Finished cream of mushroom soup is not safe to home can. Cream, milk, butter, flour, and cornstarch block heat, and no tested home process exists for the finished soup. What you can do is pressure-can a thin mushroom base, then stir in dairy and thickener when you open the jar. Freezing is the better route if you want the soup already creamy.
Commercial cans are not a model for a home kitchen. Plants use retorts, controlled formulas, and sometimes aseptic filling. A dial-gauge or weighted-gauge pressure canner cannot copy that.
Why the finished soup fails a home canning test
Cream of mushroom soup is a low-acid food. Clostridium botulinum spores can survive in low-acid food packed without oxygen. They are destroyed only if the coldest spot in the jar reaches a high enough temperature for long enough. A pressure canner is what produces that temperature. A boiling-water bath cannot.
Thickness is the problem, not just the ingredient list. Flour, cornstarch, and other starches slow heat as it moves toward the center of the jar. Fat and cream do the same. If the center never gets hot enough, spores can survive, then grow and produce toxin on the shelf. The toxin does not reliably change the smell or look of the food.
University extension guidance is consistent on this point. The National Center for Home Food Preservation (NCHFP) tells home canners not to add noodles, pasta, rice, flour, cream, milk, or other thickening agents to canned soup. Penn State Extension says creamed soups are not suitable for home canning because those ingredients interfere with heat transfer, and that butter, milk, cream, and cheese should be added only at serving. Ohio State lists cream-of soups the same way: broth, vegetables, and seasonings may go in the jar; butter, milk, cream, and thickeners wait until the jar is opened.
Quality fails even when someone ignores the safety rule. Dairy often separates or curdles under pressure-canner heat. Flour can break down or leave a pasty texture. A sealed jar is not proof the food was processed safely.
Online recipes that pressure-can the finished soup, cream and flour included, are untested. Some authors label them “rebel canning” and note that USDA and NCHFP have no process for them. An untested time, even a long one, does not establish a safe process.
Two pantry options that stay inside tested rules
You do not need a jar of finished cream soup to stop buying it.
Plain canned mushrooms. NCHFP has a tested hot-pack process for domestic mushrooms in half-pints or pints. Use those jars later as the mushroom portion of a fresh pot of soup.
A thin mushroom soup base. Build a broth soup from ingredients that already have canning recommendations, leave out dairy and starch, and process it as soup. Ohio State’s cream-of table is built on that idea: the jar holds the savory base; the “cream of” finish happens in the saucepan.
Neither jar will match a can of condensed soup straight from the shelf. Condensed soup is reduced and thickened. A home-canned base is a cooking ingredient, not a ready thick sauce.
What can go in the jar
Use only cultivated domestic mushrooms. NCHFP’s caution is direct: do not can wild mushrooms. Choose small to medium mushrooms with tight veils and no dark spots. Trim stems and soft spots, soak briefly in cold water to lift grit, then rinse. An average yield is about 2 pounds per pint.
Broth or water is the liquid. Salt, pepper, and dried herbs are optional seasonings. Small amounts of onion or garlic are commonly used for flavor in extension-style soup bases, but they should not become the bulk of the jar. Do not add vegetables that have no current canning recommendation, such as broccoli, cabbage, cauliflower, celery, or summer squash.
Leave these out until serving:
- Milk, cream, half-and-half, evaporated milk, cheese, sour cream, and butter
- Flour, cornstarch, arrowroot, and other starches
- Pasta, rice, noodles, and barley
- A puree used as a thickener
If you sauté mushrooms first, use oil rather than butter. Penn State treats butter as a dairy ingredient that belongs in the pot at serving time, not in the jar.
Fill rules matter as much as the ingredient list. For soup, NCHFP says to fill the jar only halfway with solids, then add hot liquid to 1 inch of headspace. That liquid space is what lets heat circulate. A jar packed solid with mushrooms is not the same product as the tested soup process.
Pressure-canning the base
This is a pressure-canner process only. Work from a current NCHFP or extension publication if you have not pressure-canned before, and use a canner with an accurate gauge, not a pressure cooker and not a boiling-water canner.
Prepare mushrooms as for a hot pack: cover with water and boil 5 minutes. If you are making soup rather than plain mushrooms, combine the hot mushrooms with hot broth, boil the mixture 5 minutes, and do not thicken it. Keep the solids at about half the jar. Leave 1 inch of headspace, remove bubbles, wipe rims, and apply lids fingertip-tight.
Process times below are for a hot pack, from NCHFP soup guidance. Start timing only after the canner has vented steam for 10 minutes and then reached the pressure for your altitude.
