Canning 101

Click here for step by step instructions to pressure canning.


People occasionally ask, “why use a water bath method or a pressure canner? My grandmother used to and she never got ill!” That may be true, just as there are occasionally smokers who live to 100 or people who cross the street without looking and don’t get hit by a car. But no rational person would recommend these either…

The US Department of Agriculture hash extensively studied food safety and home canning. To summarize; if you are canning at home, including making jams and jellies, you must use a water bath or pressure canner, if you want to avoid food poisoning. By food poisoning they are referring to varieties of bacteria, such as botulism, that grow in a sealed environment. Without hyperbole, death is one of the outcomes of such poisoning. See this page for detailed information about botulism food poisoning.

Some recipes, particularly old family recipes and many floating around on the internet don’t call for processing. The food is cooked in an ordinary pot or kettle, then packed into hot jars and sealed without processing (this is called the “open kettle method”).. The temperatures obtained in open kettle canning are not high enough to destroy all spoilage and food poisoning organisms that may be in the food. Also, microorganisms can enter the food when it is transferred from the kettle to jar and cause spoilage. These are obsolete and may be dangerous.

Canning recipes prior to 1990 should not be used. Many old recipes do not include instructions for processing foods. The foods are canned by the open kettle method, sealed and stored. This method for canning, the open kettle method, is not recommended for it presents a serious food safety hazard. All high acid foods should be processed in a water bath canner and all low acid foods in a pressure canner.

Grandma says I don’t need to use a water bath, just fill and seal the jars! Explain it to me, again, slowly…

Okay, at the time you sealed the jars, the temperature of the contents had already dropped way below 212 F. Meanwhile, the jars were exposed to the air (temp 70f to 90 F) which is not a sterile environment and does contain floating spores of Clostidium Botulism and other harmful bacteria. Therefore, the sealed jars now contain small amounts of live lethal bacteria.

Also, microorganisms can enter the food when it is transferred from the kettle to jar and cause spoilage. (Surprisingly to some, the air in your kitchen is not sterile).

Over time, those spores would grow. Typically, some bacteria grow that consume remaining oxygen and create an environment ideal for Clostidium Botulism to then grow as a secondary bloom. How fast and to what extent, is subject to many variables.

Instead, Water bath processing would kill the vast majority of those spores, creating a sealed sterile environment.



Equipment for heat-processing home-canned food is of two main types–boiling-water canners and pressure canners.

Most are designed to hold seven quart jars or eight to nine pints. Small pressure canners hold four quart jars; some large pressure canners hold 18 pint jars in two layers, but hold only seven quart jars. Small capacity pressure canners are treated in a similar manner as standard larger canners, and should be vented using the typical venting procedures.

Low-acid foods must be processed in a pressure canner to be free of botulism risks. This is because botulism-producing bacteria produce spores that can survive boiling water temperatures, but are destroyed using a pressure canner with the appropriate time and pressure, which reaches temperatures between 240 and 250 degrees F. Low-acid foods include meats, dairy, sea food, poultry, all vegetables (except tomatoes) and many fruits (notably figs).


Higher acid foods (and those which have been acidified and tested) that may be safely canned in a boiling water bath canner include jams, jellies, pickles, applesauce, apple butter, peaches, peach butter, pears, pear butter, spaghetti sauce without meat, tomatoes, ketchup and tomatoes.







There are advantages and disadvantages of Pressure and Boiling Water Bath Canners. Which is best for you depends upon what you want to can and your budget.

Water bath canners are faster for higher acid foods


Although pressure canners may also be used for processing higher acid foods, boiling-water canners are recommended for this purpose because they are faster. A pressure canner would require from 55 to 100 minutes to process a load of jars; while the total time for processing most acid foods in boiling water varies from 25 to 60 minutes. A boiling-water canner loaded with filled jars requires about 20 to 30 minutes of heating before its water begins to boil.

A loaded pressure canner requires about

  • 12 to 15 minutes of heating before it begins to vent;
  • another 10 minutes to vent the canner;
  • another 5 minutes to pressurize the canner;
  • another 8 to 10 minutes to process the acid food; and, finally,
  • another 20 to 60 minutes to cool the canner before removing jars.

