- 1 What foods is pressure canning used for?
- 2 What is it about pressure canning that helps preserve the food inside the jars?
- 3 How safe is my pressure-canned food?
- 4 It is steam, not water, that does the heat processing
- 5 What if I add some acid, can I water bath safely instead?
- 6 How is process time for pressure canning determined?
- 7 Venting a pressure canner
- 8 Why does most pressure canning happen at 10 instead of 15 lbs as the base sea-level standard?
- 9 Is it pressure or temperature that kills the nasties?
- 10 What temperatures are reached in pressure canning?
- 11 Maintain a steady pressure for your pressure canner
- 12 The cool-down process is important
- 13 Keeping Your Pressure Canner Clean
- 14 Further reading
What foods is pressure canning used for?
- low acid;
- vegetables (generally all);
- mixtures of low-acid and acid foods (e.g. spaghetti sauce).
What is it about pressure canning that helps preserve the food inside the jars?
- Removal of oxygen;
- Sealed jar or can (so nothing can re-enter);
- Heat (pressure temperatures).
Elizabeth Andress, head of the National Center for Home Food Preservation (NCHFP), says
There is not enough acidity inside the jar to rely on inhibiting harmful microorganisms, so since we can’t inhibit them, we must destroy them by raising the heat sufficiently. The heat also inactivates enzymes.” 1
How safe is my pressure-canned food?
In their blog, the National Center for Home Food Preservation says that you can trust the results of your pressure canning if you are following recommendations:
…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…… You can trust that your home-canned foods will receive adequate heat treatment by using proper canning methods and following recommended process times.”2
The University of Alaska says,
“As long as the canner is…. working well and foods are processed for the prescribed times and temperatures, pressure canning is a perfectly safe and efficient method for canning low-acid foods.” 3
Author Lisa Rayner says,
“Pressure canning recommendations are designed to kill spores while still maintaining the sensory and nutrient value of the food…”4
As far back as 1933, the University of Michigan was saying:
“…the greatest argument in favor of the pressure [canner] is the safety of the products canned. The Bureau of Home Economics does not recommend any other method for home canning of non-acid vegetables and meats.” 5
It is steam, not water, that does the heat processing
When pressure canning, you don’t immerse the jars fully in water, as you do with water-bath canning. Instead, you will be leaving lots of air space inside the canner which will become filled with super-heated steam. You will put in just enough water sufficient to create and maintain a head of steam for the processing time. The reason for this is that water can’t get any hotter than the boiling point, but steam can, and it’s that higher heat that we are after.
Your pressure canner manual will tell you how much water is needed to generate the steam. Follow its recommendation. The amount of water can vary between brands and models.
What if I add some acid, can I water bath safely instead?
No. For low-acid foods that need to be pressure-canned, adding some acidity does not excuse you from having to process for the entire required time in a pressure canner, nor let you switch to doing the processing with a waterbath or steam canner instead. The only exception is if you add so much vinegar that you are now canning a pickled preserve: then you can water bath that, but that is a whole different food product altogether.
Mississippi State University Extension Service says,
Q: Is it safe to can green beans in a boiling water bath if vinegar is used?
A: No. You must use recommended processing methods to assure safety. You cannot shorten recommended processing times if you use vinegar in canning fresh vegetables (this does not refer to pickled vegetables). 6
You may see a few foods for which you see both water bath and pressure canning options. Those are really water-bathable food products, for which pressure canning is just used to reduce processing time required. Dr Elizabeth Andress of the National Center for Home Food Preservation explains;
Whenever you see pressure canning processes for some foods such as tomatoes and apples, it’s an equivalent process for an acid food — that particular pressure process time is not intended for low-acid.” 7 [Ed: in other words, the pressure canner is just being used to speed up the processing time on already-acidified foods; it’s not being use as a proper sterilization process in itself.]
How is process time for pressure canning determined?
What factors affect process times?
