Canning Methods That Are Unsafe and Not Recommended: Inversion Canning, Steam Canners, Microwave Canning, Oven Canning and Others
Canners and home canning methods that are unsafe and NOT
recommended for home use
People occasionally ask, "why use a water bath method or a
pressure canner?" adding:
- "my grandmother just filled the jars with hot fruit, vegetables or jam
then just sealed and inverted the jars"; or
- "I have a steam canner that my grandmother used and she never got ill!"
- My family just uses a water bath canner to can vegetables like beans and
corn. What's wrong with that?" or
- "We use the oven to heat the filled jars, it gets plenty hot and the
jars seal" or
- "Why can't I just use the microwave to blanch or heat the food then
quickly seal the jars?" or even
- I've used the dishwasher to process the filled jars and that's always
worked fine for me!"
As preposterous as the "dishwasher method" may seem, all of
these methods are unsafe. And it may be true, that no one in their family
has died (yet) from their canned food, just as there are occasionally smokers
who live to 100 or children who play in the street and don't get hit by a car,
it's hardly something a rational person does (yes, for the record, I AM
saying that smokers are behaving irrationally or are in addicted denial) .
Botulism food poisoning is nothing to mess with!
See this page for detailed information about botulism food poisoning.
But no rational person would recommend these either.. (note:
don't take MY word for it - click here to
see this list of references from major universities and US government, all
saying the same.)
Now, for those of you non-smokers who are still reading (the
smokers are now busy writing me hate mail, or by now need a nicotine break, so
they've gone outside); here are detailed explanations and references from
- Open Kettle Canning (aka, inversion canning)
The open-kettle method means placing hot food in jars and sealing with no
further heat treatment. This is the method that many grandma's used in which
granny fills a jar (sanitized or not) with hot fruit, pickles, etc., puts
the lid and ring on, then turns it upside down. The jar will cool and seal,
BUT it is NOT sterile, as the contents were exposed to the air (and airborne
bacteria) just before sealing. From the moment the jars were filled, the
contents started cooling, so airborne bacteria contacting the cooling
surfaces will still be viable. They were not exposed to a heat high enough,
nor long enough to destroy them. Then granny gives the jars away, playing
Russian Roulette. Maybe you'll get sick, maybe not. Again, this method is
NOT recommended for home canning because the amount of heat applied may not
be sufficient to destroy bacteria and the product may spoil quickly or cause
illness when consumed.
The USDA and many, many universities have warnings against the use of this
method (see the bottom of this page for references). Here's a typical
statement, from the University of Georgia:
"An old out-dated method of canning - the open-kettle method - is now
considered unsafe. In this method, foods were heated in a kettle, then
poured into jars, and a lid was placed on the jar. No processing was
done. With this method there was often spoilage, because bacteria,
yeasts, and molds that contaminated the food when the jars were filled
were not killed by further processing. The growth of these
microorganisms, in addition to spoiling the food, often caused lids that
did seal to later come unsealed. This method resulted in a very real
danger of botulism."
- Steam Canners The steam canner was designed as a means to
process foods using steam without the aid of pressure. The manufacturer
claims this process uses less water, saves time and energy, and recommends
identical processing times as those required for boiling-water bath
According to the National Home Food Preservation Center (USDA / U.Ga.):
"Steam canning is not recommended at this time for either acid or low
acid foods. Processing times for use with current models have not been
adequately researched. Today's steam canner looks like an upside-down
boiling water canner. The base is a shallow pan with a rack that is
covered with a high dome lid. After the jars of foods are placed on the
canner's base, a small amount of water in the base is brought to a boil
and the dome fills with steam. The jars and foods in them are heated by
the steam surrounding them. However, steam canners do not heat foods in
jars exactly the same as boiling water canning does. Low acid foods are
potentially deadly because Clostridium botulinum bacteria could survive
the steam canning and produce the poison that causes botulism. Acid
foods may also be underprocessed and therefore could spoil."
Studies have concluded that:
- Atmospheric steam canners result
in significantly lower product temperatures at the beginning and end of the
scheduled process when compared to water-bath canning.
- Use of steam canners as instructed by the manufacturer would result in
under processing and considerable economic spoilage.
- Because steam canners may not heat foods in the same manner as boiling
water canners, using boiling-water process times with steam canners may
result in spoilage. There is no tested nor approved conversion factor.
- Micro-Dome Food Preserver
- Micro-Dome Food Preserver Recalled Washington, DC--The U.S. Consumer
Product Safety Commission (CPSC) in cooperation with Micro-Dome of San
Ramon, CA, has warned consumers of certain safety hazards associated with
the use of the "Micro-Dome Food Preserver" manufactured by Micro-Dome and
sold and distributed to consumers after August 1987. The CPSC has also urged
consumers to destroy all food that has been preserved using a Micro-Dome
- Solar Canning The heat generated from captured
sunlight is not a reliable method to process acid foods and should never be
used to can low-acid foods.
- Oven Canning Oven-canning is extremely hazardous.
The oven canning method involves placing jars in an oven and heating. In
oven canning, product temperatures never exceed the boiling point, and
uniform heat penetration cannot be assured. It is, therefore, not considered
safe to use for home canning.
Because this process fails to destroy the many bacteria, including the
spores of Clostridium botulinum, it can cause the food to become toxic
during storage. Also, canning jars are not designed for intense dry heat and
may explode resulting in serious cuts or burns. Of "oven canning",
the USDA says:
"This also is not 'canning'. There is not sufficient, research-based
documentation to support that 'canning' any food in a dry oven as described
on this web page or any page that proposes oven canning is even sufficient
heating to destroy bacteria of concern, let alone enough to produce a proper
seal with today's home canning lids. "
- Microwave Processing
Microwave oven cannot be used for home canning. Microwaved food reaches 212
F but heating is not uniform. There is also a danger of explosion of the
jars within the microwave oven or as food is being removed from the oven.
