How to process jars of jam in a boiling water canner
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To process jam in a boiling water canner, set filled, lidded jars on a rack in water heated to about 180°F, add hot water until the jar tops sit under at least 1 inch, cover the canner and bring it to a vigorous rolling boil. Start the timer only then, keep the water boiling for the whole time your recipe gives for your elevation, turn off the heat, take the lid off and wait 5 minutes before lifting the jars out upright [1]. This page walks through each stage in order, with the reasons behind the steps that people most often skip.
What the water bath does for jam
Jam in a jar that has only been filled hot and capped has not been heated all the way through after the lid went on. Boiling the closed jars does two jobs. Covering the jars in boiling water drives oxygen out of the headspace, which would otherwise feed mold [4], and a process of 10 minutes or more sterilizes the jars themselves [2]. As the jars cool, the lids pull down into a vacuum seal [10]. Fruit jam has enough acid and sugar that boiling water, at 212°F at sea level, is enough heat; that is why jam never needs a pressure canner [3].
The time in the canner is set by tested recipes and by your elevation. That number has its own page: how long to process jam. This page covers how to run the canner so the time you set actually counts.
The equipment, and what can stand in for it
| Item | What it has to do | If you don't own one |
|---|---|---|
| Canner or deep pot | Hold jars on a rack with at least 1 inch of boiling water over the tops, plus room to boil without spilling over [1] | A tall stockpot with a tight lid works for half-pints and pints [4] |
| Rack | Keep jars off the bottom so water circulates under them [1] | A round cake rack or trivet that fits flat in the pot [4]; a folded towel is not a rack |
| Lid | Keep the heat in so the boil holds | Any lid that fits the pot snugly |
| Jar lifter | Lift jars level, gripping below the neck [1] | Tongs slip on wet glass; a lifter is worth buying |
| Thermometer | Check the 180°F preheat [1] | Optional; water just starting to steam with small bubbles on the bottom is close |
| Timer | Time from the boil, not from loading | A phone timer |
The canner has to suit the burner. UGA's canning bulletin asks for a canner no more than 4 inches wider than the burner, overhanging it by no more than 2 inches on any side, and warns against straddling two burners because the jars in the middle will not heat enough. On electric and smooth-top ranges the bottom should be fairly flat [1].
For the full first-batch kit, see equipment for your first batch.
Step by step, from cold pot to cooling rack
Before the jam is ready
- Put the rack in the canner and add water. For a load of pints, UGA's starting point is a canner half full of clean warm water; for other sizes, aim for a level that will end up 1 to 2 inches over the lids once the jars are in [1]. Half-pints displace less water than pints, so you may need more.
- Center the canner on the burner and preheat the water to 180°F. That is the figure for hot-packed food, and jam is always hot-packed [1]. Water at this temperature keeps hot jars from cracking when they go in and shortens the time to a boil.
- Have your jars hot and ready. Use jars you have sterilized if your recipe's time is under 10 minutes, or clean, hot jars if it is 10 minutes or more [2]. The reasoning, and how to sterilize, are on do you need to sterilize jars.
Filling and loading
- Fill each hot jar with hot jam, leaving 1/4 inch of headspace [5]. Wipe the rim with a clean damp paper towel and put on a lid and band [5]. Filling technique is covered on filling jars; headspace has its own page.
- Load the jars one at a time with the jar lifter, gripping below the neck and keeping each jar upright. If your rack has side handles, you can hook it on the canner rim above the water, fill it, and then lower it, as long as it sits steady [1].
- Top up the water so the jar tops are under at least 1 inch. Pour hot water around the jars, never onto them. UGA asks for 2 inches when a process runs longer than 30 minutes, which does not happen with ordinary jam [1].
Processing
- Cover the canner, turn the heat to high and bring the water to a vigorous, rolling boil [1].
- Start the timer when the water is boiling hard, not when the jars go in. The time your recipe gives is time at a boil [1].
- Keep the lid on and hold a gentle but complete boil for the full time. You may turn the heat down once the boil is going, but the water must keep boiling across its whole surface [1].
- If the water level drops below 1 inch over the jars, add boiling water, again pouring it around the jars [1].
- If the boil stops at any point, start over on the time. Turn the heat back to high, return to a vigorous boil, and begin the full processing time again from zero [1]. A boil that stalls for a few minutes means the jars spent those minutes below the temperature the time was tested at.
Finishing
- Turn off the heat, take the canner lid off and wait 5 minutes [6]. UGA notes that this rest is not needed for safety when you use USDA or UGA processing times [1]. The Ball recipes most home cooks follow include it; University of Wisconsin food safety specialist Barb Ingham explains that it lets the contents settle as the heating rate drops and may reduce jar breakage from thermal shock [7]. It costs nothing, so do it.
