In home canning, the phrase “processing time” refers to the time period that a jar, filled with food and with a lid put on and intended for shelf stable storage, is subjected to the full force of the heat treatment for the length of time called for by the recipe.
Processing achieves two things:
- The sterility and safety of the food inside the jar; and
- A tremendously strong hermetic seal to keep the food safe.
In home canning terms, ‘processing’ means heating filled jars at the temperature and time required to destroy spoilage microorganisms, inactivate enzymes and vent excess air from the jar. As the filled jar is heated, headspace pressure increases until air is vented from the jar. After processing, the atmospheric pressure outside the jar is greater than inside due to this ‘venting’. This pressure difference causes the lid to be pushed down onto the jar, resulting in a true vacuum seal. This seal prevents microorganisms and air from entering and contaminating the food.”  https://www.bernardin.ca/pages/faq/33.php#43 . Accessed June 2015.
- 1 What if the recipe I found doesn’t give a processing time?
- 2 What does the processing method depend on?
- 3 What does the processing time depend on?
- 4 Using a jar size different than sizes that processing times are given for
- 5 Jar mouth size and processing time
- 6 How do you know when full heat been reached?
- 7 My processing time got interrupted
- 8 What about the end of the processing time?
- 9 Water-bath canning processing notes
- 10 Miscellaneous facts about USDA recommended processing times
- 11 Final tip about processing time
What if the recipe I found doesn’t give a processing time?
If a recipe does not indicate a heat processing time for jars of canned food intended to be stored unrefrigerated, then run, don’t walk, away from that recipe, no matter how pretty the pictures in the book and no matter how long the organization that put it out has been around, and yes this includes the WI and your grandmother’s handwritten recipe book. “Heat processing ALL filled jars is still a necessity for safe home canned foods.”  https://www.bernardin.ca/pages/faq/33.php#19 Accessed June 2015.
Don’t look for similar recipes to guess a processing time, because the chances are there are other safety problems with the recipe, too.
And one of those processing methods must be either with a boiling water canner (or its equivalent, steam canning) or a pressure canner. “There are only two methods for heat processing home canned foods that are considered safe: boiling water canner for high acid foods and pressure canner for low acid foods.”  https://www.bernardin.ca/pages/faq/33.php#27. Accessed June 205. The only exceptions are for refrigerator products or items such as freezer jams — but then while those are preserves, they aren’t “canned” preserves.
What does the processing method depend on?
The processing method depends on the acidity of the food product. If it’s acidic, with a pH of 4.6 or less, then it is water bathed (or steam canned.) If it is low-acid, with a pH higher than 4.6, then it is pressure canned. The processing method a recipe requires will always be made explicit. (Note that there are a few items such as some fruits and tomatoes where you have a choice of which method: the USDA 2015 Complete Guide tells you when there is such a choice.)
What does the processing time depend on?
The processing times vary based on factors such as jar size, density of the food and what the food is. “Times vary depending on jar size, food density and chemical make-up.”  https://www.bernardin.ca/pages/faq/33.php#12 . Accessed June 2015 The processing time allows for the contents of the jar to be sterilized to be made safe for shelf storage.
The processing time will usually be indicated by a phrase such as “process for x minutes.”
It’s important to note that processing time does not refer to warm-up or cool-down time, or waiting for the water to come to a full boil time, but only to the actual process time called for after the full heat has been reached.
Using a jar size different than sizes that processing times are given for
You can use smaller jars than a tested recipe gives a processing time for, but not larger.
When you use a smaller jar, process for the same processing time as for the next-up suggested jar size
The reason is that larger jars usually requires a longer processing time for proper heat penetration to the food in the centre of the jar, and thus would require a longer tested and and verified processing time to ensure quality and food safety.
You may always use a smaller sized jar, but it is not recommended to use a larger jar.”  Treiber, Lisa. Jam and jelly season is here! Blog posting 1 July 2016. Michigan State University Extension. Accessed July 2016.
When you use a smaller jar, process for the same processing time as for the next-up suggested jar size, unless a separate processing time for the smaller jar is given.
Processing times have not been developed for many foods in half-pint, 12-ounce or 24-ounce (one and one-half pint jars.) If the recipe does not specify processing in one of these jars, process half-pint and 12-ounce jars for the same time as pints. Jars that hold 24 ounces (one and one-half pints) will need to be processed using quart jar times.”  Andress, Elizabeth L. and Judy A. Harrison. So Easy to Preserve. University of Georgia Cooperative Extension. Bulletin 989. Sixth Edition. 2014. Page 23.
Jar mouth size and processing time
Jar mouth size does not affect the processing time for either water bath processing, steam canning or pressure canning. Processing time is instead based on the volume capacity of the jar.
How do you know when full heat been reached?
In a water bath canner, when the jars are placed in the pot and submerged in water, you start counting processing time when the water reaches / returns to a full rolling boil. (For a steam canner, you start when your thermometer or gauge tells you the inside of the canner has come to temperature: see steam canning for full info.) For altitudes above 1,000 feet (300 metres) above sea level, you must increase the length of processing time to compensate for the altitude. Process with the lid on the pot.
