USDA and the Cooperative Extension recommend a boiling water canning process for jams and jellies even though some entrepreneurs or small batch processors are told to just fill the jar hot and close it. Why? Some other sources say processing isn’t necessary for any home canning of jams and jellies — just fill the jar hot and close it. Some others will say to invert the jar (turn it upside down) after putting the lid on, with various numbers of minutes recommended for this inverted position. Why?
A “process authority” advising someone packing jars for sale may indeed not recommend a boiling water process, but then other controls should also be specified for the particular recipe and full preparation procedure, such as a measured minimum hot fill temperature with every batch, specific acidity levels that would allow for no process with that specific product, perhaps measured and documented water activity control or degrees Brix for each batch that allows for no processing, and/or specific processing steps that would support this recommendation. Home canning recommendations for the general public have to cover a wide variety of recipes and methods of cooking the jam or jelly.
Even with a very acid fruit jam or jelly that has good water activity control, there can be some benefits to even a short boiling water process if food safety isn’t the reason. For one thing, molds can be airborne and settle into your jars at filling. The retained heat from filling jars in many home situations may not be enough to kill airborne mold spores. The goal in this case for a boiling water process would be to minimize the risk of spoilage during storage and thereby prevent “economic loss” by product having to be thrown out. Many larger scale commercial operations, in addition to tighter batch-to-batch cooking and filling temperature controls, have equipment systems that force an injection of superhot steam into the headspace before the lid is placed on the jar. This helps with closing temperature, but also forcing air out of the headspace.
So that leads to a second benefit to the home boiling water process. With a properly applied lid, the processing helps force some of the retained air of the headspace in jar. When the jar then cools after processing, this evacuated headspace can lead to better vacuums in the sealed jars (if lids are applied correctly) than just hot filling. Not all vacuum seals are equal. A jar may seal with a weak vacuum. A weaker vacuum is related to more occluded (trapped) air, and therefore oxygen, in the headspace of sealed jars. That oxygen can lead to discoloration and possible flavor changes over time, so quality retention is often better with a processed product than one just hot filled at home.
Finally, one of the considerations discussed when USDA was making recommendations is the potential for burns or leaking jars with the inversion process vs. a boiling water process. Although not in published journals or other sources, there are documentations from some research experiences that leaking and failure to seal are higher risks with inversion than processing. I have read one journal article using inversion that caused the researchers to abandon it in their future studies because of leaking and needing better control of temperature for safety reasons with their particular recipes. Other concerns with inversion include individual variation in practicing this procedure or that unexpected interruptions can result in delays between filling jars, getting lids screwed on, and inverting the jars. If the product cools down too much, the temperature of the product can become low enough to no longer be effective in sealing jars or preventing spoilage. Not ALL jams and jellies/spreads even fit criteria for even a 5 or 10 minute boiling water process; they might need a longer process!
USDA made the decision to recommend processing for all jams and jellies and very acidic foods for reasons of preventing economic loss and physical injury from burns as well as food quality during storage, even if it might not be needed for food safety. We in Extension consumer education firmly believe the best practice to recommend even for very high acid foods is a boiling water process vs. hot fill only and inversion even though the other may be successful and safe for some food products.