- Lift the jars straight up and out without tilting them [1]. Leave any water on the lid where it is; it evaporates as the jar cools.
- Set the jars on a towel, cooling rack or other padded surface, at least 1 inch apart, away from cold surfaces and drafts [1].
- Leave them upright and alone for 12 to 24 hours while the vacuum seal forms and the jam sets up [9]. Do not tighten the bands or press the lids until the jars are completely cool [1]. What to do next is on cooling jars and checking the seal.
When something goes wrong in the canner
| What happens | What it means | What to do |
|---|---|---|
| Water never reaches a full boil | Canner too large for the burner, or too much cold water | Use a burner that matches the canner [1]; preheat the water before loading |
| Boil stops partway through | The jars were below boiling for part of the time | Return to a vigorous boil and restart the full time [1] |
| Water drops below the jar tops | Evaporation over a long process or a loose lid | Add boiling water around the jars [1]; if tops were exposed, restart the time once covered and boiling |
| A jar breaks | Cold jar into hot water, a scratched or nicked jar, or a jar set straight on the canner bottom | Finish the load; discard the jam from the broken jar; check the rest for glass shards on the outside before handling. Scratches in glass can cause cracking in processing [8] |
| Jam has leaked into the water (siphoning) | Too little headspace or a loose band [8] | Finish processing; check those jars' seals carefully after cooling |
| White film on jars and canner | Minerals in hard water | Harmless. Clean the canner with 1 tablespoon of vinegar per quart of water, boiled covered (SDSU) [6] |
Batches back to back
For a second batch, top up the canner with hot water and bring it back to 180°F while you cook. Never put cold jars into water that is already boiling. Keep jars waiting to be filled hot, in hot water or a closed dishwasher that has just finished its cycle [2]. When you are done, UGA asks you to wash the canner in warm soapy water, dry it well, and store it with crumpled paper towels on the bottom and around the rack to soak up moisture and odors [1].
Canner steam and full canners are the main burn risks in jam making. If children are around, or you are lifting a full canner off the stove, read working safely with boiling sugar first.
Your processing time comes next
Every step above assumes you have the right number of minutes for your recipe, your jars and your elevation. That number depends on whether you sterilized the jars and where the recipe came from. Find it on how long to process jam.
References
- University of Georgia Cooperative Extension, Schwan, Beltran and Andress, Preserving Food Series: Using Boiling Water Canners, Circular 1344-01, May 2025. https://nchfp.uga.edu/papers/factsheets/Preserving_Food__Using_Boiling_Water_Canners.pdf
- National Center for Home Food Preservation, Do I need to pre-sterilize my jars before canning?, May 21, 2014. https://nchfp.uga.edu/blog/do-i-need-to-pre-sterilize-my-jars-before-canning
- Penn State Extension, Zepp, How to Use an Atmospheric Steam Canner, updated March 17, 2025. https://extension.psu.edu/how-to-use-an-atmospheric-steam-canner
- Oregon State University Extension, Hyde, Safe canning practices for jam?, June 2013, reviewed 2024. https://extension.oregonstate.edu/ask-extension/featured/safe-canning-practices-jam
- USDA and National Center for Home Food Preservation, Complete Guide to Home Canning, Guide 7: Preparing and Canning Jams and Jellies, revised July 2015. https://nchfp.uga.edu/papers/guide/GUIDE07_HomeCan_rev0715.pdf
- South Dakota State University Extension, Kleine and Botzet, A Guide to Water Bath Canning (P-00255), 2022. https://extension.sdstate.edu/sites/default/files/2022-10/P-00255-v2.pdf
- University of Wisconsin-Madison Extension, Winnebago County, Ingham, At the End of Canning, Why Wait 5-10 Minutes?, August 3, 2020, updated September 4, 2020. https://winnebago.extension.wisc.edu/2020/08/03/at-the-end-of-canning-why-wait-5-10-minutes
- National Center for Home Food Preservation, Recommended Jars and Lids (from USDA Complete Guide to Home Canning, Agriculture Information Bulletin 539, revised 2015), viewed October 2026. https://nchfp.uga.edu/how/can/general-information/recommended-jars-and-lids/
- University of Georgia Extension, Andress, Preserving Food: Processing Jams and Jellies, FDNS-E-37-1, revised June 2011. https://openscholar.uga.edu/record/23661/files/FDNS-E-37-1.pdf
- National Center for Home Food Preservation, Cooling Jars and Testing Jar Seals (adapted from USDA Agriculture Information Bulletin 539, revised 2015), viewed October 2026. https://nchfp.uga.edu/how/can/general-information/cooling-jars-and-testing-jar-seals