- how large the food pieces are;
- bones / boneless in meat;
- how tightly it’s packed in jar;
- initial temperature of jar;
- size and shape of jar. 8
Factors that slow heat penetration in pressure canning
- adding extra sugar or fat;
- larger pieces of food than called for;
- adding thickeners.
Marilyn Swanson at the Pacific Northwest Extension says,
At a temperature of 240°F (115 C), the time needed to destroy bacteria in low-acid canned food ranges from 20 to 160 minutes. The exact time depends on the kind of food, the way it is packed into jars, and jar size. The exact times provided in recipes for specific foods have been determined by laboratory research. Be sure to use them!” 9
Determining the Process Times for Foods
The length of time required to process a food is based upon:
- the acidity of the food;
- how the food is prepared (left whole, cut into halves, chopped into 1 inch cubes, etc.);
- the composition of the food (the viscosity of the product; how tightly it is packed into the jars; how much liquid versus solid is present; the amount of starch, fat or bone that is present);
- whether the heat will be transferred by convection or conduction;
- the temperature of the food when it is placed into the jar or can;
- the temperature at which the food is processed;
- the size of the jar or can;
- the shape of the jar or can.
The National Center for Home Food Preservation Self Study Course teaches,
Process times for specific products in particular sizes of jars are calculated based on how heat penetrates through the jar to reach the coldest spot of the jar. The consistency and size of the food, as well as the amount of liquid present, determine whether the food will heat by conduction, convection or a combination of both. Therefore, process time is affected by how a food heats. Heat penetration studies are used to scientifically determine safe processing times for foods. In order for the food to be safe, the coldest spot in the jar of food must reach the correct temperature for the correct length of time to destroy target pathogens. When canning, it is important to follow the tested recipe exactly. Heat penetration can be slowed down by adding extra sugar or fat, oversized pieces of food or thickeners.” 10
Venting a pressure canner
Venting is also called “exhausting” the canner. It’s a process of letting steam come out of your canner for a period of time before you seal the canner down to bring it up to pressure. Without proper venting, up to 30% of the sterilizing value of a 20-minute process may be lost. 11
And whatever air is in the canner when the lid goes on, is added to by air that escapes from inside the jars in the canner. The most “jar air” comes from those with raw-packed foods (that haven’t had a pre-blanching to help release air from them):
The highest volume of air trapped in a canner occurs in processing raw-packed foods in dial-gauge canners. These canners do not vent air during processing.”12
If you see pressure canning directions in a book that omit a specific air venting process at the start, find another canning guide to follow instead.
Here’s how it happens
- As water boils in a canner with its lid on, the empty space / spaces inside the canner become a mixture of steam and air;
- The air comes not only from being inside the canner when the lid is put on, but, it also gets released from inside the jars as they heat;
- This steam / air mixture won’t get as hot as a pure steam environment, and that’s an issue, because USDA process times are intended only for a pure steam environment.
- So, we have to let the steam drive all the air out, in order to create a pure steam environment.
- Steam must flow freely from the open ventport in the lid for 10 minutes prior to pressurizing.
NOTE: The venting time required can be longer if you are pressure canning fish or seafood. Follow those venting procedures from your tested recipe.
The following is from an NCHFP slide presentation, ” Pressure Canning and Canning Low Acid Foods at Home” 13
- After putting filled jars in the pressure canner, fasten the lid in place;
- Leave the ventport open;
- Pipe where weighted gauge or dead weight will go;
- Turn the heat on high;
- When water boils, steam will start to come out of open vent;
- Wait until there is a constant, strong funnel of steam, then start timing 10 minutes;
- At the end of the 10 minutes, place weight in place to start pressurizing the canner.
Should anything happen such as a gasket reveal itself to be faulty, replace it with a fresh one, reheat the canner and start the venting timing all over again. (For this reason, it’s a good idea to keep a spare gasket to hand — gaskets generally only reveal themselves to be worn out during use!)