But Water Bath Canners cannot be used for meats, dairy, sea food, poultry, vegetables and many fruits.

And the food quality and storage time is better with a pressure canner. Because they get hotter (240F vs 180F-212F) pressure canners result in a better flavor and the ability for to store for a longer time.

A pressure canner can be used as a boiling water bath canner, just remove the gauge and weight. That way you have 2 canners in one!


If you’ve tried some water bath canning; making jams or applesauce, you may have been tempted to can some other vegetables from you garden. If they are low-acid (and pretty much all vegetables other than tomatoes are low acid, not counting pickles – to which vinegar, which is acidic – is added) you’ll need a pressure canner! And you definitely need a pressure canner for anything containing any meat. There are limits to what you can safely can at home, though. Home canning of dairy is not recommended (USDA, Ball, etc.) , even with a pressure canner.

The open water bath canners can’t get water any hotter than 212 F and that’s not hot enough to kill the bacteria that can grow in low acid foods. A pressure canner hits 240F which allows for home canning of many more foods, like corn, beans, meats, etc.).



A water bath canner is fine for acidic fruits and vegetables, such as jams, jellies, applesauce, apple butter, and tomatoes, but for almost all other vegetables, like carrots, squash, green beans, squash, corn, okra, etc. you’ll need a pressure canner.

Clostridium botulinum is the bacterium commonly found in vegetables and meats. It is harmless until it finds itself in a moist, low-acid, oxygen-free environment or a partial vacuum. In other words, a sealed jar! Under these conditions, the bacterium can grow and produce toxins dangerous to people and animals. Yes, we have ALL heard about someone’s grandmother who canned without a pressure canner and lived to 90. And we all know people who smoked 2 packs a day and lived to 90. But neither is smart to do, and the odds will catch up with most people. You’re literally gambling with your life when you eat low acid foods that were canned using the open water bath, rather than pressure canning method.

Do not process (low acid) vegetables using the boiling water bath because the botulinum bacteria can survive that method.


Modern pressure canners are lightweight, thin-walled aluminum or stainless steel kettles. Most have twist-on lids fitted with gaskets. There are still one or two that have screw-down knobs around the lid on the canner. They have removable racks, a weighted vent port (steam vent), and a safety vent. They also have either a dial gauge for indicating the pressure or a weighted gauge (which both regulates the pressure and indicates, by rattling). Pressure canners can usually handle either one layer of quart or smaller size jars, or deep enough for two layers of pint or smaller size jars.

Unlike a water bath canner, the jars do not need to be completely covered with water. The directions that came with the pressure canner tell you how many cups of water to add in order for it to generate the right amount of pressure. You vent the pressure canner a considerable length of time while the water boils (with the jars in and the lid on). This causes steam to push out all the air. So the jars are in a space filled with only boiling water and steam. In theory, both will have the same temperature, which will be 240 F, substantially high than 212 F of an open water bath canner, due to the increased pressure. And since there is no air, just water vapor, the heat will be easily conducted to all sides of the jars.

Conclusion: Pressure canners cost more to buy, but ultimately, you can “can” more foods in them, store the foods longer, and use the same canner as a pressure canner or without sealing the lid, as a boiling water bath canner.


Some old bad habits of canning:

  • Fill the jars with hot fruit, vegetables or jam then just sealed and invert the jars.
  • Use the steam canner that my grandmother used and never got sick.
  • Use a water bath canner to can vegetables like beans and corn.
  • Use the oven to heat the filled jars, it gets plenty hot and the jars seal.
  • Use the microwave to blanch or heat the food then quickly seal the jars.
  • Use the dishwasher to process the filled jars.

As crazy 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 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. Botulism food poisoning is nothing to mess with! See this page for detailed information about botulism food poisoning.

Here are detailed explanations:

  • 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. 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. National Center for Home Food Preservation
  • 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 treatments.
    • 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.


  • 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 National Center for Home Food Preservation says:
  • “…This can be dangerous because the temperature will vary according to the accuracy of oven regulators and circulation of heat. Dry heat is very slow in penetrating into jars of food. Also, jars explode easily in the oven.” 
  • 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. 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 more dangerous.”
  • 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 National Center for Home Food Preservation says:
    “Because of possible mold contamination, paraffin or wax seals are no longer recommended for. any sweet spread, including jellies.”



