Safety margins in home canning recipes

It certainly can seem sometimes that a lot can go wrong in home canning.

And it can, if you don’t first learn the modern, science-based rules behind it. But, we all need to focus on this fact: if you are following a tested recipe from a reputable source, then you have zero worries. They won’t tell you this, because they don’t want people “testing the limits”, but these tested, reputable recipes have huge safety margins built into them. You can be 190% confident about the safety of the food product you have just canned.

Elizabeth Andress, head of the National Center for Home Food Preservation, writes:

 You can trust that your home-canned foods will receive adequate heat treatment by using proper canning methods and following recommended process times.” ^[1] 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/

USDA safety margins

Home canning recipes that have been tested to USDA standards (which includes those by Ball and Bernardin) are written for people even with a flawed or incomplete understanding of health and safety, and allow, frankly, for a high level of stupidity, distraction, and stubbornness.

Independent researchers tested some of the various recipes and recommended processing times in the 2009 USDA Complete Guide to Home Canning. They concluded:

Thus, as long as up-to-date, research-tested home-canning practices are followed for high-acid foods, consumers can be reassured by the inherent safety of the product that they are producing for themselves and their families.” ^[2] They found that “calculated lethality was far greater than the required 5-log reduction of spores in tomato juice and vegetative cells in cranberries, suggesting a wide margin of safety for approved home-canning processes for high-acid foods.” Etzel, M. R., Willmore, P. and Ingham, B. H. (2014), Heat penetration and thermocouple location in home canning. Food Science & Nutrition. doi: 10.1002/fsn3.185 https://onlinelibrary.wiley.com/doi/10.1002/fsn3.185/full

They found an extremely wide margin of safety that far surpassed any usual safety requirements.

It’s because of these massive safety margins that the volume measured recipes (such as 4 cups of chopped carrot), which have a huge possibility for measurement errors and wildly varying interpretation, can be safe.

Oftentimes the reasons the USDA either withdraws recommendations, or won’t make recommendations, for canning a certain food item, such as unpeeled potatoes or carrots, is NOT that there have been any issues, but rather that they haven’t had the funding to research and document a 110% safe process for that food item in that particular state. Given their position, they can’t possibly say, “oh, you’re probably safe.” They tried that approach in the first few decades of the 1900s and people were dropping like flies. Because of that, they have to be able to say, “you will be safe.”

Broad USDA safety recommendations for all equipment

The USDA gives broad, simple recommendations that will work for everyone: “It has been suggested that separate process recommendations for multiple models of canners … would only confuse the public and increase risks in home canning practices.”  ^[3] 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#pressure

Reviewers in the Journal of Science wrote,

The generation of process recommendations based on every canner, load size, and water level combination would be a cumbersome, confusing, and costly endeavor that would create chaos in the canning community.”  ^[4] Montville et al., 1983. Inherent variability in the efficacy of the USDA raw‐pack process for home‐canned tomatoes. J. Food Science 48:1591-1597.

Processing times are always given in 5 minute intervals, and rounded up, never down, to the next 5 minute mark.

What are the margins of safety?

“Up-to-date pressure canning times are designed to achieve a 12-log reduction in spores. In plain English, this means a survival probability rate of one survivor in 10 to the 12th power jars, or one live spore per one trillion jars.”  ^[5] Rayner, Lisa. The Natural Canning Resource Book. The Natural Canning Resource Book. Flagstaff, Arizona: Lifeweaver LLC. 2010. Page 35.

…the underlying safety assumption is that when starting to sterilize a food product, there will be an impossible 60 billion C. botulinum spores present that need to be killed off: ‘The processing effect of a 12-log inactivation, or the ‘botulinum cook,’ is based on the extrapolated values (121.1uC) of data obtained by Esty and Meyer (33), who studied the resistance of 60 billion of the three most heat-resistant C. botulinum spores in phosphate buffer at 120uC and established a z-value of 10uC. The log-reduction process design assumes that the initial level of the contaminant will never go above a predetermined maximum expected level and that to deliver a specific level of safety a known log reduction of the organism of concern must be delivered by the process. There is generally considered to be an inherent safety factor in the assumption that there will be 60 billion resistant C. botulinum spores present. The application of such a large safety factor may compensate for an occasional can of food that heats more slowly than anticipated and for the occasional mechanical or human error that occurs in production.”  ^[6] N.M Anderson et al. Food Safety Objective Approach for Controlling Clostridium botulinum Growth and Toxin Production in Commercially Sterile Foods. Journal of Food Protection, Vol. 74, No. 11, 2011, Pages 1956–1989 doi:10.4315/0362-028X.JFP-11-082. Page 1960

While you should always give the seal on your jar a final test before opening, and give the contents a quick sniff once opened, if you have followed tested recipes based on USDA procedures, and the jar stayed sealed, experts say that contents of that jar will never be unsafe. You can reach for it with confidence from your pantry shelf:

“If you follow a tested recipe, a properly sealed canning jar will never become unsafe.”  ^[7] Ingham, Barb. Interviewed by Matt Hrodey. Preserving Agent. A food science expert explains how to safely preserve foods for the pantry. In: The Epicure’s Guide to Milwaukee. 24 March 2015. Milawaukee Magazine. Accessed March 2015 at https://www.milwaukeemag.com/2015/03/24/epicures-food-lovers-guide-milwaukee/

None of this is to say, try to nibble away at the safety margins, or, that you can make a few variations and you’ll be covered. The safety margins are there for a reason and have to be maintained. Thus, most USDA Extension Agents refuse to condone even the simplest change to a recipe, even if it is patently safe, because they fear that “if you give people an inch they will take a yard.”

