Home
canning can be an economical and safe way of preserving favorite
foods. Only up-to-date methods based on
current research
should be used in home canning. Methods, including recipes,
found in old cookbooks or home canning leaflets should not be
used. Reliable sources of up-to-date methods are Virginia Cooperative
Extension, United States Department of Agriculture (USDA), and
major manufacturers of home canning equipment. The following
information will help you can foods safely and prevent home
canning problems.
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Microorganisms,
physical damage, water loss, and enzymes can spoil home-canned
foods. Microorganisms that cause spoilage include molds, yeasts,
and bacteria. They are found in the air and soil and on people
and animals. Molds grow well on high-acid foods, such as those
listed in Table 1 below, but they are destroyed by heating foods
to temperatures between 140 and 190 degrees F. Yeasts also grow
on high-acid foods but are destroyed at temperatures between
140 and 180 degrees F.
Bacteria
can grow at cold, warm, or hot temperatures, depending on
the particular type of bacteria. Some need air (oxygen) to
grow, and others do not. Most grow well on low-acid foods,
such as those listed in Table 1 below. Although most are destroyed
by heat, a few can form spores that are destroyed only by
temperatures above boiling (212 degrees F).
In home
canning, foods are heated in jars to temperatures that destroy
microorganisms that cause spoilage and foodborne illness.
There are two safe methods of canning: the boiling water
bath method and the pressure canner method. The
method to use depends on the type of food being canned.
The
boiling water bath method is used to can high-acid foods (see
Table 1 below). In this method, jars of food are completely
covered with boiling water and heated for a specific period
of time. The time depends on the food. Reliable sources of
canning methods will indicate the time needed to heat the
food.
The
pressure canner method is used to can low-acid foods (see
Table 1 below). In this method, jars of food are heated under
pressure to 240sF, a temperature above boiling. This temperature
is needed to destroy the spores of Clostridium botulinum,
the bacterium that causes botulism. These spores grow well
in low-acid foods and in the absence of air. When they grow
they produce the deadly botulism toxins (poisons). Even a
taste of food containing these toxins can be fatal. Because
the spores survive at 212sF, low-acid foods must be heated
under pressure to achieve a temperature high enough (240sF)
to destroy the spores.
Table 1. High-acid and low-acid foods.*
|
| High-acid
foods | Low-acid
foods
|
Apples
Applesauce
Apricots
Berries
Cherries
Cucumbers
Fruit juices |
Peaches
Pears
Pickled beets
Plums
Rhubarb
Tomatoes
Tomato juice
| Asparagus
Beans, shelled
Beans, snap
Beets
Carrots
Corn
Hominy
| Mushrooms
Okra
Peas
Potatoes
Pumpkin
Spinach and greens
Squash
|
* Process
high-acid foods in a boiling water bath canner and low-acid
foods in a pressure canner according to up-to-date methods.
Most
high-acid foods can be canned with the boiling water method
because they contain enough acid to prevent the growth of
Clostridium botulinum. Although tomatoes are usually
considered a high-acid food, some varieties are lower in acid
than others. Therefore, tomatoes must be acidified with lemon
juice when they are canned with the boiling water bath method.
Two tablespoons of bottled lemon juice should be added to
each quart, and one tablespoon should be added to each pint
of tomato .
Salt,
sugar, and vinegar are frequently used in home canning because
these ingredients inhibit the growth of microorganisms that
cause spoilage and illness. Therefore, it is important
not to alter recipes for home-canned foods. Only recipes
from reliable sources of canning methods should be used. It
is possible, however, to can foods at home for people who
are watching their sugar and salt intakes. Recipes and methods
have been developed for people on special diets.
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Most
large metal containers can be used as a boiling water bath canner
if deep enough. Water must cover the top of jars by 1 to 2 inches,
and there must be another 1 to 2 inches of space to allow for
brisk boiling of water. The canner must have a rack and a tight-fitting
lid. The rack keeps jars from touching the bottom of the canner
and allows for water to circulate under jars. If it has dividers,
jars will not touch each other or bump against sides of the
canner during processing. A deep pre sure canner can be used
as a water bath canner. The lid of the canner should not be
sealed, and the safety valve must be left open to allow steam
to escape, preventing buildup of pressure in the canner.
A canner
with a flat bottom must be used on an electric range, but
a canner with either a flat or ridged bottom can be used on
a gas range. To insure uniform processing of all jars with
an electric range, the canner should be no more than 4-inches
wider in diameter than the element on which it is heated.