  • Do not use overripe fruit. Canning doesn’t improve the quality of food, so if you start out with low quality, it will only get worse in storage.
  • Do not add more low-acid ingredients (onions, celery, peppers, garlic) than specified in the recipe. This may result in an unsafe product.
  • Don’t add substantially more seasonings or spices, these items are often high in bacteria and excess spices can make a canned item unsafe. I doubt whether increasing a spice from 1 teaspoon to 2 in a batch of 7 quarts will have any adverse effect, but use some common sense and don’t go overboard.
  • Do not add butter or fat to home-canned products unless stated in a tested recipe. Butters and fats do not store well and may increase the rate of spoilage. Adding butter or fat may also slow the rate of heat transfer, and result in an unsafe product.
  • Thickeners – With the exception of “Clear-Jel” which has been tested in USDA and university food labs, do not thicken with starches, flour, or add rice, barley or pasta to canned products – this applies to both savory products (such soups and stews), sauces and pickled items. Items that thicken products will absorb liquid during processing and slow the way in which the food heats. Under-processing and unsafe food could result.
  • DO add acid (lemon juice, vinegar or citric acid) to tomato products when directed in the recipe. In 1994, food scientists proved the risk of botulism poisoning from canned tomato products, and acid is now added to canned tomatoes, even to those canned commercially. Lemon juice is widely available, but will add a sharp note to canned tomatoes; citric acid will change the flavor less noticeably, and vinegar is part of many recipes anyway. If necessary, you can balance the tart taste by adding sugar.
  • Heat process (water bath canning or pressure canning, as called for in the recipe) all canned items that will be stored on the shelf. Some recipes, especially those for jams and jellies, instruct you to simply seal hot-filled jars, or to invert the jars as the final step. I know of no reputable source (university food science departments, the USDA, FDA, National Home Canning Center, etc.) that recommend either “open-kettle canning” or inverting jars as the final step, as unsafe final products may result.
  • Never process the jars in any oven (electric, gas or microwave). Steam canning is also, pretty broadly NOT recommended. There ARE a couple of manufacturers selling steam canners, but you’ll find virtually no credible authorities recommending them, for a variety of reasons, starting with basic heat transfers properties of steam vs. water.
  • Increase water-bath processing times at altitudes of 1,000 feet or more to compensate for the lower temperature of boiling water at high altitude. I’ve tried to be sure to include the conversion charts in all recipes for this.
    • Use a commercial ascorbic acid mixture like “Fruit-Fresh”, which is available at the grocery and drug stores. Sprinkle it over the cut fruit and mix well. OR
    • Put the cut fruit in a solution of 1 teaspoon ascorbic acid (vitamin C, available in a powdered form from the drug store) and 1 gallon water. Drain before canning.
    • Put the cut fruit into a lemon juice solution (3/4 cup lemon juice to 1 gallon water). Drain fruit before canning.To prevent darkening: Some peeled or cut fruits (such as peaches, apples, nectarines) will darken when exposed to the air. Any of these simple treatments will help prevent darkening:
  • Canning jars. Use standard mason / Ball / Kerr (etc.) jars for home canning. Commercial food jars that are not heat-tempered, such as mayonnaise jars, often break easily (although, I’ve had great luck with “Classico” brand quart spaghetti jars. Note that the Classico’s manufacturer does not recommend reuse of their jars: here is what they say on this page). Sealing also can be a problem if sealing surfaces do not exactly fit canning lids. Be sure all jars and closures are perfect. Discard any with cracks, chips, dents or rust. Defects prevent airtight seals.
  • Do not use jars larger than specified in the recipe, as an unsafe product may result. It’s almost always ok to go smaller. Generally speaking, quart jars are the largest size you should use.
  • To remove scale or hard water films on jars, soak several hours in a solution of 1 cup vinegar (5 percent) per gallon water. Keep the jars warm until ready to fill (to reduce breakage from thermal shock).
  • Prepare the two-piece metal canning lids by washing them in water and following the manufacturer’s instructions for heating the lids (some need to be covered with hot water for a minute or more – in steaming, but not boiling water).
  • The flat lids can be used only once, but the screw bands can be reused as long as they are in good condition. Read Do not reuse lids from commercially canned foods.
  • Check jars for seals within 24 hours of canning. Treat jars that fail to seal properly as if they are fresh (refrigerate and eat soon).