History

At first, the USDA’s canning advice was just based on guesses and hunches, because there were no hard solid facts yet to base their thinking on. They would pass along information based on collective wisdom or other people’s experiences. Here’s some very unscientific 1914 advice being passed along by USDA Extension Agent Louise Stanley (1883 – 1954), who later became head of the US Bureau of Home Economics in 1923:

The method of cooking three successive days may be applied to all types of green vegetables, such as peas, beans, etc., with slight differences in the preparation and the length of time of cooking. This method has been described by a practical housewife as follows : ‘Spinach, Swiss chard, lambs quarter, are cooked down until tender in salt water in open kettle in order to get more bulk into cans. Then put into the jars until nearly full and pour on enough hot water to fill the jars full; to this add a scant teaspoonful of salt; put on the lid and screw down loosely without rubber (to allow steam to escape); put into steamer, which should be kept about two-thirds full of water, and boil one hour for each of two days. On the second day of boiling, about fifteen minutes before time is up, unscrew tops, put on rubbers and screw down tight and place back in steamer for fifteen minutes so as to seal rubbers. See also that the jars arc full of water at the last cooking.'”  ^[8] Stanley, Louise and May C. McDonald. The Preservation of Food in the Home. University of Missouri Bulletin, vol 15, # 7. Extension series 6. 6 March 1914. Page 20.

When problems arose from this type of advice being given out, the USDA first denied (for a decade from about 1916 to 1926) that the problem was with their advice:

It has been previously mentioned that recommendations from USDA for home canning processes were modified in response to outbreaks of botulism after 1916. There were indications as early as 1917 (Dickson, 1917) that home-canning recommendations for vegetables were suspect. However, bacteriologists in the Bureau of Chemistry and States Relation Service of USDA denied this, apparently for almost 10 years (Tanner, 1934).”

By 1926, the tone of the USDA advice had already sharply changed. They had a lot more research-based evidence under their belts, and the same Louise Stanley was issuing more specific recommendations (which we know now to be headed more in the right direction):

It was not until Farmers’ Bulletin No. 1471, ‘Home Canning of Fruits and Vegetables,’ [Ed: Canning Fruits and Vegetables at Home] (Stanley, 1926) was issued in 1926 that pressure canning was the only method recommended for low-acid vegetables. ”  ^[9] Andress, Elizabeth L and Gerald Kuhn. Critical Review of Home Preservation Literature and Current Research. Athens, GA: University of Georgia, Cooperative Extension Service. 1983. https://nchfp.uga.edu/publications/usda/review/earlyhis.htm

In the decades following, to be safe, processing times and safety steps were very broadly padded to provide extra safety: “However, it was customary to lengthen the recommendation considerably to insure a generous margin of safety to overcome the lack of precision in home methods (USDA, 1946c).”  ^[10] Andress, Elizabeth L and Gerald Kuhn. Critical Review of Home Preservation Literature and Current Research. Athens, GA: University of Georgia, Cooperative Extension Service. 1983. https://nchfp.uga.edu/publications/usda/review/earlyhis.htm

The USDA Complete Guide was first compiled by a team led by a Dr Gerald Kuhn, who set the new uncompromising safety standards. In 1986, an extension agent wrote:

Dr. Kuhn explained that home canning is a safe technique because it is based on sound scientific principles. “This takes guts to say,” he stated, “but if you follow the right directions in home canning, the food is perfectly safe to eat. If it wasn’t, we would be in trouble. That would be an admission that we’re only playing a guessing game, and we’re not.” ^[11]Jenkins, Kathryn. New guidebook available this fall giving up-to-date home canning tips. Frederick, Maryland: The Frederick News-Post. 19 June 1986. Page F-1.

Criticism of the USDA safety margins

Over the years, there have been debates whether the safety margins go too far: “Cover et al (1956) reported that the selected sterilizing values for some products were excessive, particularly for whole corn. Apparently, they felt the F0 values applied to raw-packed home-canned foods contained excessive margins of safety.”   ^[12]Andress, Elizabeth L and Gerald Kuhn. Critical Review of Home Preservation Literature and Current Research. Athens, GA: University of Georgia, Cooperative Extension Service. 1983.

For instance, in 1926, the recommendation was for a US pint size jar of corn kernels to be processed for 75 minutes.  ^[13] Stanley, Louise. Canning Fruits and Vegetables at Home. USDA Farmer’s Bulletin No. 1471. May 1926. Page 19. By 1975, that had been reduced to 55 minutes.  ^[14] Acker, Geraldine. Canning Fruits and Vegetables. Illinois Cooperative Extension Service. Circular 1112. May 1975. Page 10.

Processing times have been refined over the decades since, some up, some down. The current processing times in the USDA 2015 Complete Guide should be considered the required processing times for safety and quality, and therefore be followed exactly. Never, ever vary recommended processing times or pressures, including altitude adjustments.