The best part of any food preservation class (besides the food) is always the discussion. Usually, I have quick answers for a fairly regular set of questions about food poisoning, the physics of pressure cookers, storage, and the differences between jams, preserves, conserves, and compotes. But recently, I’ve started to hear some new ones. This post is an attempt to round up some of the answers to both. Feel free to add more questions in the comments, or drop us a line.
Why is canned food shelf-stable?
When you can something, you’re doing two things. First, you’re killing bacteria, molds, and yeasts through the addition of heat. Second, by creating a seal, you’re preventing new bacteria from getting in.
What’s the difference between pressure canning and water-bath canning?
For the purposes of canning, all foods are either high-acid or low-acid foods. The kinds of food-born pathogens that can live in a high-acid environment can be killed fairly easily with moderate heat, so you can process them in a boiling water bath. Low-acid foods, on the other hand, can harbor botulism spores that are not destroyed at 212°F. Low-acid foods must be processed in a steam pressure canner than can reach much higher temperatures (usually 240ºF).
How do I know if a food is high or low acid?
All fruits except for figs and tomatoes are acidic enough to can in a water-bath canner. Figs and tomatoes can be safely canned this way with the addition of a small amount of lemon juice. Everything else—including vegetables, meats, fish, and mixtures of high acid and low acid foods (for instance, salsa)—either has to be pressure canned or made more acidic. The short answer to this question is to follow the recipe. The longer answer, recommended only for experienced canners, involves comparing fruit/vegetable/acid ratios and densities from trusted sources. Please be cautious when using internet canning recipes, and consider comparing unfamiliar instructions to published guidelines, such as those of the National Center for Home Food Preservation and those listed in the Ball Blue Book of Preserving.
What’s the processing time?
The term “processing time” refers to the amount of time that you either boil or pressure can your jars. The amount of time depends on the food—check your recipe. In a boiling water bath, you start timing when the water returns to a boil. In a pressure canner, you start timing when the canner reaches pressure.
I have a pressure cooker. Can I use it as a canner?
That depends. Small pressure cookers are not necessarily guaranteed to reach and maintain the appropriate temperatures necessary to kill botulism spores. Most pressure canners are 23 quarts or larger—large enough to hold 7 quart jars on a rack, or 14 jelly jars, stacked. If you’re not sure, contact the manufacturer. My attempt to explain the science of pressure cookers is here.
Do I have to sterilize the jars?
If the recipe calls for a processing time of more than 10 minutes, no. Otherwise, yes. In either case, manufacturers generally recommend that you pre-heat your jars to reduce the chance of breakage, particularly before processing in a water-bath canner.
Can I use a dishwasher to sterilize my jars? What about to process them?
If your dishwasher has a sterilization setting, you may use it to sterilize your jars. You may not, however, process your jars in the dishwasher. You need to use a boiling water bath or a steam pressure canner, as per the recipe.
If I have a low-acid food, but I just want to keep it for a few weeks, can I water-bath can it?
No. Food is either safe for water-bath canning, or not. If you do not have a pressure canner but have a food that needs to be pressure canned, you either need to refrigerate it or find some other way to preserve it.
How long will canned food keep once it’s opened?
Once the jars are opened, canned food is just like regular food, with similar keeping times. Something pickled might last months; a highly sweetened jam might last several weeks; and a tomato sauce might last only a few days.
Can I reduce the sugar or salt in a recipe? What about sugar substitutes?
YES. Contrary to what you may have heard, it is perfectly safe to reduce the sugar in a canning recipe so long as you are using an appropriate processing time. (The highly liability conscious company that sells Ball jars backs me up on this.) Sugar is a preservative, which means that food (particularly fruit) with sugar added will retain more of its original color, taste, and texture. It also means that the jars will keep longer once you’ve opened them. You may not get the texture you’re expecting if you reduce or substitute sugar—for instance, your jams might not set. But since sugar does not affect acidity, it doesn’t affect safety. Same thing with salt: the amount of salt in canning recipes is not enough to act as a preservative. It’s there for flavor. If you need to reduce your salt, just leave it out.
How will I know if a jar has gone bad?
A broken seal, a bulging lid, moving bubbles, mold, foam, bad smells, funky texture, and sliminess are all signs that you should not eat the contents of a jar.
Why do you store jars without their rings?
See previous question. If you’ve got an active bacterial population in your jar, they will produce various gasses. If the rings are removed, the pressure inside the jars can eventually build up to the point that the lid pops off. This is your signal, months later when you find the jar in the basement, to not eat it. If the ring is attached, the lid might not pop off. In rare cases, the jar might even explode. On a more mundane level, you should remove the rings because moisture trapped between the jar and the lid will cause them to rust.
How long can you store your jars?
That’s a good question. The USDA generally says 1 year. Many experienced canners will tell you that they fairly regularly keep their canned goods for longer than that, and just as many will tell you that food begins to lose its flavor much sooner (say, 6 months). Since the whole point of preserving foods is to hold you over until the next year’s harvest, shoot for a year.
Got more questions? Bring ‘em on!