_by Carole Davis and Annetta Cook_[20]
Consumers frequently have questions regarding home food preservation
practices. Below are some questions commonly asked, and the answers.
They concern canning fruits, vegetables, pickles, and jellies, and
freezing fruits and vegetables.
_Why is open-kettle canning not recommended?_
In open-kettle canning, food is cooked in an ordinary kettle, then
packed into hot jars and sealed. The food is not processed after packing
in the jars.
Open-kettle canning is unsafe because temperatures reached are not high
enough to destroy all the spoilage organisms that may be in low-acid
foods, such as meat and vegetables, other than tomatoes.
Spoilage bacteria may also enter the food when it is transferred from
kettle to jar, making it undesirable as well to can other foods such as
fruits, pickles, preserves, and jams by this method.
_Why is oven canning unsafe?_
Jars may explode, causing personal injury or damage to the oven. Also,
temperatures obtained in the oven are not high enough to insure adequate
destruction of spoilage organisms in low-acid foods.
Times specified for boiling-water-bath processing of foods do not apply
to oven processing since the rate of heat penetration would be different
in the oven and the products could easily be underprocessed.
_Should jars and lids be sterilized before canning?_
No, not when the boiling-water-bath or pressure-canner method is used,
because the containers and lids are sterilized during processing. But be
sure jars and lids are clean.
_Why is no liquid added when tomatoes are canned?_
Because tomatoes provide their own juice if pressed gently when packed
raw, or when heated before packing hot into jars.
_Is it safe to add celery, green pepper, and onion to tomatoes when
canning them?_
No. Adding other vegetables lowers the acidity of tomatoes. Acidity
helps protect against the growth of botulinum bacteria, which can
produce a fatal toxin in canned foods. Specific recipes, times, and
temperatures determined scientifically for vegetable mixtures need to be
used for their safe canning.
_Why is headspace important in canning?_
Headspace--the distance between the surface of food and the underside of
the lid--allows for expansion of solids or bubbling up of liquid during
processing. If headspace is not adequate, some food in the container
will be forced out, leaving food particles or sirup on the sealing
surface and preventing a seal.
When too much headspace is allowed, some air may remain in the jar after
processing, causing food at the top of the jar to darken.
_What causes jars to break in a canner?_
Breakage can occur for several reasons: (1) Using commercial food jars
rather than jars manufactured for home canning, (2) Using jars that have
hairline cracks, (3) Putting jars directly on bottom of canner instead
of on a rack, (4) Putting hot food in cold jars, or (5) Putting jars of
raw or unheated food directly into boiling water in the canner, rather
than into hot water (sudden change in temperature--too wide a margin
between temperature of filled jars and water in canner before
processing).
_What causes liquid to be lost from jars during processing?_
Loss of liquid may be due to packing jars too full. Headspace must be
allowed between the top of the food and lid as specified in the
instructions for each food. Food expands when processed, so headspace
must be adequate or liquid will be forced out of the jar.
Liquid may be lost if the canner's pressure fluctuates during
processing. Lowering the pressure too suddenly after processing may also
cause liquid to be lost. Pressure canner should be removed from the heat
and allowed to cool normally at room temperature.
_If liquid is lost from jars during processing, can more be added to
fill them again?_
No, because if the jar is opened and liquid added, this would allow
bacteria to enter the jar and you would need to process again.
Loss of liquid does not cause food to spoil, though food not covered by
the liquid may darken.
_Why does fruit sometimes float in the jar after canning?_
Fruit may float because it is packed too loosely, sirup is too heavy, or
because some air remains in tissues of the fruit after heating and
processing.
_How do you test the seal on home-canned foods?_
After jars have cooled, check two-piece lids by pressing the center of
the flat metal lid; if lid is down and will not move, jar is sealed.
For porcelain-lined caps, check seal by turning each jar partly over in
your hands. If no leakage occurs, the jar is sealed.
_What causes lids not to seal?_
If food has not been sufficiently heated, a vacuum may not be drawn on
the jar of food and the lid will fail to seal. Presence of food
particles or sirup on the jar rim could also prevent obtaining an
airtight seal. Each jar rim should be wiped clean of all food and sirup
before putting the sealing lid in place.
If food has been packed too tightly in the jars or if sufficient
headspace has not been allowed, expansion of the food during heating
could force sirup or food out of the jar, thus causing poor contact
between the lid and jar.
_Why should flat metal disks and rubber rings be used only once?_
Depressions in the rubber compound made when the lid or ring was first
used can prevent obtaining an airtight seal if used a second time.
