FAQs about Mushrooms: Are they Very Dirty or Very Clean?

MushroomQ. I’ve read and been told that mushrooms are grown in a sterile environment. I’ve also read that they’re grown with manure. Considering the manure, how carefully must they be washed?  Are they quite clean or very dirty and full of pathogens? 


 Q. One box that I bought recommended transferring them to a paper bag before refrigerating. Is a paper bag better than the packaging they come in?  Why don’t they sell them in a paper bag if that’s a better container for them?


A.  Food scientist Dr. Luke LaBorde answers both questions as he describes how mushrooms are commercially grown and how best to care for them.


Mushrooms are grown indoors on beds or trays under generally clean, but not sterile, conditions. They grow on a special soil called growth substrate. This consists of 2 layers. The underlying layer (about 2 feet deep) is a mixture of organic materials including hay, corn cobs, grains, and leaves, along with some chicken and horse manure. The proportion of each used is based on an optimal carbon:nitrogen ratio that will encourage rapid composting.  The composting process includes a pasteurization step where mushroom pests (e.g. insects and worms) and disease organisms (e.g. molds and bacteria) are killed. Any bacteria present that are harmful to humans are completely killed during this step.


After composting, grains previously inoculated with Agaricus bisporus (the common button mushroom) are mixed in. On top of this layer, a thin layer of peat moss is added to help promote mushroom growth. The dark specks you might see on mushrooms in the store are bits of peat moss. Many mushroom packers wash the mushrooms before packing to improve their appearance.


If they’re grown and packed correctly, mushrooms should be free of pathogens. There are no reported cases of anyone getting sick from eating commercially-grown fresh mushrooms.


Mushrooms can be washed at home to improve their appearance, but it is recommended that this be done only just before using. Wet mushrooms quickly deteriorate in the refrigerator due to rapid growth of spoilage bacteria.


Mushrooms respire rapidly, so it is best to store them in the refrigerator in a high oxygen environment. They can be stored in the package they came in or transferred to a paper bag which will maintain a high oxygen level while preventing them from drying up.  Some mushrooms are sold in bulk, but most are sold in plastic containers overwrapped with plastic wrap that has small holes to prevent build-up of carbon dioxide, which will accelerate deterioration. We recommend storing them at home in the plastic container they were packed in because it protects the very fragile mushrooms from bruising and browning during storage in the home refrigerator.


You can find more information about mushrooms at http://mushroominfo.com/history-and-background/




Luke LaBorde, Ph.D., Penn State University, associate professor, Department of Food Science



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