A wash kettle that fits over two burners should not be used
on either a gas or electric range because jars in the middle
of the kettle will not get enough heat.
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Low-acid
foods
must be processed in a pressure canner for the
correct time and temperature to avoid the risk of botulism.
Temperature must be adjusted when food is processed at altitudes
of 1,000 feet above sea level or higher (see Adjusting for Altitude).
Foods can spoil if processed at a lower pressure than specified
or processed for a shorter time than specified. All types of
pressure canners must be vented 10 minutes before they are pressurized
because air trapped in a canner lowers temperature, resulting
in under processing. Canners must be cooled at room temperature
until they are completely depressurized. Foods can spoil if
canners are improperly exhausted or if canners are cooled too
quickly with water.
Pressure
canners have either a weight or a dial gauge that controls
pressure. For weight-gauge models, the sound of the weight
rocking or jiggling during processing indicates that the canner
is maintaining the recommended pressure. Weight-gauge models,
however, cannot correct pressure precisely at altitudes of
1,000 feet above sea level or higher.
Dial-gauge
models must be checked for accuracy before use each year.
Gauges can be checked at county Cooperative Extension offices.
Replace the gauge if it reads high by more than 1 pound at
5, 10, or 15 pounds of pressure. A low reading indicates that
the accuracy of the gauge is unpredictable.
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Because
altitude affects pressure and the boiling point of liquid, adjustments
must be made when canning foods at altitudes of 1,000 feet above
sea level or higher. When using the boiling water bath method,
processing time must be increased. Add 5 minutes to processing
time for altitudes between 1,000 and 6,000 feet above sea level.
When using the pressure canner method, pressure must be increased.
If using a dial-gauge pressure canner, process foods at 12 pounds
pressure for altitudes between 2,000 and 4,000 feet and at 13
pounds pressure for altitudes between 4,000 and 6,000 feet.
If using the weight-gauge pressure canner, use 15 pounds of
pressure rather than 10.
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Only
glass jars that have been tempered (strengthened) for heat and
cold should be used in home canning. The Mason jar is the most
common type. Mayonnaise jars and other jars from commercial
foods should not be used. Jars with cracks or chips should not
be used because these defects will prevent sealing. The two-piece
lid, consisting of a lid and screwband combination, is used
most often in canning. The lid can be used only once but the
screwband may be reused. Follow the manufacturer's instructions
for use to insure formation of a good seal during processing.
Examine all lids for dents, gaps, or defects in the sealing
gasket. Do not use lids with defects, and buy only the quantity
of lids needed for 1 year because gaskets in older, unused lids
may fail to seal.
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Test
the seal of jars within 12 to 24 hours after processing by pressing
the center of the lid or tapping it with a spoon. The lid should
stay down and give a clear, ringing sound when tapped. If it
makes a dull sound, the lid is not sealed.
Foods
from jars that did not seal should be reprocessed within 24
hours. Empty the food from the jar and start over as if the
food were fresh. Reheat the liquid if a raw pack is used or
reheat the food and liquid if a hot pack is used. Pack the
food in clean jars free of nicks or cracks, adjust new lids,
and process for the full time. Label jars of food that have
been reprocessed and use them first. Texture of the food will
be softer than that of food processed only once.
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The
acidity of pickled products is important because it helps prevent
the growth of Clostridium botulinum. Only recipes with tested
proportions of ingredients should be used in pickling. Proportion
of ingredients should never be altered. Vinegar (cider or white)
having an acidity of 4 to 6% (40 to 60 grains) should be used.
This is the range of acidity for most commercial vinegars. Homemade
vinegars or other vinegars with unknown acidity should not be
used. Vinegar should never be diluted unless this is specified
in the recipe. "Pickling" or "canning" salt and Kosher salt
can be used. Iodized salt can darken pickles, and table salt
contains anti-caking agents that can make the brine cloudy.
Alum and pickling lime should not be used. Alum, if used in
excess, can cause digestive upsets, and lime decreases the acidity
of pickled products.
Brined
pickles can be fermented in stone crocks, glass jars and bowls,
and food-grade plastic containers. Non-food-grade plastic
containers can be used only if they are lined with a food-grade
plastic bag. Garbage bags or trash liners should never
be used. Products should not be fermented in aluminum,
copper, brass, galvanized or iron containers.