  • Fill the canner at least halfway with water. A little practice will help you to know how much water you will need to start out with to ensure the jars will be covered by at least 1 inch of water.
  • Preheat water that is added to the jars (when called for) to very warm but not boiling (around 140 degrees F) for raw-packed foods (the lower temperature helps to reduce jar breakage) and to boiling for hot-packed foods.
  • Put the filled jars, with lids and rings on, onto the canner rack and use the handles to lower the rack into the water. Or you may fill the canner, one jar at a time, with a jar lifter. Obviously, you’d need to be quick, or the first jar could be in the bath for substantially longer than the last jar you add. If you don’t use a jar rack, then a flat rack on the bottom helps to reduce break. One of these comes with each canner.
  • Always add more boiling water, if needed, so the water level is at least one inch above the tops of the jars.
  • Turn heat to its highest position until the water boils vigorously, and then set a timer for the minutes required for processing the food.
  • Cover the canner and if necessary, lower the heat setting to maintain a full but gentle boil throughout the processing time. Generally, I find I need to keep the burners on high.
  • If one burner doesn’t produce enough heat to keep the water boiling, you can usually straddle two burners with the canner.
  • When the jars have been boiled for the recommended time, turn the heat off and use a jar lifter to remove the jars and place them on a towel in an area that is not drafty, leaving at least one inch between the jars during cooling.
  • Do not retighten the jar lids – it may break the seal.
  • Do not leave the jars in the boiling water after processing time is done, because the food will become overcooked.
  • Check the jar seals 12-24 hours after processing for leaks and broken seals. Just press down on the lid. If it seals, it will be sucked down tight. If it did not seal, it will flex and make a popping sound each tip. See this page for more information.
  • To store, remove the screw bands and wipe the jars clean. Otherwise, the rings may rust tight to the jar!
  • Any jar that fails to seal can be reprocessed in a clean jar with a new lid. Reprocess within 24 hours. Generally, it is better to refrigerate the jar and use it within several days. The jar may also be stored in the freezer if the headspace is adjusted to 1-1/2 inches to allow for the expansion of the product.




Most two-piece lids will seal with a “pop” sound while they’re cooling, as the lid gets sucked down by the vacuum created by the contents cooling and contracting inside the jar. After cooling jars for 12 to 24 hours, remove the screw bands and test seals with one of the following options (do not test the jars while they are still hot!):


  • Option 1: Press the middle of the lid with a finger or thumb. If the lid springs up when you release your finger, the lid is unsealed.
  • Option 2: Tap the lid with the bottom of a teaspoon. A clear ringing sound means a good seal. If it makes a dull sound, the lid is not sealed. If food is in contact with the underside of the lid, it will also cause a dull sound (that is not a problem nor a sign of spoilage). If the jar is sealed correctly, it will make a ringing, high-pitched sound.
  • Option 3: Hold the jar at eye level and look across the lid. The lid should be concave (curved down slightly in the center). If center of the lid is either flat or bulging, it may not be sealed.

If a jar is not sealed, refrigerate it and use the unspoiled food within two to three days. Other options are to reprocess (see below) the food within 24 hours or to freeze it.


If, by the time that a home canned jar reaches room temperature (normally within 12 to 24 hours of canning), a lid fails to seal on the jar, remove the lid and check the jar-sealing surface for tiny nicks. If necessary, change the jar, add a new, properly prepared lid, and reprocess within 24 hours using the same processing time. Headspace in unsealed jars may be adjusted to 1½ inches and jars could be frozen instead of reprocessed. Foods in single unsealed jars could be stored in the refrigerator and consumed within several days.


If lids are tightly vacuum sealed on cooled jars, remove screw bands, wash the lid and jar to remove food residue; then rinse and dry jars. Label and date the jars and store them in a clean, cool, dark, dry place. For best quality, store between 50 and 70 °F. Can no more food than you will use within a year.

Do not store jars above 95° F or near hot pipes, a range, a furnace, in an uninsulated attic, or in direct sunlight. Under these conditions, food will lose quality in a few weeks or months and may spoil. Dampness may corrode metal lids, break seals, and allow recontamination and spoilage.