- Dishwashing Processing
Processing canned foods in a dishwater cycle is dangerous. The temperature
of the water during the cleaning and rinsing cycle is far below that
required to kill harmful microorganisms. Thus the product will be
underprocessed and unsafe to eat. Note that it is fine to use the
dishwasher to clean and sanitize the empty jars, especially if your
dishwasher has a "sanitize" setting - the empty jars will get hot enough.
- Aspirin / Salicylic acid - So-called canning powders are
useless as preservatives and do not replace the need for proper heat
processing. You may have heard of someone's grandmother canning corn by
boiling the corn, adding aspirin or salicylic acid from the drugstore, then
sealing the corn in jars with no further processing. According to the
University of Illinois, a recipe circulated several years ago, using
aspirin to acidify tomatoes and beans for canning. Aspirin is not
recommended for canning. While it contains salicylic acid, it does not
sufficiently acidify low acid foods like tomatoes or beans for safe hot
water bath canning. Low acid foods (without added acids) should only be
processed safely in a pressure canner. Lemon juice or vinegar is recommended
to acidify tomato products for safe water bath processing. You can also see
an article in JAMA (the Journal of the American Medical Association, Vol.
289 No. 13, April 2, 2003, titled "Is salicylic acid as a food preservative
harmful?"; from which the abstract states: "salicylic acid, in
the ways in which it is used in the preparation of food products,
is not only not harmful, but is a preservative to health, inasmuch
as the process of decomposition which it prevents would be far
- Using Paraffin or other wax to seal jars, like jams, preserves and
jellies: This is an outdated
method from 50 years or more ago, that is considered unsafe. The lid and
ring method with a boiling water bath (usually on 5 minutes for jams and
jellies) is much safer. The USDA says:
Because of possible mold contamination, paraffin or wax
seals are no longer recommended for. any sweet spread, including jellies.
The University of Minnesota's Extension says:
Note. Jelly jars and paraffin are no longer recommended. An
incomplete seal with paraffin and the absence of a heat treatment may result
in mold growth and toxin production in the jelly. Persons continuing to use
the paraffin / no water bath method should be aware of the potential health
See this page for why you should use a canner and how to choose one.
Equipment Not Recommended
In addition to the methods above being considered unsafe, some, particular
outdated equipment has also been found to be unsafe:
- Jars with wire bails and glass caps make attractive antiques or storage
containers for dry food ingredients but are not recommended for use in
- One-piece zinc porcelain-lined caps are also no longer recommended. Both
glass and zinc caps use flat rubber rings for sealing jars, but too often
fail to seal properly.
- Pressure saucepans (which are much smaller than a pressure canner),
because of poor temperature control and risk of inadequate heat processing,
are not recommended.
- Devices for canning food in microwave ovens are not recommended because
of incomplete destruction of bacteria due to non-uniform heating.
University of Minnesota
Temperatures and Food Safety
See this page for water bath canners
and see below for prices, descriptions and ordering options for pressure
canners. For water bath
canners and other supplies, see this page! If you have a glass top radiant
Canners for glass top stovers?
This is just a small sampling of the many authorities who concur that the only
safe home canning methods are the water bath canner (for jams and acidic fruits
and vegetables) and the pressure canner (for low acid fruits and vegetables,
meats, and dairy). Click on the links to see their articles.
For other supplies:
- Books on canning, freezing,
drying, preserving and jam making
- Strainers, pit removers, seed-skin-stem
removers, jelly strainers, etc.
All types, makes and prices
(from $19 to $350)
- Vacuum Foodsealers for
freezing, dried foods, and refrigerated foods - the FoodSaver line
Canning Lids and Rings,
- Canning jars,
- Canning mixes,
- And just for fun, the history of
the canning jar
You can also find free information from the USDA in this PDF
file (it will take a while to load!) about
selecting and using canners here!
For more information, and NO obligation to buy, just click on
the links in the Amazon boxes on the left!
If you want to can low-acid foods such as red meats, sea food, poultry,
milk, and all fresh vegetables with the exception of most tomatoes,
you will need a pressure canner. These foods fit into the low
acid group since they have an acidity, or pH level, of 4.6 or greater. The
temperature which must be reached and maintained (for a specified amount
of time) to kill the bacteria is 240 F. Pressure canning is the only
canning method recommended safe by the U.S.D.A. for low-acid foods such as
vegetables, meats, and fish. Ordinary water bath canners can only reach
212 F and cannot to kill the types of bacteria that will grow in low acid
foods. This temperature can be reached only by creating steam under
pressure as achieved in quality pressure canners.
There are several manufacturers of pressure canners. The two
leading ones are Presto and All American (Wisconsin Aluminum). They are more
expensive than water bath canners, but extremely well built - I bought mine
in 1988 and it still looks and works like new!
Presto 01781 23-Quart Pressure Cooker/Canner
This is usually about $80 PLUS SHIPPING. (which is a GREAT
price for a pressure canner). There is also a 16 quart version for about
$69. Click on the links at left or above for more info and current
pricing. It is also available from Amazon .com (click on the box link at
left) (and below from Target)
- 17 by 15-1/2 inches; 12-year warranty
- Heavy-duty 23-quart aluminum pressure canner and
- Comfortably ergonomic, stay-cool black plastic
- Strong-lock lid with pressure regulator, dial
gauge, and overpressure plug
- Comes with canning rack to protect jars during