Some manufacturers of some weighted gauge canners may say venting is not necessary. The USDA instructs instead to vent ALL pressure canners. This is the one time they tell you to ignore manufacturer’s directions if the manufacturer says otherwise.
The National Center for Home Food Preservation Self Study Course notes:
Although some manufacturers of weighted gauge canners say venting is not necessary, USDA recommends venting all canners. Without proper venting up to 30% of the sterilizing value of a 20 minute process may be lost.” 14
If the model of pressure canner you are looking at has no provision for venting, that may be an area of concern. The USDA gives these directions:
Air trapped in a canner lowers the temperature obtained at 5, 10, or 15 pounds of pressure and results in under processing. The highest volume of air trapped in a canner occurs in processing raw-packed foods in dial-gauge canners. These canners do not vent air during processing. To be safe, all types of pressure canners must be vented 10 minutes before they are pressurized.
To vent a canner, leave the vent port uncovered on newer models or manually open petcocks on some older models. Heating the filled canner with its lid locked into place boils water and generates steam that escapes through the petcock or vent port. When steam first escapes, set a timer for 10 minutes. After venting 10 minutes, close the petcock or place the counterweight or weighted gauge over the vent port to pressurize the canner.” 15
History of the venting recommendation
The venting recommendation has been made since at least 1944. Dr Elizabeth Andress says,
Since 1944, USDA has recommended that pressure canners be vented for 10 minutes prior to pressurization. A mixture of air and steam will create gauge pressure without the necessary corresponding high temperature (Esselen, 1944; Esselen and Fellers, 1950), and could result in understerilization” 16
The venting procedure was firmly codified in the very first USDA Complete Guide in 1988. An extension agent from Pennsylvania wrote:
If using a pressure canner, be sure to follow the new recommendation of venting the pressure canner for ten minutes before allowing the pressure to build. Venting means to allow steam to escape freely from the canner.”17
NOTE: The venting time required can be longer if you are pressure canning fish or seafood.
Why does most pressure canning happen at 10 instead of 15 lbs as the base sea-level standard?
The recommendation is partly arbitrary, and only because the appropriate research has not been done yet as of this point in time. Dr Elizabeth Andress notes in her review of the literature available:
A few recommendations have been made over the years for home canning of meats at 250F / 121 C (15 psig) (Cover et al, 1943). It was later found to be unnecessary, but the higher temperature was and is used successfully, especially with commercial products. At present, 240F / 115 C (10 psig) is the most commonly recommended temperature. More recent interest in conserving nutritional and textural characteristics in canned food has led to some research with higher pressure (temperature) and shorter time processes. Use of 250F / 121 C in home canning has been limited due to a lack of research at this temperature. Shorter process times would be required at 250F / 121 C, as lethality accumulates about three times faster than at 240F / 115 C (Zottola et al, 1978). These authors and Nordisen et al (1978) recently investigated heat penetration rates for low acid foods processed at 15 psig. Convection-heated foods required about one-third the time as specified for processes at 10 psig. Those foods heated by conducting or a mixed mechanism appeared to require equal time at 10 and 15 psig. It was concluded that each product be investigated before recommendations could be made.” 18
Is it pressure or temperature that kills the nasties?
Note that it’s not pressure that kills the nasties by squishing them or anything, but rather it’s the temperature that the pressure brings with it.
