All jars of preserves meant for shelf-stable storage require heat processing of the filled, sealed jar. No matter how much you sterilize a jar in advance, or how much you cook the food before, or how tight you screw a lid on, or how much sugar or vinegar is in your heritage recipe, or whether your Great-Aunt Matilde did or not. None of that counts: filled jars intended for shelf-stable storage must be heat processed.
It is not acceptable to just whack a lid on, let a jar cool, and if a seal, however weak, forms, call it a day.
In terms of kitchen sanitary practice, it’s the equivalent of licking a raw chicken.
Why just a seal by itself doesn’t count
The National Center for Home Food Preservation says,
A seal does not indicate that a canned product is safe. A seal indicates that new contaminants cannot get in, but it tells you nothing about the presence of microorganisms (mold, yeast, bacteria) that were already in the jar. Heat from a proper canning process is needed to make sure any microorganisms in the jar of food are killed.”  Christian, Kasey. Your Favorite Salsa Recipe…Is it Safe to Can? Preserving Food at Home blog. 13 August 2013. Accessed January 2016.
Heat from a proper canning process is needed to make sure any microorganisms in the jar of food are killed. Kacey Christian at the National Center wrote about the method of just slopping heated food into a jar and slapping a lid on, and calling it “job done”:
The ‘canning’ process you describe is not actually a recommended canning method — it is sometimes called “open-kettle canning”. In open kettle canning, food is cooked in an ordinary stockpot or kettle, then packed into hot jars and sealed without processing. 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.”  Christian, Kasey. Your Favorite Salsa Recipe…Is it Safe to Can? Preserving Food at Home blog. 13 August 2013. Accessed January 2016.
Note as well that scientific testing has shown that sterilizing preserve jars (except for jars processed for less than 10 minutes) doesn’t actually achieve anything. You’ve wasted your time. It doesn’t replace the need for proper processing of the filled jar, and, the jar gets sterilized anyway during the processing step. So, modern advice based on research says: don’t bother.
Three ways to process your jars
There are three acceptable ways to process your jars, depending on the food product inside the jars.
- For high-acid foods, the jars can be water-bathed, or steam-canned;
- For low-acid foods, the jars must be pressure-canned.
Always use a tested recipe from a reputable source, which will give you the correct processing time and method for that particular food item being made. If a recipe doesn’t give you a processing time, run, don’t walk, away from that recipe source.
Don’t guess at a processing method. Do the method that the recipe says — pressure canning your jam is overkill.
Whether you call your craft “canning” or “bottling”, process your jars.
Benefits of processing
- Eliminates risk of food poisoning, when used in conjunction with a tested recipe;
- Eliminates risk of spoilage and loss of food, when used in conjunction with a tested recipe;
- Lengthens storage life and quality;
- Gives longer protection against undesirable colour and flavour changes, especially with fruit ”More complete removal of oxygen from the headspace also offers some longer protection from undesirable color and flavor changes with some types of fruit products. The canning process is therefore a more foolproof method of making jams and jellies that will not spoil.” Andress, Elizabeth. Preserving Food: Processing Jams and Jellies. University of Georgia Cooperative Extension. June 2011. ;
- Creates a very strong, lasting vacuum seal essential to preserve safety and quality of the food;
- Replaces sterilization (for items processed over 10 minutes), saving you a lot of labour intensive futzing at the start of the preserving process;
- AND, as a minor point, prevents the embarrassment when a friend says that jar of unprocessed jam you gifted to them had lovely psychedelic blue mould growing in it.
Proper processing is paramount
How important is proper processing? It’s so important that — while in-the-know experts do have firm opinions on many other things such as what type of jars and lid closure systems are best from an operational point of view — when push comes to shove, their concern about properly researched canning (i.e. processing) procedures being used makes it to the very top of the list.
The authors of So Easy to Preserve, who help to form the USDA’s guidelines, write:
As long as the proper jar type, size and shape is used with properly researched canning procedures, the lid choice itself (e.g, two-piece metal, plastic or one-piece metal lids) does not affect the microbiological safety of the canning process.” Andress, Elizabeth. So Easy to Preserve. Page 24
The worry about lid systems on jars is whether you will get a jar that stays sealed next month; the worry about improper processing procedures is “will you make people very, very sick tomorrow.”
That’s just how important processing your jars is.
It’s not just the public sector officials who are saying this. Reputable companies from the private sector are adamant about it, as well. Bernardin says:
Heat processing of all filled jars of home canned foods is not optional! It is essential to create an adequate hermetic or vacuum seal required for food safety as well as delicious taste and quality. All jars of home canned foods must be heat processed by the appropriate method for the correct time for the food type and jar size. Failure to adequately heat process jars can result in seal failure, food spoilage and substantial health risks.”  Bernardin Guide to Home Preserving. Toronto, Canada: Bernardin Ltd. 2013, page 4
Not processing often leads to false seals
Dr Martha Zepp, from PennState Extension, warns that not processing jars often leads to false seals on jars, which soon leads to food wastage when the jars lose their seals on the shelf:
“One of the most common occasions for a false seal occurs when hot food is poured into jars, lids are applied and the jars of product are not heat processed. This method is called open kettle canning or the hot fill method; it is not recommended for home food preservation and is not safe. A false seal will often occur with applesauce that is canned this way. You may also find this with jams and jellies that are put into jars without processing.” Zepp, Martha. When a Jar Becomes Unsealed. PennState Extension. 3 November 2020. Accessed February 2021 at https://extension.psu.edu/when-a-jar-becomes-unsealed
The heat processing also drives out air, which can expand and weaken the seal:
“Heat processing helps to ensure a safe product by destroying microorganisms and forcing air from the jar as indicated above. If the products are not processed for the entire recommended time, air can be trapped in the jar. In a warm storage space that trapped air can expand and pop the lid off the jar. Under-processing may also allow some microorganisms to survive. They can become active at storage temperatures producing a gas that can pop the lid off the jar.” Zepp, Martha. When a Jar Becomes Unsealed
Why do you have to process jars of jam?
|↑1||Christian, Kasey. Your Favorite Salsa Recipe…Is it Safe to Can? Preserving Food at Home blog. 13 August 2013. Accessed January 2016.|
|↑2||Christian, Kasey. Your Favorite Salsa Recipe…Is it Safe to Can? Preserving Food at Home blog. 13 August 2013. Accessed January 2016.|
|↑3||”More complete removal of oxygen from the headspace also offers some longer protection from undesirable color and flavor changes with some types of fruit products. The canning process is therefore a more foolproof method of making jams and jellies that will not spoil.” Andress, Elizabeth. Preserving Food: Processing Jams and Jellies. University of Georgia Cooperative Extension. June 2011.|
|↑4||Andress, Elizabeth. So Easy to Preserve. Page 24|
|↑5||Bernardin Guide to Home Preserving. Toronto, Canada: Bernardin Ltd. 2013, page 4|
|↑6||Zepp, Martha. When a Jar Becomes Unsealed. PennState Extension. 3 November 2020. Accessed February 2021 at https://extension.psu.edu/when-a-jar-becomes-unsealed|
|↑7||Zepp, Martha. When a Jar Becomes Unsealed|