Ask the Goats is a weekly feature in which we, the goats, attempt to answer your questions about growing, making, eating, and preserving food. Got a question? Drop us a line at email@example.com.
Q: When you have a boiling-water bath recipe, how much do you adjust the processing time for different size jars? Most jam recipes have processing times for 1/2 pint jars, but what if I wanted to can pint size jars? Or vice versa? Sometimes I don’t have all the same size jar for one recipe and I don’t want to over or under process.–MP
A: Great question! When you’re processing the jars, you’re doing two things. First, you’re heating up the air inside the jars and thereby forcing some out. This creates a vacuum when the jars cools. In a good rolling boil, you can accomplish this in about 5 or 10 minutes, regardless of the size of the jars.
The variable processing time for different size jars has to do with the second function of heating up the jars: killing food-borne pathogens like yeasts, molds, and bacteria. The length of time to accomplish this depends on the size and contents of the jars. For some foods, like applesauce or grape juice, the processing time will be the same regardless of whether you’re making half-pints, pints, or quarts. In most cases, though, you have to add time—typically 5 or 10 minutes—for larger jars.
If you’re moving to smaller jars, you’ll definitely be safe using the recommended processing time, but—depending on what you’re making—you map run the risk of over-processing. This isn’t an issue for things that have already been cooked a long time, like ketchups, chutneys, and most sauces. Things get a little more dicey when texture matters, for instance, peaches and pickles.
So, to make a long story short: there is no rule of thumb for whether and/or how much time to add or subtract when switching between jar sizes. With jams, you should feel free to move between quarter-pint, half-pint, and pint jars. For everything else, you’ll want to check your recipe against a published canning reference. Both the Ball Blue Book and the National Center for Home Food Preservation are pretty good about listing different times for different sizes. And besides, it’s not such a bad idea to check their guidelines anytime you’re using a recipe you’ve found online.