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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Canned Mushrooms
Wash the mushrooms carefully, but do not peel them. Pack them closely in glass jars; adjust the rubbers, and lay the lids on loosely. Stand the jars in the sterilizer. Cover. If hot-waterbath canner is used, bring the water to the boiling point and boil one hour and a half. If water-seal canner is used, boil one hour. If steam-pressure canner is used, boil thirty minutes. By this time, the mushrooms will have wilted down; so fill two jars by means of a third. Fill the jars to overflowing with boiling water.
Return to sterilizer and process one-half hour,
This is a low-acid canning procedure from the era before the conditions that permit botulinum growth were understood, and the page carries a warning for good reason. The notes below explain where the method fails rather than how to correct it.
- Mushrooms are a low-acid foodCultivated mushrooms sit around pH 6, well above the 4.6 threshold below which Clostridium botulinum cannot grow. Nothing in this recipe adds acid, so the sealed jar is close to an ideal anaerobic environment for the organism.
- Boiling water cannot reach the required temperatureA hot-water-bath canner holds the jars at 212F at sea level, no matter how long it runs. Botulinum spores can survive hours of boiling; reliable destruction requires the higher temperatures reached under steam pressure. Ninety minutes at a boil is ninety minutes at the wrong temperature.
- The water-seal canner is not a pressure cannerPeriod water-seal outfits operated at little or no gauge pressure and are functionally boiling-water processing. They are no longer among USDA-recommended methods.
- The pressure instruction has no pressure in it"If steam-pressure canner is used, boil thirty minutes" specifies no gauge pressure, no altitude adjustment and no jar size. A pressure process without those three numbers is not a process at all.
- Loose lids and no headspaceRubbers adjusted with lids laid on loosely, then jars filled "to overflowing" with boiling water, leaves no measured headspace and no defined seal step. Modern practice depends on a specific headspace and a verifiable vacuum seal so that failures are visible.
- The topping-up step reopens the jarsFilling two jars from a third after the first long boil exposes the contents to recontamination partway through processing, so the timing that follows bears no fixed relationship to what is actually in the jar.
- The toxin is not detectable by eyeBotulinum toxin gives no reliable smell, taste, gas or discolouration in canned mushrooms. There is no inspection you can perform on the finished jar that would tell you it is safe.
- If you want canned mushroomsUse a current USDA or National Center for Home Food Preservation procedure, or buy them. Freezing or drying cleaned mushrooms avoids the low-acid canning problem entirely.
The three-way instruction "hot-water-bath, water-seal, steam-pressure" mirrors the equipment hierarchy described in period home-canning bulletins, with the water-seal outfit sold as an affordable middle option; the assumption throughout was that boiling long enough was equivalent to boiling hotter. That assumption was undermined by botulism outbreak investigations in the years following 1919, and by the 1930s USDA guidance directed that low-acid vegetables be processed under steam pressure.