TL;DR - the feds provide minimal support for safe home food preserving research. Gotta science it ourselves.
Now that it’s running, I want to provide some background on why we’re doing this whole project.
The majority of safe canning practices for consumers were developed by land grant university extension programs and the USDA in the early to middle of 20th century - they were refinements on practices that were developed by commercial processors and those developed by consumers as the tools came available to them. Much like other government work, the rules are written in blood - until the development of pressure cookers (really not a Thing for consumers until 1939), people were routinely sickened and killed by home-canned foods, especially those of a low-acid nature like meats and vegetables. Bringing the research capabilities of home economics and food science departments together in the service of the public was considered a good thing for government to do - the USDA largely funded the research but the work was done in places where the crop to be preserved was common - so, for example, the definitive reference on canning seafood at home is from Oregon State, and the definitive method for corn comes out of the Midwest.
Almost all of this research was done when data acquisition was analog - either a grad student with a thermometer or paper chart recorders. The situation in homes was worse - accurate thermometers were expensive and high-precision ones simply didn’t exist. Additionally, the tools available for home pressure canning and heating the pressure canner were dictated by what was out in the market - largely “pressure saucepans” similar to today’s stovetop pressure cookers like Fagors, and “pressure canners” with multi-gallon capacities and either a rocking weight or a dial gauge for pressure regulation.
Safe canning requires particular temperature exposure for specified periods. Because we understand how water vapor behaves, we can extrapolate pressure vessel internal temperature from the pressures reached. Then the question becomes if the food in the jars hits the required temperature with a given combination of time and temperature. Since heat moves through the food through conduction, convection and radiation, the density and nature of the food dictates the time and temperature to a great degree.
The problem is that jar size and closure method, how the food is crammed in, starting temperature, behavior of starchy materials (thickened liquids don’t conduct heat the same as water-thin), altitude, the thermal mass of the canner, the controllability of the heating appliances, etc, all modify the thermal profile of the food. So every recipe recommended for safe canning has to be independently validated - a mushroom and rice soup might not behave the same way as one with carrots and barley, for example.
Great. As long as there are food science grad students there will be *some* growth of safe food preserving recipes - home-canned salsa being an excellent example of a late-20th-Century example of a new safe recipe, and substituting one chile for another is probably just fine. But if you want to do tomatillos instead of red tomatoes? Start over from scratch.
Same goes for heating methods. Since open-kettle methods like sealing jam jars with paraffin (the wax kind, not the kerosene kind, for those in the Commonwealth) or just slapping hot lids on to hot jars filled with hot food were proven unsafe, the methods left to home canners were either “boil covered by two inches of water at atmospheric pressure” (aka Hot Water Bath) or pressure canned. But some failures in the 70s and 80s seemed to indicate that using the smaller pressure saucepans was unsafe, and the raw experimental data were lost to the sands of time, so they fell off the recommended list.
Add in the relative decrease in food prices relative to wages, women in the workforce, increasing urbanization and you just get to a place where the research lags, badly, because the government isn’t interested in funding it and there’s a perception that it appeals to a very small segment of the population.
So things you can’t officially can safely at home with any technique include mashed pumpkin, cream soups, smoked shellfish, thickened pie fillings other than those from a single USDA-developed recipe, celery or anything containing celery. . . the list goes on. And until University of Wisconsin - Madison validated the use of steam baths instead of covering the whole container +2” of water, you had to deal with a giant mass of hot water and metal in your kitchen during the hottest time of the year.
Enter the Instant Pot and ilk. Because they rely on direct electronic temperature measurement rather than inference of temperature from steam pressure levels, they are not recognized as safe methods for canning. But they’re wildly popular, relatively power efficient, don’t steam up the home kitchen and don’t take up a ton of storage space for use once or twice a year. The typical six-quart model does seven half-pint or four pint jars, and they’re AMAZING for steam canning.
But what about pressure canning? Do they get hot enough? Do they heat evenly enough? Do they cool at a similar rate to a pressure canner? More importantly, do the jars of food get to the right temperatures in the right time?
Good question.
I have no grad students. Drilling holes in jar lids and then hermetically sealing them up, and doing the same through pressure vessels is just way too much work. But I have Jon, and I have the need it of digital test and measurement technology, and now I can record this crucial data from inside sealed jars during cooking and processing to figure out if Instant Pots and their ilk are patently dangerous, just need modification from USDA methods or are drop-in replacements. My first project is probably albacore tuna - the home canned version is so incredibly superior to commercial stuff that it’s not even the same fish, and I’d love to be able to do a loin after dinner and not take the whole day making 30 jars.
So that’s why.