The USDA approves two methods for canning: boiling-water processing and pressure processing. Both are performed atop any residential stove and are well supported in Federal literature as well as almost all preserving books that one will find online or in the store.
The boiling-water process is recommended for acid foods, those with a pH lower then 4.6, which typically includes fruits, pickles, sauerkraut, jams, jellies, marmalade and fruit butters. I have noticed that most recipes these days for acid foods will include a tablespoon or two of fresh lemon juice, the additional acid will help to fix the color of most fruits and will tilt towards the conservative to ensure a truly acid product. The pH of fresh lemon juice may vary, however, and if you are concerned about the ultimate acidity of the end product then use bottled lemon juice whose acidity is more consistent and lower then fresh lemons.
The pressure process is recommended for non-acid foods, those with a pH higher then 4.6, which typically includes meats, stocks, fish, milk and vegetables. A pressure canner works by boiling water in an enclosed vessel that vents the air inside thereby raising the internal pressure, at sea level, to 10.5 lbs achieving an internal temperature of 240°F. Processing times are measured once 10.5 lbs of pressure is attained and the amount of time to process will vary on whether the product was hot or cold packed and whether you are using pint or quart jars. This could become a fairly drawn out process if you had plans to can 2 or 3 quart cases of fresh produce in one sitting.
Some thoughtful person gave me The Preservation Kitchen by Paul Virant with Kate Leahy last Christmas. I was quite struck by an insert on page 10:
“Europeans have always taken a slightly different approach to canning than Americans. This book follows the FDA- and USDA-approved processing procedures, in which jars are boiled in a water bath or pressure-canned for a set amount of time. French jam make Christine Ferber uses a different technique; she processes filled jars in the oven until the aigre-doux liquid begins to bubble up through the lid. The idea behind this method is that a filled jar left in the oven long enough will eventually reach an internal temperature hot enough to kill spoilers. There are benefits to using the oven for processing; it’s gentle on the fruit, allowing it to retain its shape. It’s also easier – just put the jars on a large sheet tray and place in a preheated 215° F oven for about 1 hour. The down side is that it is less precise. While water always boils at 212°F at sea level, oven temperatures vary widely and hot dry air doesn’t transfer heat as quickly as hot water or steam. I’ve experimented with the process and I’ve liked the results – especially with the blueberry aigre-doux (page 91). but I’m still waiting on the canning authorities to give this method a closer study.”
This European method appear more useful as I have two ovens and usually process 18 to 24 pints at a time. I tried to find more information about the European method but had no luck, everyone seems too scared of the liability from talking about anything other than the FDA/USDA approved processes. I bought a copy of Christine Ferber’s Mes Confitures and then quickly discovered that all of her recipes were the acid style. She describes the following method for processing.
“I use standard, faceted glass jars. The jars should be in perfect, unchipped condition. They should close very simply with a screw top. In the United States, Ball makes jars with two-part, sealing screw-top lids. Before making jam, I sterilize the jars , either by putting them in boiling water for a few minutes or by putting them in a 225°F (108°C) oven for 5 minutes.
As soon as the jam has finished cooking, I fill the jars, using the small ladle and the jam funnel. I fill them right up to the top. Any drips should be carefully wiped off. I close the jars while they are hot and turn them upside down. I wait until they are cool to put the labels on and then put the jars in a cool, dry place, out of the light.”
Still not satisfied, I continued my search for the European method and fortunately found The Blue Chair Jam Cookbook by Rachel Saunders, owner and founder of Blue Chairs Fruit Company. All of her recipes end with the comment to “Pour the jam into sterilized jars and process according to the manufacturer’s instructions or as directed on page 42.” Referring to the ominous page 42:
“There are many ways to sterilize jars, including putting them in a canning kettle or a sterilizing dishwasher, but my preferred way is in the oven. This method is easier than the other methods and, if you use an oven thermometer, is virtually foolproof. To sterilize jars and lids in the oven, first be sure they are perfectly clean. Place the clean jars upright with an equal number of clean unused lids on a baking sheet or sheet pan in a preheated 250°F oven. they should remain in the oven for a minimum of 30 minutes to ensure that they are heated through. Remove them from the oven right when you need to fill them. After you have filled them, leaving 1/4 inch of room at the top, wipe the rims with a clean, damp cloth. Put the lids on, being careful to screw them on just until they are snug, and replace the jars in the oven for 15 minutes or so to ensure they are completely sterilized. They will seal as they cool.”
Christine Ferber’s and Rachel Saunder’s books focus primarily on jams and jellies while Paul Virant’s book tackles preservation more broadly. I can easily recommend all three as excellent reference books for recipes as well as techniques. The USDA Complete Guide to Home Canning is a good place to begin if you are new to canning and might want to consult with your County Extension Service if questions arise during the canning process.
I have read many comments online searching for the European method and they are negative on the process and consider it a dangerous method. From you can’t know what the temperature really is to American jars are not manufactured to withstand an oven environment, etc. etc. Granted, glass is a poor conductor of heat and the pressure canner’s steam heat environment is more efficient than an oven’s dry heat at warming the jars, there is nothing inherently wrong with canning in an oven. The main point of the pressure method is to bring the foods to 240°F and hold for the length of time specified in the recipes in order to destroy all bacteria, their spores and the toxins they produce.
I have digital controlled convection ovens, I set the temperature and the oven beeps when it is at temperature. However, the temperature being measured is the internal oven air temperature, you should preheat the oven for at least 30 minutes after it has reached the set temperature to ensure that the oven’s metal has been heat-soaked otherwise once you open the door to load the jars, all of the hot air will escape and you will have a cool oven. It is also a good idea to buy a quality mercury oven thermometer to verify that after heat-soaking, your oven really is at the temperature that you want. Put a baking stone on the bottom rack of your oven to assist temperature recovery after loading the oven and closing the door.
I am not sure why the FDA/USDA doesn’t not review the oven method. I doubt that the Pressure Canner lobby is that active. The pressure method requires additional equipment and takes longer for large batches. You can buy a Taylor 5921N 5* Commercial Oven Thermometer at Amazon for under $15.00, and there are numerous other reputable oven thermometers available. Sterilizing the jars and lids in the oven seems much easier than in boiling water and faster than in most dishwashers. I had the bottom of a Ball jar fall off last year as I was removing a jar of salsa from a hot water bath, manufacturing defects and accidents can happen regardless of the process. Given the grease and spatter that collects in my oven, I am not particularly troubled by the idea of an exploding jar.