Why Can’t I Just Guess at a Process Time for Canning?

Mold on JamIf you guess at a process time for canning, you run the risk of underprocessing your food, which could lead to food poisoning and/or product loss due to spoilage.

All reliable recommendations for canning include process times that have been determined by or based on results of laboratory testing. The exact time and temperature combinations of tested canning processes are needed to assure the destruction of microorganisms that may be present in the filled jars. Sure, it’s possible that you could use unsafe canning practices for some time without causing waste or harm, but it only takes one batch of food with destructive microorganisms in it to ruin your streak of luck. And especially if you are canning low acid foods, the consequences could be severe and irreversible.

While some microorganisms are apparent just by looking (think molds growing on the surface of jelly), others remain invisible to your bare eyes (think pathogenic bacteria which cause food poisoning). Many different types of mold, yeast, and bacteria dwell on food. Given their preferred conditions of moisture, acidity, oxygen levels, and temperature they will grow, and some will even product toxin.

Fortunately, all microorganisms can be destroyed by breaking their threshold for heat. The process times provided in each of our home canning recommendations have been found to deliver enough heat to destroy microorganisms of concern in that particular food. Characteristics such as consistency, pH level (acidity), size of food pieces, presence of protective nutrients, size and shape of jars, and solid to liquid ratio all influence the ability of heat to move through and thoroughly penetrate the entire contents of a filled jar. You can trust that your home-canned foods will receive adequate heat treatment by using proper canning methods and following recommended process times.

Why does each recipe have a process time table with multiple times or amounts of pressure? (See Crushed Tomatoes for a good example.) Because atmospheric pressure decreases as altitude increases, causing water to boil at lower temperatures as altitude increases. In order to achieve the same overall heat treatment as at sea level, more time is needed in a boiling water canner at higher elevations (since the temperature of the water distributing the heat is less). Pressure canning relies on temperatures inside the canner building even higher than that of boiling. So, likewise, more pressure needs to be applied to a pressure canner at higher elevations so that the temperature inside can reach higher.

If you’d like to learn more about how process times are determined for home-canned foods, please read ‘Backgrounder: Heat Processing of Home-canned Foods’ by Elaine M. D’Sa.

2 thoughts on “Why Can’t I Just Guess at a Process Time for Canning?

  1. Jo-Ann

    Does this also apply to the time of boiling water bath in jelly/jam making? I’ve seen recipes that call for BWB that range from 5 minutes to 20 minutes.

    1. nchfp Post author

      You have correctly identified a low-risk category of canned foods, IF the jam or jelly is a traditional, pectin-gelled fruit product. However, not all jams and jellies are the same in term of ingredients and final consistency to allow for the same process time for the whole category. If you find some that say 20 minutes, there may be a very good reason in that it does not meet the traditional characteristics of those that only require a 5 or 10 minute process. People are making all kinds of spreads they call jams or jellies but may not technically fit that category for preservation purposes. (For example, some chefs publish a “bacon jam” that has none of the needed characteristics for canning like a 5-10 minute processed fruit jam.)

      So, yes, please follow recommended process times for all home-canned products, including jams and jellies. An adequate process time is important to the safety and quality of canned products, whether they be jams, jellies, pickles, or fruits in a boiling water canner or low-acid foods like meats or vegetables in a pressure canner.

      The process times vary for jams and jellies (or spreads people are calling that) just like for other foods because different lengths of time are needed to destroy microorganisms based on many characteristics, as explained in the posting (heating patterns are affected by texture, liquid to solid ratio, acidity, jar size, etc.). There is not an across-the-board process for a whole loosely-defined category of foods. Underprocessing is a risk because mold, yeast, and bacteria may survive and grow on the food. Overprocessing, which happens when you leave filled jars in a canner for longer than the recommended amount of time, does not present a safety concern but you’d want to avoid overprocessing for quality reasons. Overprocessing can cause undesirable texture and color changes.

      In traditional fruit jams and jellies gelled by pectin and sugar or a no-sugar-needed commercially purchased pectin, the most common microbial activity is from mold and yeast. Mold growth is most often identifiable by the visible appearance of fuzz, spots, and/or unusual coloring and yeast growth is identifiable by off-odors caused by fermentation. In addition to an unpleasant taste, some types of mold will cause digestion problems and sickness if consumed. Fermentation by yeast creates lactic acid or alcohol which have a sour flavor and also the potential to lead to undesirable effects if consumed.

      Clostridium botulinum, the bacteria which causes botulism, is not a concern in these fruit jams and jellies because it cannot survive in the high sugar content and acidity of jams and jellies. However, it is still possible that underprocessed jam or jelly could lead to food poisoning, as well as spoilage, from other microorganisms. To reiterate: “If you guess at a process time for canning, you run the risk of underprocessing your food, which could lead to food poisoning and/or product loss due to spoilage.” And then there are the cases of even low-acid mixtures of vegetables or bacon cooked down to spreading consistency being called “jams” and then there are many more food safety risks in canning including botulism.

      By the way, if you do see mold at the surface of even your fruit jam or jelly, you may not also see that it also grows underneath the surface, but it does. If you notice mold on jam or jelly, or signs of yeast spoilage, discard the entire or rest of the container of any jams or jellies.

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