Q & A: Canning Tomatoes

Posted by on September 13, 2010 at 6:28 pm.

Around this time of year I notice I get a lot of people searching my photo essay on canning tomatoes for answers to very common questions. Occasionally it’s people trying to trouble-shoot a problem, but most of the time it’s people trying to cut corners.  I am a huge fan of making food as simple as possible to prepare, but I have to emphasize that when it comes to canning there are very few corners you can cut safely. So, I thought I would take this time to bring up some of the search queries I see most often and explain the reasons for the process.

Why are my jars breaking?

Most likely your jars are breaking because you are putting cool jars in hot water. The most common break is for the bottom to pop off or for it to split up the side. This usually only happens with cold-packed or if you wait too long to put the jars in the water (ie. you made more than you can process in one batch and the second batch has cooled too much before the first one is finished).  If you are using the cold-pack method you need to put the jars in warm (not hot) water and bring it gradually up to boiling. Begin the processing time once the boiling point is reached. If you are using the hot-packed method and need to process a second batch, dump out most of the water and replace it with cold water so that result is something around bath water temperature.  Heat the water to boiling with the jars in it, start the processing time once the water has reached a boiling point.  On occasion a jar will have a flaw in it, if you are certain the break is not a result of your process this is probably what happened. Not a big deal, just a casualty of canning. It probably won’t happen again for a while.

Can I process tomatoes by turning them upside down instead of boiling the jars?

NO! I’m sorry but you can’t. It is completely unsafe. The processing time of high-acid canning is based on the acid levels of the contents. Tomatoes are on the border of high and low acid canning, meaning they require a longer processing time in order to prevent the growth of harmful bacterias such as botulism.  In some cases of high-acid canning the acid levels are strong enough that even without the processing time there is sufficient acidity to prevent the growth of harmful bacterias. In these cases turning a jar upside down will most often sterilize the lids properly and prevent any bacterial or mold growth. In the even that something still grows, it is very unlikely to be harmful. THIS IS NOT THE CASE FOR TOMATOES. Without the proper processing time there is a chance you will fail to kill all of the bacteria spores, including dangerous spores that create botulism. The risk is not worth it. If you don’t want to water process your tomatoes you need to refrigerate them, though I strongly recommend water processing as refrigeration will compromise the flavor of your tomatoes, making all that effort not very worth it.

Do I have to add citric acid or processed lemon juice to can tomatoes?

YES YOU DO. The short answer is that the acid levels in tomatoes are unpredictable and right on the border of what is considered safe for water bath processing. Without the boost in acidity you run the risk of getting Botulism. The long answer: In the 80s, the USDA revised their standards for canning tomatoes based on new varieties which were bred to be more meaty and less acidic. This means that the safe level of acidity you need to process tomatoes as “high acid canning” in a water bath was no longer standard or certain.  According to new standards, when water processing any sort of canned tomato product, you are required to boost the acid levels to a safe standard of 4.6 PH or lower (I know that sounds weird, but higher acid content means lower PH).  The NDSU extension service researched this and found that not a single variety of tomato they tested had sufficient acid levels to defeat the growth of clostridium botulinum, the bacteria that causes botulism.

Why does the lemon juice need to be processed?

Part of the processing includes checking the juice and assuring it has a consistent acid level. The amount of juice required is based on this acid level, meaning if you use fresh juice it could contain a lower acid level, your tomatoes will not be safe.

Do I have to use salt in tomato canning?

No. It is entirely a preference, though I strongly suggest using some salt. Adding it to recipes later means the salt will not entirely infuse the tomato and it will make a difference in taste.

Why does my tomato sauce have a chemical or metallic taste?

This could be one of two things. Either you have decided to use processed lemon juice instead of citric acid to boost the acid levels of your tomatoes, or your cooking equipment isn’t sound for high acid cooking.  You should never cook tomatoes in aluminum. This means if you have a non-stick pot with scratches you need to get something else, as most of those are aluminum. Also, if you have an iron pot that you are cooking the tomatoes in for a long period of time, the iron will start to leach into the sauce. This is not particularly harmful, but it will effect the flavor. I strongly recommend using stainless steal or enamel cooking equipment for canning tomatoes.

Why are my tomatoes floating to the top of the jar? or It looks like water got into my jars!

Two reasons. One is that the tomatoes still have small amounts of air in them. The second is that there is actually an enzyme in tomatoes that activates during the canning process and causes a separation of the liquid and pulp. As your jars cool, give them a gentle shake and everything will normalize.

Why do I have to remove the skins from tomatoes before canning?

The truth is you don’t. The main reason to remove the skin is flavor and versatility.  The skin can change the flavor and texture of sauce, giving a sometimes bitter or biting flavor, and adding chewy little bits of skin that stick in your teeth. The main reason I remove the skin is because I use a lot of tomatoes to make sauce or soup that has cream in it. Leaving the skin on the tomatoes will make milk or cream curdle when added. Besides, the process for removing skin is quick and satisfying, making it worth the trouble to me.

