This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
In food preservation, the growth factors that are important are:
Sugar - enough sugar will stop the growth of most organisms
Salt - enough salt will stop the growth of most organisms
Acid - enough acid will stop the growth of most organisms
Water - dehydration. Lack of water inbibits all organisms. Below 35%
moisture, even C. botulinum, is directly inhibited in growth.
** Too little sugar, salt or acid will permit spoilage.
Air - most organisms must have air to grow, BUT the most dangerous bacteria in
home food preservation, Clostridium botulinum, will only grow without air.
Temperature - most dangerous microorganisms grow best at room temperature or
a little above. But in preserving food, we are interested in killing the
organisms and their spores, not just in slowing their growth.
The death rate of microorganisms depends on:
Microorganisms: They die at different rates. The number of cells or spores
present initially in the food the more there are, the longer it will take to
kill them all. The medium (food) that they are in most die faster in acidic
food than low acid food, and in wet food than dry food.
The temperature in canning: the important temperature is the temperature at
the coldest spot in the jar.
The length of time at that temperature when we heat the food: not all
the organisms will die at the same time, they die gradually, and the full
process time is necessary to be sure that all, even the most heat-resistant
ones, have died.
These last two factors, temperature and time, depend on how much solid vs.
liquid is in the jar, and on how tightly the food is packed. Heat from
the steam or water in the canner penetrates into different foods at different
rates. Liquids circulate in the jar and carry the heat into the center of
the jar. Solids must heat slowly from the outside in. A process time for
randomly packed green beans, which have spaces for water to circulate, will
not be adequate for "tin soldier" green beans, when the tightly packed,
vertically aligned beans leave no room for water to circulate.
The most important microorganism in home canning is Clostridium botulinum.
The toxins it produces damage the nervous system, producing paralysis and
possible death. The damage to nerve cells is permanent. Minute amounts of
contaminated food can carry enough toxin to cause death. This bacteria
produces spores which are very resistant to heat. It is also very sensitive
to acid, and will not grow in acid foods. Other pathogenic bacteria are usually
killed by much less heat and in a shorter period of time than Cl. botulinum.
Most require air, so will not grow in a sealed jar. They are of less concern
in home canning.
Molds and yeast are of concern because if they grow they can reduce the
amount of acid present in the food. If that occurs Cl. botulinum may be able
to grow. Some molds, particularly those that grow on fruits and fruit
products are known to produce toxins that cause damage to the nervous system and
kidneys, or cancer in research animals. The likelihood is that they will
cause some damage in humans if consumed often enough. (Toxin-producing molds
grow well on grains and peanuts, but these products are not home-canned.)
Molds and yeasts will also spoil the taste, texture, color and overall
appearance of the food, making it unfit for consumption.
Jams, Jellies, Sweet Spreads
In a jam or jelly recipe made with regular pectin, not the low or no-sugar
variety: If the jam or jelly sets properly (stiffens into jam or jelly) it
has enough sugar to inhibit the growth of bacteria and all but a few sugar
tolerant molds and yeasts. This will also be true for marmalades and
preserves, and for jellies made the long-boil method without added pectin.
The fruit blend used is not crucial.
However, mold growing on a fruit spread is a problem. It should not be
scooped off, rather the entire product should be discarded. To avoid mold
problems, all jellies, jams and sweet preserves should be packed in
pre-sterilized jars and processed 5 minutes or more in a boiling water
The exceptions are some of the sugar-free types which explicitly state on
the package of jelling agent that they should not be processed. These
contain preservatives to prevent mold growth, and the heat of processing
would cause soft jelly. In addition, heat will cause the sweetener to break
down and lose its sweet taste.
Pickles and Relishes
The pickle recipe is more complicated. The proportion of acid
(vinegar) to the amount vegetable is crucial. Enough vinegar must be added to
change the low-acid cucumber into a high-acid pickle to be safe. There is no
formula or set proportion to decide if the recipe provides for adequate vinegar.
The best thing to do is to find a recipe with similar procedures in the
USDA Guide to Home Canning and compare the amounts. This is especially
true of pickle relishes or vegetable relishes where several vegetables are
Similarities to look for include:
1. Similar recipes will use the same presoak - soak in ice water, or in
salt water, or no soak.
2. They will call for the same size cucumbers - 4", or 6", or 8", or
specify small or large.
3. The maturity of the cucumber influences how much acid it will take to
pickle it. Smaller, less mature cucumbers have the capacity to neu-
tralize more acid per unit of weight than do larger, more mature ones.
4. Similar recipes will also specify similar procedures with the brine: Are
the slices or spears packed in the jar raw and the brine poured over, are they
merely heated in the brine, or are they simmered before packing? Is the
simmering or boiling time the same? Each of these will influence how rapidly
the acid penetrates the cucumber and how much the cucumber juice will dilute
5. Similar recipes will call for similar proportions of onion or other
6. Quantities of salt are critical in fermented pickled products; proportion of
salt to vegetable to vinegar should be very similar to USDA recipe to be sure
that it will be safe. Proportions of spices are not crucial and may be adjusted
to suit tastes without danger.
If too little salt is used the cucumbers will spoil, get slimy, float, smell
foul, and the fermenting mixture may support the growth of hazardous
microorganisms. If too much salt is used, there will be no fermentation,
just shriveled cucumbers sitting in salt water. Either case is obvious: the
recipe is not good.
In quick-pack pickles the amount of salt is not critical. Salt may be
omitted, or a reduced sodium salt-type product used. The flavor and
texture may be noticeably different, and probably less acceptable but,
the product will be safe.
All pickle products should be processed in a boiling water bath to reduce the
likelihood of mold or yeast spoilage. Old recipes for whole or sliced
pickles that have been used for generations without processing and without
spoilage should at least be given a 10 minute process.
Pickle relish products must also adhere to the USDA proportions and process
times. Quantities of vegetable and vinegar, heating prior to packing, and
process time must be similar to a USDA recipe. An old, tested and trusted
recipe may be used if the 10 minute process time is used. Other recipes may
be changed, or the product refrigerated.
For comparison of quantities, note the following equivalencies:
1 lb 5" cucumbers = about 5 cucumbers
1 lb mushrooms = about 6 cups chopped = 1 1/2 cups sauteed
1 lb onions = about 3 cups chopped = about 4 medium
1 lb green peppers = about 3 C chopped = 8-9 peppers
1 lb sweet red peppers = about 3 cups chopped = 6-7 peppers
1 lb celery = about 4 cups chopped)
1 lb tomatoes = about 3 medium = about 1 1/2 cups chopped
22-23 lb tomatoes = about 7 quart or 28 cups cooked juice
The use of alum is unnecessary. The slight increase in crispness that it
provides is lost after about 2 months of storage. Few pickles are consumed
within 2 months of processing. However, since alum is usually used in very
small amounts, its use does not constitute a safety problem.
The use of grape leaves might contribute slightly to flavor. They have no
significant effect on safety.
Lime does cause a significant increase in the crispness of pickles. If it is
used, all excess lime must be rinsed away before the vinegar is added since it
will neutralize the vinegar. After the soak in lime water, the cucumber slices
should be soaked in fresh water then drained, re-soaked and drained two more
times (3 rinses in fresh water).
Honey may be used safely, but quantities will need to be adjusted for taste, and
color may be darker. One cup of sugar is equivalent to 3/4 C + 1 T honey
(or 1 C less 3 T).