Accidental freezing of canned foods will not cause spoilage unless jars become unsealed and recontaminated. However, freezing and thawing may soften food. If jars must be stored where they may freeze, wrap them in newspapers, place them in heavy cartons, and cover with more newspapers and blankets.


Do not taste food from a jar with an unsealed lid or food that shows signs of spoilage.

You can more easily detect some types of spoilage in jars stored with the rings (the screw bands that hold the lids down) removed. Growth of spoilage bacteria and yeast produces gas which pressurizes the food, swells lids, and breaks jar seals. As each stored jar is selected for use, examine its lid for tightness and vacuum. Lids with concave centers have good seals.

Next, while holding the jar upright at eye level, rotate the jar and examine its outside surface for streaks of dried food originating at the top of the jar. Look at the contents for rising air bubbles and unnatural color.

While opening the jar, smell for unnatural odors and look for spurting liquid and cotton-like mold growth (white, blue, black, or green) on the top food surface and underside of lid.

Spoiled low-acid foods, including tomatoes, may exhibit different kinds of spoilage evidence or very little evidence. Therefore, all suspect containers of spoiled low-acid foods, including tomatoes, should be treated as having produced botulinum toxin and handled carefully in one of two ways:

  • If the swollen metal cans or suspect glass jars are still sealed, place them in a heavy garbage bag. Close and place the bag in a regular trash container or bury it in a nearby landfill.
  • If the suspect cans or glass jars are unsealed, open, or leaking, they should be detoxified before disposal.

If you did not add any acid (lemon juice, vinegar, citric acid, etc.) to tomato products and you did not follow an approved recipe or process for the right amount of time, they may be contaminated. Outside of a lab test, there is no way to know with certainty.


  • A bulging lid or leaking jar is a sign of spoilage.
  • When you open the jar, look for:
    • spurting liquid,
    • an off-odor
    • mold


Improperly canned low-acid foods (most vegetables, tomatoes, meats, seafood, etc.) can contain the toxin that causes botulism without showing signs of spoilage. Low-acid foods are considered improperly canned if any of the following are true:

  • The food was NOT processed in a pressure canner.
  • The canner’s gauge was INACCURATE.
  • Up-to-date researched processing times and pressures were NOT used for the size of the jar, style of pack and kind of food being processed.
  • Ingredients were added that were NOT in an approved recipe.
  • Proportions of ingredients were CHANGED from the original approved recipe.
  • The processing time and pressure were NOT correct for the altitude at which the food was canned.

Because improperly canned low-acid foods can contain the toxin that causes botulism without showing signs of spoilage, they should also be detoxified as directed below and then discarded.

Surfaces that come in contact with spoiled or questionable food should be cleaned with a solution of one part chlorine bleach to five parts water. Wet the surface with this solution and let stand five minutes before rinsing.



This is what the National Center for Home Food Processing recommends:

  1. Carefully place the suspect containers and lids on their sides in an 8-quart volume or larger stock pot, pan, or boiling-water canner.
  2. Wash your hands thoroughly.
  3. Carefully add water to the pot. The water should completely cover the containers with a minimum of a 1-inch level above the containers. Avoid splashing the water.
  4. Place a lid on the pot and heat the water to boiling.
  5. Boil 30 minutes to ensure detoxifying the food and all container components.
  6. Cool and discard the containers, their lids, and food in the trash or bury in soil.
  7. Thoroughly scrub all counters, containers, and equipment including can opener, clothing, and hands that may have contacted the food or containers.
  8. Discard any sponges or wash cloths that may have been used in the cleanup. Place them in a plastic bag and discard in the trash.

Honestly, the USDA process above looks pretty darn rigorous and time-consuming for the home canner. I usually pour the contents down the garbage disposal, rinse the jars and boil them in water for 10 minutes. I then pour the boiling water into the sink, then spray the sink with a Clorox solution and let it stand for 10 minutes before wiping up with paper towels. I put the lids and rings in a sealed trash container where children and animals cannot come in contact with them.


Click here for step by step instructions to pressure canning.


  • Inclilegente and simplicity – easy to understand how you think.

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