The USDA says,
Pressure does not destroy microorganisms, but high temperatures applied for an adequate period of time do kill microorganisms. The success of destroying all microorganisms capable of growing in canned food is based on the temperature obtained in pure steam, free of air, at sea level. At sea level, a canner operated at a gauge pressure of 10.5 lbs provides an internal temperature of 240°F / 115 C. 19
The authors of Putting Food By help to explain temperature differences:
The type of heat is vitally important, because the ability to transfer heat varies between wet heat and dry heat. Illustration: hold your hand in a steady, strong flow of steam from the spout of a teakettle for one minute, and it will be burned enough to blister badly; hold it in the dry air of an oven at the same temperature reading for a minute and it will be pleasantly warmed. To carry the idea further, the 240 F/116 C steam in a Pressure Canner at 10 pounds psig (pounds per square inch by gauge) at sea level has a much greater effect on the heat transfer process than does the atmosphere of an oven operating at 240 F/116 C. So Never Forget This: only under GREATER THAN ATMOSPHERIC PRESSURE can you produce wet heat that is hotter than the boiling point of water at your altitude. And you will need wet heat under pressure to reach deep enough inside a container of low-acid food to destroy the spoilers that can make it nasty, even dangerous, to eat. If you remember this Why, you’ll always be able to keep track of the How in canning food at home.” 20
What temperatures are reached in pressure canning?
Temperatures of 240 F / 115 C or above are needed to keep process times down to a reasonable amount of time.21
The following is a table of temperatures that can be reached at sea level at given pressures:
Maintain a steady pressure for your pressure canner
As you get into the first few minutes of your processing time, you want to be at a good steady heat. Otherwise wild heat variations can cause siphoning from the jars.
Elizabeth Andress from the National Center for Home Food Preservation says,
[A] drop in pressure during processing means the sterilizing value of the process will be decreased == Underprocessing. Foodborne illness (botulism) and/or spoilage could result. If pressure drops below target anytime during the process time, bring the canner back up to pressure and start timing the process over, from the beginning. 22
Large and/or quick variations in pressure during processing may cause loss of liquid from jars.”23
The cool-down process is important
A lot of the lethality (killing of the nasties) occurs during the cool-down process, so it’s important to let your pressure canner cool down naturally.
Keeping Your Pressure Canner Clean
Carolyn Ainslie from the National Center for Home Food Preservation gives this easy tip for cleaning the inside of your canner:
The darkened surface on the inside of an aluminum canner can be cleaned by filling it above the darkened line with at mixture of 1 tablespoon cream of tartar to each quart of water. Place the canner on the stove, heat water to a boil, and boil covered until the dark deposits disappear. Sometimes stubborn deposits may require the addition of more cream of tartar. Empty the canner and wash it with hot soapy water, rinse and dry. (Hint: deposits from hard water may be reduced if you add 1 tablespoon of white vinegar to the water in the canner while you process your jars.)” 24
Andress, Elizabeth. “History, Science and Current Practice in Home Food Preservation.” Webinar. 27 February 2013. (42:00) Accessed January 2015 at https://nchfp.uga.edu/multimedia/video/nchfp.wmv ↩
Andress, Elizabeth. Why Can’t I Just Guess at a Process Time for Canning? NCHFP. Preserving Food at Home Blog. 26 February 2014. Accessed October 2016 at https://preservingfoodathome.com/2014/02/26/why-cant-i-just-guess-at-a-process-time-for-canning/ ↩
Dinstel, Roxie Rodgers. Food Preservation. Back to Basics, Lesson 1. University of Alaska Fairbanks Cooperative Extension Service. FNH-00562A. Revised March 2013. ↩
