Home Canning FAQ

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!

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54 comments to Home Canning FAQ

  • I have what is probably a very basic and silly question. I understand that you should remove the ring from the jar when you store it. However, once you open the jar and then want to refrigerate the remaining contents, is it ok to put the ring back on? Or is the vacuum lid sufficient after you pop it off and it loses its vacuum seal?

  • dorisandjilly

    Jenni–Not only is it OK to put the ring back on when you refrigerate it, it’s recommended. Once you’ve broken the seal, there’s nothing holding the lid in place. If we’re talking about something that has a lot of vinegar in it, you might want to switch to a plastic lid to reduce corrosion.

  • I’ve been reading up, and both bananas and green mangoes seem to be too low in acid to trust in a hot water bath without added acid.

  • Oh Doris and Jilly, I have SO many questions, I love this post!

    To follow up on Jenni’s question – I’m also confused as to another aspect of removing the ring for storage. When you remove the ring before storing a can of preserves, why doesn’t the vacuum seal break? I would just imagine that removing the ring without breaking the seal would be hard.

    And another question – I always have trouble getting the lid off the jars themselves. What do you use? It hurts my fingernails usually to pry stuff open.

    Related to the lids, I just want to confirm that you’re not supposed to use a lid twice (I think I read this somewhere but I don’t remember where). I usually write “used” in sharpie marker on my lids when I clean them off after finishing a jar of preserves. (I do, however, re-use them when I’m using ball jars and lids to tote my lunch salad, store a salad dressing in the fridge, etc., because I figure the seal doesn’t matter since I’m refrigerating the contents.)

    Finally (for now, anyways) – re:/ canned preserves going bad. I think I read somewhere that if a canned preserve has botulism from not high enough acid, that you won’t be able to tell (i.e. that there aren’t any indicators that the food has gone bad, like off-coloring, smells bad, slimy, etc.) I’m not particularly worried about this considering I always follow the recipe and processing times, but still, I was just curious what your thoughts are on this.

  • dorisandjilly

    @Sarah: Good find! I wonder if this actually goes for other unripe fruit, too?

    @Carter: Excellent questions all. Let’s start from the top. The ring and the lid are not actually connected. The seal is created by suction from the vacuum that results as the hot air in the jars shrinks as it cools off (think ideal gas law: PV = nRT). The ring is there merely to hold the lid in place until it seals. Once it seals, you don’t need the ring. This is related to why you’re not supposed to use the lids twice–the rubber sealant is only guaranteed to work once. An easy way to keep track of new lids vs. used lids is to leave the new lids in the box until you use them. This way, you know that if you just find a lid in a drawer, you can’t use it for canning. The fact that you can’t get the lids off easily is a good sign. You want to use the flat side (as opposed to the pointy end) of a bottle opener, aka, a church key. Sometimes handheld can openers also have a piece that can do this.

    And yes, alas, botulism can be invisible. In practice, it does usually result in some bulging, which is why it’s recommended that home canners use the two piece lids rather than glass lids with clamps. Following the recipe and processing times is by far the best way to avoid botulism.

  • Fantastic entry. It’s basically what I’ve spent hours re-telling to my friends who are interested in canning. Now I’ll just send them here. Thanks!

  • Oh, I know they’re not connected. My question was more just, I always worry that the pressure I exert to get the ring off the lid before storage will break the seal!

    I think I’ll just become a ring-removing daredevil. I guess I can always re-process them if the seal breaks, right?

  • Great FAQ! My biggest canning issue has been failure to set. Several times when making a jam recipe using pectin, my jam has failed to set. I believe I am following the recipe to the letter, but still I have this problem. What am I likely doing wrong? Do you have to boil the jam mixture for longer than the one minute usually called for?

  • dorisandjilly

    Emily: Achieving a good set can be very difficult. To be honest, I don’t usually use pectin (I think it changes the flavor and I don’t like using so much sugar), so I have learned to be somewhat less attached to the idea of a “good set.” I’ve become more tolerant of looser jams that I call “preserves.” Using pectin will usually get your more predictable results, but other things still matter: the ripeness of your fruit, their water content, the ambient humidity, etc. In general, you want to use a mixture of perfectly ripe fruit and slightly underripe fruit, even with pectin.

    Anybody out there with more experience with pectin have a better answer?

  • My mom has been canning for years, and I found out last week that she repeatedly reuses the vacuum seal rings. I was a bit horrified because I’ve read so many books/blogs about this being a huge no-no. But she maintains that if they seal, she’ll keep reusing them (she mostly pickles and makes apple jam). Are there any other naught canners out there re-using the vacuum seals? Also, how the heck do you recycle them? (I don’t think you can throw them in the recycling bin.)