_Why should metal bands be removed after jars have cooled?_
If bands are not removed soon after cooling, moisture between the ring
and jar may cause the ring to rust, thus making later removal of the
bands difficult. The band is no longer necessary after the jar has
cooled because the seal has been provided by the flat metal lid with
sealing compound and the vacuum created during cooling.
_What causes canned foods to change color?_
Darkening of food at the top of the jar may be caused by oxidation due
to air in the jar, or by too little heating or processing so that
enzymes are not destroyed. Overprocessing may cause discoloration of
foods throughout the jar.
Pink and blue colors sometimes seen in canned pears, apples, and peaches
are caused by chemical changes in the fruit coloring matter.
Iron and copper from cooking utensils or from water in some areas may
cause brown, black, and gray colors in some foods.
_Why do undersides of metal lids sometimes discolor?_
Natural compounds in some foods, particularly acids, corrode metal and
make a dark deposit on the underside of jar lids. This deposit on lids
of sealed, properly processed canned foods is harmless.
_Is it safe to use canned foods which have been frozen as the result of
storing them in an unheated storage area?_
Freezing does not cause the food to spoil unless the seal is damaged or
the jar broken. If the jar is no longer sealed, the food may still be
safe to eat if the jar is not broken and the food is still frozen and
has not been subjected to thawing and refreezing.
Remove the frozen canned food from jars as carefully as possible. The
food may need to thaw slightly to ease its removal from jars, but it
should be left in as large blocks as can be removed through the jar
opening.
Examine jars for breaks and hairline cracks. If any are found, discard
food from those jars. If no cracks are found, food may be transferred
from jars into freezer bags or containers and stored in the freezer, or
it may be kept in the refrigerator for use within a day or two.
Home-canned foods which have been frozen may be less palatable due to
texture changes than properly stored canned foods. Do not recan
home-canned foods which have been frozen.
_How do you protect canned foods against freezing?_
Wrap the jars in paper or cover them with blankets. However, if the
storage area temperature is expected to be below freezing (32° F) for
more than a day or two, move the food to a warmer storage area.
_What does mold on canned food indicate?_
It means the jar has not sealed and the food is spoiled. Even if mold
appears to be only on the surface, discard all food in the container
because parts of the mold may not be visible in the food.
_Is it safe to can foods without salt?_
Yes. Salt is used for flavor only in canned vegetables and is not
necessary for safe processing. Since the characteristic flavor and
texture of pickled vegetables depend on salt, do not omit this
ingredient from recipes for pickles and relishes.
_What kind of salt should be used in pickling? Why?_
Use pure granulated salt. Uniodized table salt can be used, but
materials added to the salt to prevent caking may make the brine cloudy.
Do not use iodized table salt because it may darken pickles.
_What type of vinegar should be used for making pickles? Can it be
diluted?_
Use cider or white distilled vinegar of 4% to 6% acidity (40 to 60
grain). Do not use vinegar of unknown acidity. Do not dilute vinegar
unless the recipe so specifies. If a less sour product is preferred, add
sugar rather than decrease vinegar.
_Why should pickles be processed in a boiling-water bath?_
Pickles require heat treatment to destroy organisms that cause spoilage,
and to inactivate enzymes that may affect flavor, color, and texture.
Heat processing in a boiling-water bath is recommended for all pickle
products. There is always danger of spoilage organisms entering the food
when it is transferred from kettle to jar. This is true even when the
utmost caution is observed and is the reason open-kettle canning is not
recommended.
_Why should plastic containers not be used when brining pickles?_
Vegetables being pickled undergo physical as well as chemical changes
during brining or fermentation. As a result of these changes, the
plastic may be affected, causing undesirable compounds to be formed or
leached from the plastic.
For fermenting or brining pickles, use a crock or stone jar, unchipped
enamel-lined pan, or large glass jar, bowl, or casserole.
_What causes pickles to be hollow?_
Hollowness in pickles generally results from poorly developed cucumbers,
holding cucumbers too long before pickling, too rapid fermentation, too
strong or too weak a brine during fermentation.
_What causes jelly to be too soft?_
Too much juice in the mixture, too little sugar, mixture not acid enough
(overripe fruit), or making too big a batch at one time.
_What makes jelly tough?_
Mixture was cooked too long to reach jellying stage because too little
sugar was used in proportion to the pectin and acid in the juice.
_What makes crystals form in jelly?_
Crystals throughout the jelly may be caused by too much sugar in the
jelly mixture, or cooking the mixture too little, too slowly, or too
long. Crystals on top of jelly that has been opened and allowed to stand
are due to evaporation of liquid.
Tartrate crystals in grape jelly may occur if juice has not been allowed
to stand overnight and then strained through a double thickness of
cheesecloth before preparing jelly.