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A
proper ratio of fruit, pectin, acid, and sugar is needed to
make jams and jellies. Sugar prevents the growth of microorganisms
and enables gel formation. The amount of sugar called for in
a recipe should never be decreased, and artificial sweeteners
cannot be substituted for sugar in a recipe. However, recipes
developed for reduced-sugar fruit spreads can be used.
Even
though sugar helps preserve jams and jellies, molds can grow
on the surface of these products. These molds are not always
harmless. Some can produce mycotoxins (poisons). Because of
possible mold growth, paraffin or wax seals should not be
used when making jams and jellies. They should be sealed with
self-sealing lids and processed in a boiling water bath canner.
Only
one jam or jelly recipe should be made at a time because double
batches do not always gel properly. Although most jams and
jellies can be stored safely for at least a year, they lose
their flavor and color during storage. Therefore, prepare
only the quantity that can be eaten within a few months.
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Home
canned foods can be stored safely for up to 1 year. Do not eat
foods stored longer than 1 year. To store home-canned foods,
label and date jars and store them in a clean, cool, dry place.
Do not store them above 95sF or near pipes, a range, a furnace,
in an insulated attic, or in direct sunlight because these conditions
can cause spoilage. Do not store them in a damp place because
dampness can corrode metal lids, break seals, and cause spoilage.
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Look
closely at all jars of food before opening them. A bulging lid
or leaking jar are signs of spoilage. When you open the jar,
look for spurting liquid, mold, or an off odor.
Do not taste
foods that show signs of spoilage or foods from a jar with unsealed
lids. All suspect jars of spoiled low-acid foods, including
tomatoes, should be treated as containing botulism toxin. Handle
spoiled foods in one of two ways:
- Place
sealed jars in a heavy garbage bag, close the bag, and place
it in a regular trash container or bury it in a nearby landfill.
- Detoxify
unsealed, open, or leaking jars of food before disposal
to prevent the spread of toxin.
Detoxification
process: Place suspect jars of food, including lids, on
their sides in an 8-quart volume (or larger) stock pot. Wash
your hands thoroughly. Add water to the pot to cover to a
minimum of 1 inch above the containers. Do not splash the
water. Place a lid on the pot, heat to boiling, and boil for
at least 30 minutes to insure detoxification. Cool and discard
the containers of food, including lids, in the trash or bury
them in the soil. Scrub all counters, pots, and other equipment or utensils used in the process. Wash clothing and hands.
Place sponges or wash cloths that were used in clean-up in
a plastic bag and discard them in the trash.
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Open-kettle
canning and processing of freshly filled jars in conventional
ovens, microwave ovens, and dishwashers should not be used in
canning because they do not prevent all risks of spoilage. Pressure
saucepans with small volume capacity should not be used. Steam
canners should not be used as boiling water bath canners because
they do not heat foods in the same manner. Pressure processing
in excess of 15 pounds should not be applied when using new
pressure canning equipment. Canning powders should not be used
to replace the proper heat processing of foods during canning.
Jars with wire bails, glass caps, or one-piece zinc porcelain-lined
caps should not be used because they have flat rubber rings
that do not form a proper seal.
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Answer
the following questions before canning foods.
- Is
the food a high- or low-acid food?
- Should
a boiling water bath canner or a pressure canner be used?
- Is
the canning method up-to-date and from a reliable source?
- Should
the processing time or temperature be adjusted for altitude?
- Is
the gauge of the pressure canner accurate?
- Have
the jars been designed for use in home canning, and are
they free of cracks and chips?
- Are
the lids the self-sealing type, and how old are they?
More
information on home canning can be
obtained from your county
Virginia Cooperative Extension office.
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Barton,
J.A. 1993. VCE Publication 348-027,
Preserving Foods.
Virginia Tech, Blacksburg, VA.
Reynolds,
M.S. and Ybarra, P.W. 1993. So Easy to Preserve.Cooperative
Extension Service, The University of Georgia, Athens, GA.
USDA.
1989. Complete Guide to Home Canning. Agriculture Information
Bulletin No. 539. U.S. Dept. of Agriculture, Washington, D.C.
Publication #: 348-078
Denise Brochetti,
Extension Specialist, Department of Human Nutrition, Foods,
and Exercise, Virginia Tech
Disclaimer and Reproduction Information: Information in
NASD does not represent NIOSH policy. Information included in
NASD appears by permission of the author and/or copyright holder.
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