  • dorisandjilly

    Jenni: If she gets a seal, they’re safe. The issue is that you are not *guaranteed* to get a seal, and since the lids are so cheap (usually ~$2 for a dozen), it’s not worth the risk. I re-use my lids for storing dry goods in jars (beans, etc.) and for covering up leftovers (why use plastic containers when you’ve got a house full of mason jars). My ever resourceful sister figured out that you could use the lids and rings as mini-tart pans. But mainly they stack up and I never quite know what to do with them, either.


  • Doris and Jilly, you write that you don’t like using so much sugar… how much sugar do you use in cooked jam? You know there’s low-sugar pectin, right? You can use 0-3 cups of sugar for 4 cups of mashed fruit. I agree with you on full sugar pectin (7 cups of sugar to 4 cups of fruit!?), but I don’t think you should say the reason you don’t use pectin is because of the sugar content. Ball, Sure-jell and Pomona’s all either have a low-sugar version and allow you to adjust the amount of sugar you use. Thanks!!

  • dorisandjilly

    @Kate: Excellent point! I should be clear: I like a moderately sweet jam. I also like sugar’s preserving effects on flavor. OTHER people frequently ask me about reducing sugar, and I want them to know that it’s OK. You are absolutely correct in stating the low- or no-sugar pectin is a great (perhaps the only) way to get a good set if you choose this route. Personally, though, I don’t like the texture of jams made with low-sugar pectin. I also don’t like what it does to the color. This is, however, a personal choice.

  • Kathleen

    I’ve just completed my first attempt at canning (strawberry jam). When I put the bottles back into the canner, 3 of them fell over and I had a hard time getting them all upright again. Is this a problem – i.e., does water get into the jars?

    One seal did not take, so can this jar be stored like freezer jam? Thanks for any help!

  • dorisandjilly

    @Kathleen: Well, it depends. If the lids were screwed on fairly tight, not much water should have entered. It might affect your set. If they weren’t on terribly tightly, then yes, you may get something that’s more like a sauce. So long as the seal took, it’s still safe, but it may be liquid-y. if your seal didn’t take, you could either try again, or you can store in the fridge. The freezer is safe, theoretically, but you’ll need to leave room in the jar for the contents to expand. Good luck with your next batch!

  • Laura

    Thanks for such a great post! I have a quick question…

    I plan to spend the week pickling veggies from my garden. Because I am spreading my pickling out (doing a few jars a day), it would be great to be able to do all my processing at the week’s end. Is it safe to wait, or do the jars need to processed immediately?

  • dorisandjilly

    @Laura: If these are vinegar pickles, they should be fine in the refrigerator all week. Just remember that you should not stick cold jars in boiling water–they’ll break. Let your jars come back up to room temperature first.

  • All right, my turn. You can and should remove the rings from jars to store long term. But can you stack jars on top of each other? (for example, can I stack my quarter pints on top of each other to save shelf space?) I feel like this would lead to the same issue of storing jars without rings, but I’m canning 100 quarter pints for my wedding next May and space is at a premium!

  • Catherine

    I am new to canning of anything beyond jam. I have a couple of questions.
    1. What happens if you dont leave enough head space? I was canning some peaches yesterday and 2 of the jars were a bit over filled. When I took them out of the water bath some of the juice was bubbling out. However as they cooled they sealed. Would they be safe since they sealed? Or should I be making some peach cobbler in the next day or two?

    Also I found a recipe from my granny for Dilly Beans. The recipe calls for cook crumbled bacon. I made the Dilly beans and added the bacon to some of the jars as we have a vegitarian in the house. It called for a BWB for 10 minutes and not a pressure cooker. I am now worried about the bacon not going in a pressure cooker… thoughts?
    Thank you in advance.

  • dorisandjilly

    @Catherine: That is exactly what happens when you don’t leave enough headspace. You can get a siphoning action in which liquid is sucked out of your jars. So long as you have a good seal, you’re fine–they won’t look pretty, but they’re safe to eat. However, since they have less preserving fluid, the quality may deteriorate faster than the rest of your jars. Use them first. Be sure to wipe down the jars so they don’t grow mold on the outside.

    As for bacon in dilly beans, it’s hard to say without the recipe. Personally, I wouldn’t, particularly if the ratio of vinegar to water is around 1:1. If, on the other hand, it’s pure vinegar, it’s probably OK, but I can guarantee you that no home extension agent is going to give you the go-ahead on this.