Rayner, Lisa. The Natural Canning Resource Book. The Natural Canning Resource Book. Flagstaff, Arizona: Lifeweaver LLC. 2010. Page 35. ↩
Gilbert, Muriel Dundas. Successful Home Canning. Michigan Extension Bulletin No. 132. May 1933. Page 6. Accessed March 2015 https://archive.lib.msu.edu/DMC/Ag.%20Ext.%202007-Chelsie/PDF/e132.pdf ↩
Fountain, Brent. Home Canning: Questions and Answers. Mississippi State University Extension Service. Publication 99. Accessed March 2015 at https://msucares.com/pubs/publications/p0993.pdf. Page 7. ↩
Andress, Elizabeth. “History, Science and Current Practice in Home Food Preservation.” Webinar. 27 February 2013. Accessed January 2015. ↩
Andress, Elizabeth. “History, Science and Current Practice in Home Food Preservation.” Webinar. 27 February 2013. 1:00:00. Accessed January 2015 at https://nchfp.uga.edu/multimedia/video/nchfp.wmv ↩
Marilyn A. Swanson. Using and caring for your pressure canner. Pacific Northwest Extension. July 2013. PNW .421. Accessed March 2015 at https://www.cals.uidaho.edu/edcomm/pdf/PNW/PNW0421.pdf ↩
National Center for Home Food Preservation Self Study Course. Module 2. General Canning: Importance of Heat Penetration in Canning. Accessed March 2015. ↩
Andress, Elizabeth L. Pressure Canning and Canning Low Acid Foods at Home. Cooperative Extension: University of Georgia. Powerpoint presentation, slide 23. Accessed March 2015 at nchfp.uga.edu/multimedia/slide_shows/CanLowAcid_web08.ppt. ↩
United States Department of Agriculture (USDA). Complete guide to home canning. Agriculture information bulletin No. 539. 2015. Page 1-20. ↩
Andress, Elizabeth L. Pressure Canning and Canning Low Acid Foods at Home. Cooperative Extension: University of Georgia. Powerpoint presentation, slide 22-24. Accessed March 2015 at nchfp.uga.edu/multimedia/slide_shows/CanLowAcid_web08.ppt. ↩
National Center for Home Food Preservation Self Study Course. Module 4. Canning Low Acid Foods: Dial Gauge Canners. Accessed March 2015. ↩
United States Department of Agriculture (USDA). 2015. Complete guide to home canning. Agriculture information bulletin No. 539. Available at: https://nchfp.uga.edu/publications/publications_usda.html (Accessed March 2015). Page 1-20. ↩
Pressure Canning Equipment. In: Andress, Elizabeth L and Gerald Kuhn. Critical Review of Home Preservation Literature and Current Research. IV. Equipment and its Management – History and Current Issues. Athens, GA: University of Georgia, Cooperative Extension Service. 1983. ↩
Redding, Mina. Extension Home Economist. Use new guidelines when canning. Gettysburg, Pennsylvania: The Gettysburg times. 10 August 1988. Page 4B. ↩
Pressure Canning Equipment. In: Andress, Elizabeth L and Gerald Kuhn. Critical Review of Home Preservation Literature and Current Research. IV. Equipment and its Management – History and Current Issues. Athens, GA: University of Georgia, Cooperative Extension Service. 1983. https://nchfp.uga.edu/publications/usda/review/equip.htm#psig ↩
United States Department of Agriculture (USDA). Complete guide to home canning. Agriculture information bulletin No. 539. 2015. Page 1-19. ↩
Hertzberg, Ruth; Greene, Janet; Vaughan, Beatrice (2010-05-25). Putting Food By: Fifth Edition (pp. 45-46). Penguin Publishing Group. Kindle Edition. ↩
Andress, Elizabeth L. Pressure Canning and Canning Low Acid Foods at Home. Cooperative Extension: University of Georgia. Powerpoint presentation, slide 5. Accessed March 2015 at nchfp.uga.edu/multimedia/slide_shows/CanLowAcid_web08.ppt. ↩
Andress, Elizabeth L. Pressure Canning and Canning Low Acid Foods at Home. Cooperative Extension: University of Georgia. Powerpoint presentation, slide 32. Accessed March 2015 at nchfp.uga.edu/multimedia/slide_shows/CanLowAcid_web08.ppt. ↩
Andress, Elizabeth L. Pressure Canning and Canning Low Acid Foods at Home. Cooperative Extension: University of Georgia. Powerpoint presentation, slide 33. Accessed March 2015 at nchfp.uga.edu/multimedia/slide_shows/CanLowAcid_web08.ppt. ↩
Ainslie, Carolyn and Elizabeth Andress. When It’s Time to Store Canning Supplies. National Center for Home Food Preservation. December 2005. Accessed March 2015 at https://nchfp.uga.edu/tips/fall/store_canning_supplies.html ↩