In pressure canning, when the jars are in the canner, you start counting processing time when the canner hits target pressure. For altitudes above 1,000 feet (300 metres) above sea level, you must increase the pressure. When planning your schedule, depending on the size of the canner and the load in it and how fierce your burner heat source is:
- allow 10 to 20 minutes for the canner to heat up,
- then 10 minutes for it to vent,
- then another 10 to 20 minutes for it to hit target canning pressure.
At that point, you then begin your process timing.
My processing time got interrupted
If anything should happen to interrupt processing time, either in a water bath (or steam canner) or in a pressure canner, you must start counting the process timing all over again from the start. This is because sterilization requires an unbroken heat interval to ensure that as many potential nasties as possible will succumb to the relentless, unbroken heat and be killed. Even if you are worried about “overcooking” the food in the jar, you must make that concern secondary, because the nagging worry afterward about the food being safe is even worse. Your first, second and third responsibility as a good cook is to ensure that food is safe.
Always ensure that the full uninterrupted processing time has taken place. Never, ever cut it short.
What about the end of the processing time?
At the end of processing, do not rush the exit of the jars from the canner or the subsequent cooling of the jars. Besides risking shattering the jars, there is still some killing of bacteria that occurs after processing at natural air temperatures immediately at the start of cool down so you don’t want to lose that. Though the cool down does not count as part of the processing time, it is still a vital part of the processing process.
- Water-bath canning: the cool down process is important
- Pressure canning: the cool down process is important
Water-bath canning processing notes
- Allow lots of time for the water to come to a boil. The phrase “a watched pot never boils” is never more true than when you are tapping your foot waiting for it to get started for canning;
- That being said, many old timers who are still writing the USDA, Ball and Bernardin recipes still tell you to start the water boiling before starting two hours of prep work. The result of course is that you have the pot boiling needlessly for 1 ½ hours. The days when cooking fuel was cheap are long gone and not coming back in a hurry, so when the directions have you start the pot boiling ridiculously early, you’ll have to, over time, figure out what the right time actually is for your stove;
- For processing times greater than 30 minutes, you need to start off with the water level 5 cm (2 inches) above the tops of the jars in the pot. Under 30 minutes, 3 cm (an inch) is fine;
- Processing times are almost always greater for litre / US quart jars than for half-litre / US pint jars but occasionally you will run into the odd exception where they are the same and you blink, but if it’s a tested recipe, do it;
- Processing times for ¼ litre and ½ litre (½ US pint and 1 US pint) jars are often the same, if only because in the product development the lab didn’t test for the smaller size. One tip though: while Ball Blue Book and Bernardin Guide will often give the processing time for one size, the Ball / Bernardin Complete Book will give the same recipe with the processing time for the other size. So in that case, it’s just a matter of which jar size the editors decided to promote in that particular edition;
- Processing times for ¾ litre and 1 litre (¾ US quart and 1 US quart) jars are usually the same, if only because few tested times have been developed for the ¾ size;
- Processing times are generally longer for raw-packed fruits than for hot-packed fruits. “When canning in boiling water, more processing time is needed for most raw-packed foods and for quart jars than is needed for hot-packed foods and pint jars.”  United States Department of Agriculture (USDA). Complete guide to home canning. Agriculture information bulletin No. 539. 2015.USDA guide. Page I-11
Process jams and jellies for 10 minutes, even if the recipes call for 5: that way, you don’t need to waste time and energy sterilizing the jars and lids:
Heat processing time recommendations for jams and jellies was been increased from 5 minutes to 10 minutes in 2003, thus eliminating the need to pre-sterilize mason jars before filling. Tests have shown that increasing the processing time does not adversely affect the product. If you choose to continue to use high acid recipes that require less than 10 minutes heat processing time, mason jars should be boiled 10 minutes, at altitudes up to 1,000 ft (305 m), prior to filling and the subsequent shorter than 10-minute heat processing time.”  https://www.bernardin.ca/pages/faq/33.php#19 . Accessed June 2015
Miscellaneous facts about USDA recommended processing times
- USDA processing were not developed all at once, by the same team of people. “USDA process recommendations have been developed over time by different laboratories and researchers.”  D’sa, Elaine M. and Elizabeth L. Andress. Heat Processing of Home-canned Foods. National Center for Home Food Preservation. 22 December 2005. Accessed March 2015 at https://nchfp.uga.edu/publications/nchfp/factsheets/heatprocessingbackgrounder.html
- “USDA home-canning processes are rounded off to the next higher 5-minute interval… if [an item] is calculated as 11 minutes [it will] get rounded off to, and published as, a 15-minute process time.”  D’sa, Elaine M. and Elizabeth L. Andress. Heat Processing of Home-canned Foods. National Center for Home Food Preservation. 22 December 2005. Accessed March 2015 at https://nchfp.uga.edu/publications/nchfp/factsheets/heatprocessingbackgrounder.html . This reduces complexity for home canners, and often introduces an extra layer of safety.
Final tip about processing time
When you are setting the processing timer on whatever timer watch, clock or device you are using, stop, breathe out and note the time you set. It can happen on really busy, frantic days when your mind is overloaded that at the end of processing, you’ll second guess yourself whether you set 35 minutes or 45 minutes. Taking a second to be conscious of the time you set can help to avoid that moment of panic and self-doubt at the end.
Timing devices such as mobile phones are handy, in that they remember and display the starting time you punched in, to eliminate that doubt.