  • Kris

    I canned 20 pounds of apricots several days ago. I blanched them to remove the peels, quartered them, heated them in a medium syrup, and processed them for the recommended 25 minutes. Problem is, they turned completely into mush. Tasty mush, but still mush. Now I am paranoid to can any fruit. Any ideas on what I might have done wrong? If I can fruit salad, should I pick slightly under ripe pears and peaches and apricots? The apricots I used this time were at the perfect ripeness for eating. Please help, one can only do so much with fruit mush. Thanks.

  • dorisandjilly

    Kris: Canned fruit is always going to be mushier than fresh, but it doesn’t have to be *that* mushy. With ripe fruit, blanching, a hot pack, and a 25 minutes processing time, you have created a perfect storm of apricot mush. You typically want to work with fruit that’s just a hair short of dead ripe. With apricots, peeling is optional, so that can remove one round of cooking. You also have a choice between a “hot pack” and a “raw pack.” The National Center for Home Food Preservation says that raw-packed apricots “make a disappointing product,” but at least they’re not mush. In a cold pack, you pour boiling syrup over your fruit. If you’re using a hot-pack, just barely bring them to a boil, and move as quickly as possible. Finally, check your processing times and consider moving to pints. The NCHFP says you only need 20 minutes for pints if you’re using a hot pack (you still need 25 for a raw pack). Better luck next time!

  • Kris

    Thank you so much for the quick response. Just had to let you know, last night I put some ice in a blender, added some sugar, milk, a dash of vanilla, and a jar of apricot mush, and made the BESTEST smoothie in the whole wide world. Seriously, we couldn’t get enough. Maybe this was the true destiny for these apricots…:)

  • Irene

    Help! Have made many jams in the past with absolutely no problems. Made Blackberry (Marion Berries) jam this weekend (picked fresh), and they did not set. Re-processed and still did not set. Used only berries, sugar, water and lemon juice first time and did the pectin route second. Can I save my jam or do I now have a bunch of Blackberry topping???

    Thanks much / irene

  • dorisandjilly

    Irene: Blackberries contain very low amounts of natural pectin, so it’s to be expected that they didn’t set the first time. I rarely use pectin, so I’m hardly an authority, but my understanding is that you have to add the pectin at the indicated moment. Given that you already had sugar in it, it might not work properly. The good news is that blackberries do indeed make an excellent topping–I actually like the stuff better that way. Next time, if you want a set with blackberries, use your pectin and follow the directions to the letter. Sorry I can’t be of more help!

  • susan Peran

    I,ve used the 4 minute pectin for years for my jams. When I couldn’t find it I used the 1 minute type for this year. which one is better AND what happens when you overcook the 1minute pectin?

    Thanks for your help!

  • dorisandjilly

    @Susan: Alas, I don’t use pectin, so I can’t tell you! My guess is that if you overcook it you get goo instead of jam.

  • Christine

    I have a problem…I just spent all day canning peaches and I think that it is all messed up. I followed the directions to the T- or at least I thought that I did. Now that they are out and cooling I have noticed that they have lost a lot of liquid- I guess this is commonly known as siphoning. Now what should I do? I have 13 jars of peaches and many of them have less liquid meaning that the peaches aren’t completely covered anymore. They are all sealed however.
    Will these peaches be edible and safe or should I empty them all and freeze them?

  • dorisandjilly

    @Christine: So long as you have a seal, your peaches are safe. You’ll want to eat them sooner rather than later, though, because the syrup acts as a preservative for flavor and color. Those peaches will probably start losing their flavor within a couple of months. I know from experience. 🙂

    Now, as for how to eliminate that problem in the first place….I’m guessing these were quarts, right? I find that siphoning is a bigger problem with larger jars and that you are most likely to get siphoning when you remove jars from water that’s at a rolling boil. It’s caused by the dramatic change in temperature that results. Next time, turn off the heat and leave them in the water for about 5 minutes before removing them from the water bath. They’ll “cook” a little longer, but you’re less likely to get siphoning. Good luck, and report back!



  • Mary Rufener

    Hi. I am not new to canning. But I had something that happened this year that has never happened before. I canned 16 quarts of vegetable soup. I used the water bath method and processed them for 1 hour. I checked the seals and they were good. One week later the lids started pinging. What went wrong? That was an awful lot of work to throw away. I am very disgusted. P.S. My salsa, peach preserves, strawberry jam are all sealed and delicious.

  • Jen

    I forgot to remove the air bubbles while canning my peaches. Are they bad now? I cut them into quarter peices or smaller. Would love to know if they’ll be safe to eat or if I ought to toss them and chalk it up to a learning experience. Thanks!

  • dorisandjilly

    @Jen: So long as they sealed, they’re fine from a safety perspective. Air bubbles can make it more difficult to get a seal, but they also an contribute to food oxidation later. This isn’t a safety issue but a quality issue. If you’ve done several batches this summer, eat these first.

