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 Asparagus
Asparagus for canning must be fresh and tender. Select tips of uniform size and maturity and wash them. Cut into lengths according to containers to be used. Scrape off scales and tough outer skins and tie in bundles large enough for one jar.
Blanch as follows:
Immerse the lower ends in boiling water and leave them immersed for 5 minutes, then the entire stalks, leaving them in 1 to 3 minutes longer. Cold dip, drain, pack neatly, tips up, in hot sterilized jars. Add salt and cover with boiling water. Loosely seal, sterilize two hours in boiling water bath. Remove as soon as time is up. Tighten covers, invert to test seal and cool. Label and store.
This is a boiling-water-bath method for a low-acid vegetable, and that combination is the one canning failure that modern food science treats as absolute. The notes below explain the specific problem rather than offering a workaround.
- Why asparagus is the problemAsparagus runs roughly pH 5 to 6.5 depending on the part and how it is prepared, well above the 4.6 threshold below which Clostridium botulinum cannot grow. Nothing in the recipe -- no vinegar, no lemon, no brine of any strength -- brings it into the acid range.
- The two-hour water bath does not sterilizeA boiling water bath tops out at 212F at sea level. Botulinum spores survive that temperature more or less indefinitely; laboratory figures put their destruction at 240F and above. Two hours of boiling kills the spoilage organisms that would otherwise warn you the jar has gone bad, and leaves the dangerous ones in an airless, low-acid, room-temperature jar, which is precisely the environment they need.
- No warning signsBotulinum toxin does not gas, cloud, smell or sour the pack in a way that can be relied on. A jar processed this way can look and smell entirely normal. The toxin is lethal in microgram quantities.
- "Add salt" is seasoning, not preservationThe unmeasured salt in this recipe -- typically half a teaspoon a pint in books of this era -- is far below any concentration with preservative effect. It contributes nothing to safety.
- Loose seal, invert to testThe loosely-seal-then-tighten sequence belongs to the glass-lidded, rubber-ringed jars of the period. Inverting a hot jar to check the seal is no longer recommended with modern two-piece lids; it can disturb the sealing compound as it sets.
- Cold dipA period term for plunging the blanched bundles into cold water to stop the cooking. The instruction itself is sound; it is the processing that follows that is not.
- If you want canned asparagusUse a current USDA or extension-service procedure written for a pressure canner. Freezing blanched spears is the straightforward alternative and needs no special equipment.
Home canning of vegetables was widespread in American households in the decades before and during the First World War, driven by cheap mason-type jars, wartime food-conservation campaigns and books like this one that treated the pantry shelf as a household duty. The trouble was that the science lagged the practice: the link between improperly canned low-acid foods and botulism was only firmly established in the United States in the late 1910s and early 1920s, after a run of fatal outbreaks traced to commercially and domestically packed vegetables, and the pressure canner did not become standard household advice until the 1920s and 30s. Recipes of this vintage are entirely sincere, which is exactly what makes them worth reading and not worth following. Asparagus, an expensive spring luxury with a season of a few weeks, was a natural candidate for putting up, and the fussy business of tying bundles and packing tips upward reflects how much the cook was trying to protect.