Dial-gauge canner, pints
- 0–2,000 feet: 11 pounds, 60 minutes
- 2,001–4,000 feet: 12 pounds, 60 minutes
- 4,001–6,000 feet: 13 pounds, 60 minutes
- 6,001–8,000 feet: 14 pounds, 60 minutes
Weighted-gauge canner, pints
- 0–1,000 feet: 10 pounds, 60 minutes
- Above 1,000 feet: 15 pounds, 60 minutes
Do not use quart jars for this base. Plain mushrooms have a tested process only for half-pints and pints, 45 minutes at the pressures above. General vegetable soup allows quarts at 75 minutes, but that does not create a quart process for mushrooms. Stay with half-pints or pints.
If you are canning plain mushrooms in water, with no mixed soup, use the mushroom process instead: half-pints or pints, 45 minutes, same altitude pressures. A little ascorbic acid, about 1/8 teaspoon per pint, helps color. Still no cream and no flour.
When time is up, turn off the heat and let the canner depressurize on its own. Remove the weight only at zero, wait a few minutes, then take jars out and cool them undisturbed for 12 to 24 hours. Check seals, remove bands, label, and store in a cool, dark place. Best quality is generally within a year.
Turning a jar into cream of mushroom soup
Empty a pint of base into a saucepan and heat it to a simmer. Stir a tablespoon or so of flour or cornstarch into a small amount of cold milk or cream so it will not lump, then whisk that slurry into the pot. Add more milk or cream until the soup is as rich as you want. Simmer a minute or two so the starch thickens and the raw flour taste cooks off. Butter, salt, pepper, thyme, or a splash of sherry go in now.
For a casserole that calls for one can of condensed soup, the base needs reducing or thickening before it can stand in. Simmer the jar contents to concentrate them, then add a milk-and-flour slurry until the sauce coats a spoon. Expect a fresher mushroom flavor and a looser texture than a commercial can. That is normal, not a failed jar.
A jar of plain canned mushrooms works the same way: drain or use the liquid, add broth, then finish with dairy and a thickener on the stove.
Freezing the finished soup
If the goal is soup that is already creamy, freeze it. Extension sources recommend freezing over canning for creamed soups. University of Minnesota guidance on preserving soup includes freezing and a quality window of up to about 6 months.
Cool the soup quickly in shallow containers, then pack meal-size portions with headspace for expansion. Thaw in the refrigerator and reheat gently, whisking as it warms. Frozen dairy often separates; a short simmer and a fresh slurry usually bring it back together. Separation is a texture problem, not a reason to can the soup instead.
Freezing also avoids the botulism question, provided the soup stays frozen and is handled like any other leftover once thawed.
Mistakes that show up in untested recipes
- Canning the soup after the cream and flour are already in it
- Using a boiling-water bath because the jars seal
- Packing jars more than half full of solids
- Shortening the time, or failing to raise pressure for altitude
- Canning wild mushrooms
- Treating a family recipe or a blog process as tested because the jars sealed and no one got sick
- Adding pasta or rice “just a little”
- Pureeing the base before canning to mimic condensed soup
A seal only shows that the jar cooled and formed a vacuum. It does not show that the center of a thick soup reached a botulism-killing temperature.
When to discard a jar
Do not taste a jar to check it. Botulinum toxin can be present with no off smell. Discard the jar, without eating, if the lid is unsealed, bulging, or leaking, if liquid spurts on opening, or if you see mold or an unusual odor. If botulism is a possibility, follow current USDA or extension disposal guidance rather than dumping the food where animals might reach it. Suspected botulism is a medical emergency: symptoms can include double vision, drooping eyelids, trouble swallowing or speaking, dry mouth, and muscle weakness.
Frequently asked questions
Can I can condensed cream of mushroom soup the way the grocery store does? No. Home equipment and home recipes are not the commercial process. Can a thin base, or freeze the finished soup.
Is evaporated milk or powdered milk safe in the jar? No. Dairy in any form used as a soup ingredient is outside tested home processes. Add milk powders or evaporated milk when you reheat.
Can I use ClearJel to thicken before canning? Not in this soup. ClearJel appears in a few tested pie-filling and relish recipes. It is not a general substitute for flour in untested soup.
Why do some canners say they have done this for years? Absence of a known illness is not a tested process. Spore distribution and heat penetration vary by jar, recipe, and canner. Extension guidance does not make an exception for long family use.
Are half-pints processed for less time? For soup, use the pint time if the jar is smaller than a pint. For plain mushrooms, half-pints and pints share the 45-minute process. Do not invent a shorter time for a smaller jar.
Can I water-bath the base if I add lemon or vinegar? No. Acidifying a cream-style soup at home is not a tested substitute for pressure canning, and the dairy and starch problems remain.
The practical split is simple. Pressure-can domestic mushrooms, or a thin half-solid mushroom base in pints, using current NCHFP times and your altitude. Stir in cream and a thickener in the pot. Freeze the soup if you want it creamy before it ever sees a jar.