  • Susan Peran

    You may have answered this before, if so I apologize. When you buy new jars, summer heat may cause the attached new lids and bands to form a seal on the empty jars. When I lift the lids they appear to be firmly stuck to the jars and make a popping sound as I remove them. I’m afraid to use them, but they look like new. Am I wasting the lids by throwing them away in favor of using a package of new ones? Would these lids likely seal? This site is fabulous. Thanks so much for all your help!

  • dorisandjilly

    Susan: Those lids should be fine. Just be sure to clean the tops of the jars well to remove any sealing residue. It’s possible that you might have a slightly higher than usual percentage of failed seals, but not enough to panic about.

  • heide

    Hi I’m new at canning , I did not realize you should take the rings off after you jars sealed and wipe the jars down.
    Will the food still be eatable.

  • larry

    Every where I look it says I have toprocess figs in pressure cooker, but I cannot find a recipe ( processing time and pressure) anywhere.
    Can you help

    regards Larry

  • Pennie

    Is it safe to stack canned jars? Or would it prevent the lids from popping to tell me that the canned food has spoiled.
    Thank you!

  • Yes…I am a newbe to canning preserves/jams. I canned some strawberrys filled leaving a half inch (to much air) The cans all sealed however it looks like there are bubles in the strawberry preserves. I picked out all the bad strawberries however I am worried that the bubles are a bad sign…please help! Thank you for all your help.

  • Christine

    I made a batch of salsa a couple of weeks ago. After reading the posts on this site, I’m a bit concerned. I usually always follow preserving recipe exactly, but this time I was a little short of tomatoes and added extra peppers. It looks fine, but should I worry?

  • Lori T

    Proper procedure for salsa’s is that you can adjust peppers – meaning switching hot for mild or mild for hot – keeping quantities the same, but NEVER peppers for tomatoes. You should freeze your salsa for safety right away.

  • Lori T

    Has anyone seen proper proceedures for stacking canned jars? I’ve been told it is OK, but am worried about it for the same reason you do not leave lids screwed on your jars. ‘If you’ve got an active bacterial population in your jar, they will produce various gasses.’ These gasses usually open the jar and you know it when you retrieve it to use, but if stacked I’m worried the jar will re-seal as if it would if the ring was left on. I’m looking for literature indicating why stacking is OK when rings are not.
    Thank you.

  • Mary C

    Last night I canned 35 jars of peaches. I went in this morning and realized that I didn’t waterbath 7 of them. Is it safe to just go ahead and process these now? It’s only been about 7 hours at this point. Thanks.

  • Sue

    I made jalapeno jelly last night and it is awesome! However, I did something stupid…I bought new lids and bands but I forgot to wash them! I did clean and sterilize the jars in the dishwasher and did a boiling water bath for 10min and all the lids sealed..will my jelly be safe to eat?

  • Zuzann

    I was wondering if I can still eat my canned peaches if they have mold on the outside? I thought I wiped down the jars well enough but I noticed today that there is mold growth on the outside of the jars. TY

  • Shannon

    I have a similar situation as Sue in that I forgot to wash my lids (they were new). Is my jelly going to be safe to eat or do I need to throw out the batch? I processed for somewhere between 7 and 10 minutes. Or can the jars be reprocessed if I remove lids, replace with new (and washed/dried) lids? I’m concerned about the unwashed lids being on the processed jars and wonder what to do with these jars of pepper jelly?

  • I know this an older post but I was curious if you can reduce the amount of sugar from 3 cups to 2 cups in a sweet pickle relish recipe? I know you can with fruit jam recipes but was curious about relish recipes. I’ve read on some blogs it isn’t a problem, but then some others are like OMG, you can’t do that. Thanks!

  • dorisandjilly

    Yes, in general, you can reduce the amount of sugar in pickles. What you *can’t* reduce is the amount or proportion of vinegar. The sugar is there primarily to off-set the sourness of the vinegar. You have to have very high amounts of sugar (as much as a cup per pound of food) to act as a preservative.

  • Michelle

    I made two batches of crabapple jelly a few nights ago and processed the jars in a water bath. Both batches sealed right away, but they all have condensation on the lids. Is this ok? Will it cause the jelly to go bad sooner? What would have caused this? This was the first time I did a water bath.

  • Beatrice

    OMG I have never pre washed my lids before…always was told that the heating process you use before filling the jars was just fine. Boy! Now I am confused… I do wash and sterilize my jars and bands though. I was told dont mess with lids until ready to use so seal didnt get messed up. Is this ok to continue doing this? Also, I am in need of a sinple recipe for canning salsa and would like to bbe able to add some sugar to it…any ideas? Thank you for any help and advice that you can give me on these subjects.