Commercial sterilization of canned goods is done at 15 pounds pressure / 250 F [103 kPa / 121 C], which consequently requires a far shorter period of processing time. We can achieve 15 pounds pressure in our home pressure canners, but all home processing recommendations start at a sea level base of 10 pounds pressure / 240 F [69 kPa / 115 C], which requires a longer period of time.
Why not higher processing temperatures for home?
You may wonder why then there aren’t home-use recommendations for the higher pressure / shorter time. The answer might be simply: the research for our home processing times was done at 10 pounds pressure, and funding would be needed for the research to redo the times for each item the USDA currently has 10 lb times for.
In a 1983 survey of the history of home canning thus far, Dr Elizabeth Andress wrote:
A few recommendations have been made over the years for home canning of meats at 250F / 15 psig [ [121 C / 103 kPa] (Cover et al, 1943). It was later found to be unnecessary, but the higher temperature was and is used successfully, especially with commercial products.
At present, 240F / 10 psig [115 C / 69 kPa] is the most commonly recommended temperature. More recent interest in conserving nutritional and textural characteristics in canned food has led to some research with higher pressure (temperature) and shorter time processes.
Use of 250F [121 C] in home canning has been limited due to a lack of research at this temperature. Shorter process times would be required at 250F [121 C], as lethality accumulates about three times faster than at 240F [115 C] (Zottola et al, 1978). These authors and Nordisen et al (1978) recently investigated heat penetration rates for low acid foods processed at 15 psig [103 kPa.] Convection-heated foods required about one-third the time as specified for processes at 10 psig [69 kPa.] Those foods heated by conducting or a mixed mechanism appeared to require equal time at 10 and 15 psig [69 kPa and 103 kPa.] It was concluded that each product be investigated before recommendations could be made.”1
Note that though some jarred products sterilized faster at 15 lbs, some sterilized at the same rate as 10 lbs — so research would need to be done to determine which was which. And, what about people whose altitude already requires 15 lbs as a base?
In 1986, Nancy Hudson, a former extension agent in Greene County, Ohio, explained the reason for the recommendation as given to her by Dr Gerald Kuhn, who pioneered the first USDA Complete Guide. She cited the reason of too-short processing times for some items:
As of 1987, USDA will no longer recommend 15 lbs. pressure. Part of the problem is when 15 lb. pressure is used and zero or one minute processing, there is not sufficient time for all of the air to vent out of the canner and for the internal canner temperature to raise. Using 15 lb. pressure at zero or one minute processing will increase spoilage.”2
All of which is to say: in case you’ve wondered, there’s the current reasons. If you want it changed for products that can support 15 lbs processing, tell your elected representatives you’d like some money invested in researching this!
10 pounds pressure is higher than it seems
For the record, our 10 lbs pressure is actually slightly higher than that.
A pressure of 10.5 pounds at 0 to 1,000 feet of altitude is required to reach 240 F [115 C] degrees inside the canner… The pounds of pressure figure was rounded to 11 pounds in dial gauge canners to avoid the confusion of half-pound recommendations. Weighted gauge canners are already machined to operate at 10.5 pounds as a built-in safety factor when they are set at 10 pounds.”3
That doesn’t change processing times or pressures one iota, but there you go, just some poundage trivia for your next canning party!
Are there any exceptions to the 10 lbs at sea level norm?
There appears to be a small handful of exceptions in the USDA 2015 Complete Guide, for which 15 lb at sea level processing directions are given for weighted-gauge canners. This includes Tomato Juice (page 3-5), Tomato and Vegetable Juice Blend (3-6), Crushed Tomatoes (3-7), Standard Tomato Sauce (3-8), Tomatoes Whole or Halved (3-9 to 3-12)
Look at the incredible time difference when pressure canning whole tomatoes at 15 lbs:
What heats does pressure canning achieve?
Steam Canning. In: Andress, Elizabeth L and Gerald Kuhn. Critical Review of Home Preservation Literature and Current Research. Athens, GA: University of Georgia, Cooperative Extension Service. 1983. http://nchfp.uga.edu/publications/usda/review/equip.htm#psig ↩
Hudson, Nancy. New research gives tips on using jars and lids. Xenia, Ohio: Daily Gazette. 7 April 1986. Page 6. ↩
Patrick, Ruth M. Canning Green Beans. Revised November 2009. Louisiana State University Agricultural Center. Accessed March 2015 at https://www.lsuagcenter.com/MCMS/RelatedFiles/%7BB9014A9B-CD62-4F8F-92AE-FF60503C43A8%7D/CanningGreenBeans.pdf ↩