challenging because mushrooms respire at a high rate when harvested and gradually level off," Anantheswaran says. "Also, the absorbing agent cannot totally eliminate humidity, because the produce would then dry out."
Anantheswaran is testing a number of different plastic films for water vapor permeability. "The aim is to design a package that will maintain a controlled humidity," he says. His modified humidity packaging should improve the maturity of mushrooms, effectively increasing shelf-life.
Ramaswamy Anantheswaran, Ph.D. is associate professor of food science in the College of Agricultural Sciences, 111 Borland Lab., University Park, PA 16802; 814-864-3004; email@example.com. This work is funded by the Pennsylvania Department of Agriculture, the Ben Franklin Partnership, and EPL, Inc.