_Is a one-door refrigerator-freezer combination suitable for freezing
and storage of frozen fruits and vegetables?_
It may be difficult to obtain the recommended temperature of 0° F or
below for freezing and storing foods in this style freezer without
freezing food in the refrigerator part as well.
Recommended storage times are severely reduced if a freezer does not
maintain 0° F or below. If freezer temperatures are above 10°, do not
store frozen food for more than several weeks.
_Can containers for commercial foods, such as cottage cheese, margarine,
milk, yogurt, ice cream, or sour cream, be used for freezing fruits and
vegetables?_
Waxed cardboard cartons which previously contained dairy products are
not sufficiently moisture-vapor-resistant to use for packaging foods to
be frozen.
Plastic commercial-food containers are suitable if they can be tightly
sealed and do not become brittle and crack at low temperatures, thus
exposing the food to the air.
_Can citric acid or lemon juice be used to help prevent fruit from
turning dark during freezer storage?_
Although these products can be used as anti-darkening agents, neither is
as effective as ascorbic acid. Often the quantity of citric acid or
lemon juice needed to prevent darkening is so large that natural flavors
are masked or the fruit becomes too sour.
_Why is it necessary to wash and blanch vegetables before they are
frozen?_
Washing removes dirt and some of the bacteria from vegetables. Freezing
inhibits the growth of bacteria, but does not kill them. Thus it's
important that the food, as well as all surfaces it touches, be kept
clean so that the number of bacteria on the food is held to a minimum.
Bacteria can grow on food if the temperature rises during freezer
storage, and when food is thawed.
Except for green peppers and mature onions, vegetables must be blanched
to destroy enzymes which could cause undesirable changes in flavor,
texture, and color during freezer storage.
_Why can green peppers and mature onions be frozen without blanching?_
Unlike other vegetables, green peppers and onions do not lose quality
during freezer storage if their enzymes are not destroyed by blanching
before freezing.
Green peppers frozen without heating are better suited for use in
uncooked foods than are blanched peppers. Some of the characteristic
flavor of onions is lost if this vegetable is blanched before freezing.
_Why is corn which is frozen on-the-cob blanched for longer times than
cut corn?_
Longer blanching of corn frozen on-the-cob is necessary so that enzymes
present in the cob will be destroyed. Otherwise, enzymes in the unheated
cob can cause undesirable flavor changes in the corn kernels.
Corn frozen off-the-cob needs only to be blanched just enough to destroy
enzymes in the kernels.
_Can vegetables be blanched by steaming instead of by heating in boiling
water?_
The following vegetables may be heated in steam: broccoli, mushrooms,
pumpkin, winter squash, and sweet-potatoes.
To steam these vegetables, put 1 to 2 inches of water in a large kettle;
bring water to a boil. Add a basket containing a single layer of
prepared vegetable; keep the basket at least 1 inch above the water.
Cover kettle and start timing.
Steam broccoli 3 minutes; sliced mushrooms, 3 minutes; whole mushrooms
(less than 1-inch diameter), 5 minutes. Steam pumpkin, winter squash, or
sweet-potatoes until tender.
_Is it necessary to make an adjustment in blanching times for vegetables
at altitudes above sea level?_
At altitudes 5,000 feet or more above sea level, heat vegetables 1
minute longer than the time given in directions for the vegetable being
blanched.
_What can be done to prevent food from thawing if the freezer should
stop running?_
Never open the freezer unnecessarily. A fully-loaded freezer will
usually remain cold enough to keep food frozen for 2 days if the door is
not opened; a half-loaded freezer may not stay cold enough more than a
day.
If power cannot be restored or the freezer cannot be fixed before the
food would start to thaw, use dry ice. If dry ice is obtained shortly
after the failure has occurred and the freezer is fully loaded, 25
pounds of dry ice should keep a 10-cubic-foot freezer at temperatures
below freezing for 3 to 4 days; if freezer is less than half-full, for 2
to 3 days.
Place dry ice on boards or heavy cardboard on top of the packages of
food. Handle dry ice carefully--never with your bare hands. Wear gloves
to prevent burns.
Another alternative is to move food to a neighbor's freezer or to a
freezer locker plant where space can be rented.
_Can vegetables and fruits which have thawed be refrozen?_
Frozen foods that have thawed may be safely refrozen if they still
contain ice crystals or if they are still cold--about 40° F--and have
been held no longer than 1 or 2 days at refrigerator temperatures (32°
to 40°) after thawing.
Since thawing and refreezing reduces the quality of fruits and
vegetables, use refrozen foods as soon as possible to save as much of
their eating quality as you can.