Important: Safety Warning
This page describes home-canning methods from before the causes of botulism were understood. Canning low-acid foods without a pressure canner, as described here, can allow the growth of Clostridium botulinum, whose toxin is among the deadliest known. Follow current USDA home-canning guidelines instead.
This information has been left in for historical purposes but should not be acted upon.
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Mushrooms
Mushrooms may be canned as easily as fruit and much easier than some vegetables. The buttons, ranging in size from the smallest to those with the cup breaking from the stem, are most desirable for canning, as they remain firm and white after being heated. Clean by peeling, or by wiping with a cloth, removing any soiled spots or earth which may have adhered to them; cut the stems short, leaving from one half to one whole inch of stem attached to the cap.
Pack in sterilized jars, shaking down meanwhile; prepare three cans for each two to be filled. Cover the large receptacle and let cook until the mushrooms cease shrinking, then fill two jars from the third, using boiling water to finish filling the jars if there is not enough of the mushroom liquor; adjust the rubber rings, set the lids in place but do not tighten them, cover the cooker, and let cook half an hour. Tighten the covers and set aside. Often this much of sterilization will suffice, but to make sure, it is thought best to return the cans, unopened, to the boiler or cooker for one hour on the two following days. An hour is needed, as the cans are cold. With more experiments, the necessary time of cooking may probably be found to be less.
This is a boiling-water-bath process for a low-acid vegetable, and mushrooms are among the most dangerous foods to put up this way. The notes below explain the failure rather than adapting it.
- Why this method failsMushrooms have a pH around 6.0 to 6.5, well above the 4.6 threshold below which Clostridium botulinum cannot grow. A sealed jar of them is an anaerobic, low-acid, room-temperature environment, which is precisely the condition botulinum spores require to germinate and produce toxin.
- The temperature ceilingA "boiler or cooker" of the kind described here reaches 212F. Botulinum spores survive hours at that temperature; destroying them requires 240F or above, which can only be reached under pressure. No amount of time in boiling water substitutes for that pressure.
- Repeated heating does not helpThe instruction to return the sealed cans to the boiler for an hour on two following days is fractional or "intermittent" sterilization, borrowed from laboratory practice and once widely believed to kill spores as they germinated between sessions. In sealed low-acid food it is not reliable, and the intervals at room temperature are exactly when toxin can form.
- Mushrooms specificallyBecause they grow in and on soil and are handled with earth still clinging to them, mushrooms carry a heavy spore load from the start. Wiping or peeling removes visible dirt, not spores. Home-canned mushrooms have a long documented association with botulism outbreaks and are treated as a high-risk product in every modern canning guide.
- Toxin gives no warningBotulinum toxin does not sour, cloud, gas or smell in the way spoilage organisms do. A jar can look and taste correct and still be lethal, so inspecting the contents before use offers no protection here.
- The author's own hedgeThe closing line, that with more experiments the necessary cooking time may probably be found to be less, shows how provisional this process was. It was written before pressure canning was standard in home kitchens and before the required time and temperature combinations for low-acid foods had been established.
- If you want canned mushroomsUse a current tested recipe from a reputable food-preservation authority, or buy them commercially canned. Freezing or refrigerating cooked mushrooms sidesteps the problem entirely.
- For reading the original"Prepare three cans for each two to be filled" accounts for shrinkage: on the recipe's own arithmetic, roughly a third of the bulk is lost in heating. "Rubber rings" belong to the jar closures of the period, which sealed differently from today's two-piece lids.
This entry sits at an awkward moment in the history of home preserving. Cultivated mushrooms were becoming more widely available in America, grown commercially indoors, and cookbook writers were eager to add them to the list of things a household could put up for winter alongside peas and corn. The method offered here, heating sealed jars in boiling water on three successive days, is domesticated Tyndallization, a laboratory technique for sterilizing media without a pressure vessel that was widely trusted at the time to work on food as well. It did not work reliably, and low-acid vegetables handled this way -- mushrooms, beans, corn and asparagus among them -- were among those that later drove agricultural extension services to campaign hard for the household pressure canner. The confident opening claim that mushrooms can be canned as easily as fruit turns out to be exactly backwards: fruit's acidity is the reason boiling water suffices for it, and mushrooms have none.