Why is headspace important?

Headspace is important because the contents of the jar expand as you process it. If you don’t leave enough headspace the contents will burst out the top potentially compromising the seal. To be honest, I haven’t had a seal compromised from this, but what does happen is that the jars get covered with a thin film of juice and fruit sugars that make them sticky and annoying to deal with. For hot packed tomatoes you should leave at least 1 inch of headspace, for cold packed at least 1 1/2 inches.

What is the difference between sauce tomatoes and slicing tomatoes?

Sauce tomatoes have a slightly higher acid level and are generally pulpier tomatoes which makes them ideal for canning both because of the safer acid levels and their ability to hold their form during the canning process. Slicing tomatoes are less acidic and juicier, making them ideal for immediate eating.

Can I use more than one kind of tomato for canning?

Absolutely! My first time canning tomatoes I used a wide variety of heirlooms that I blended and strained for juice to pack my sauce tomatoes in. The result was fantastic!

So, these are the most common questions I see people coming to Bramblings to find out about. A few were too vague for me to know how to answer, like “canning tomatoes film”… so please feel free to post more questions and I will do my best to answer them!



  • mandy says:

    I am just starting out canning and I want to buy a pressure canner but am wondering if I use a pressure canner that is made of aluminum will it make my tomatoes taste metallic. I will only be putting the tomato jars in their to seal. I am having a really hard time finding information on this. Please help. Also if you have any recommendations as to what pressure canner to buy I would be most grateful. Thank you

  • janina says:

    Hi Mandy!
    An aluminum pressure canner won’t effect anything you can at all (the water only circulates around the jars, not inside of jars). What you might want to consider is that you can use a pressure canner as a pressure cooker, and buying an aluminum one will make it less versatile in your kitchen in general.
    Hope that helps!

  • Juno says:


    I am always scared to can anything and reading this just scares me more! Seems I need a series of tools and precision to avoid killing myself and others! I want to make tomato confit. I would like to keep the jar in the refrigerator. I thought that the seeds and skin of the tomatoes were the acidic parts– and removing the skin but keeping the seeds when I make the confit should leave a bit of acidity? Anyway, the question is actually: if I keep my jar of confit in the refrigerator (will be eaten in the blink of an eye; refrigerator is set at 37,5F), am I safe to just let tomatoes cool then cover with oil and keep in refrigerator in a sealed jar? Thanks!!! Juno

  • janina says:

    Hi Juno!
    First, let me answer your question. Your plan is totally safe! Refrigeration is a safe and easy way to prevent botulism (which I assume is what is making you nervous). Botulism occurs in low oxygen levels and warm temperatures, so as long as you refrigerate those tomatoes in oil you are doing great!

    Second, botulism is only a concern with low acid canning like canning vegetables or beans in water. Low acid canning requires a pressure cooker and very precise temperatures/lbs of pressure. Cooking tomatoes and putting them in the refrigerator is not canning so again you’ve got nothing to worry about with your plan. High acid canning which includes jams and pickles creates an acidic environment where the botulism spores cannot survive, even at warm temperatures with low oxygen levels. This includes canning tomatoes when you raise the acid levels with citric acid or bottled lemon juice. Raising the acid levels in this case is insurance since the variety and growing method can effect the natural acid levels in a tomato. This is important for making canned tomatoes “shelf stable” (meaning, stored on a shelf at room temperature) but not important for storing them in the refrigerator which is not a warm environment.

    Third! It’s possible that the seeds and skin add to the acidity, but without a PH tester I wouldn’t count on it (but again, not something to worry about with refrigeration). My experience is that the skin can make sauce/juice/etc astringent, which is not the same acidic (think, wine tannins, not lemons).

    Last!! If you are feeling nervous about canning I recommend checking out the USDA publications. The United States Department of Agriculture even has their own canning book called “The Complete Guide to Home Canning” with 100% safe and tested recipes. It is an excellent place to start. You can find out more about that here.

  • Juno says:

    Wow! Comprehensive advice. Thanks! I just bought 10 pounds of tomatoes to prepare again as I did last week– roasted in oven then put in jar filled with oil. I reckon they’ll last a week… They’re sooooooooooooooooo good that way, and the oil which is leftover is also great for condiment!!!

    I will one day get my act together and properly learn but for now I don’t have space anywhere for equipment and the likes.

    Thanks again!!! J

  • liz popp says:

    I raw packed some stewed tomatoes and they separated bad. The meat of the tomato is packed tight on top looking dry and all the clear juice is at the bottom. Are they still good? Do I need to redo? Help!!!

  • janina says:

    Hiya Liz!
    They are totally fine. As mentioned above:

    Why are my tomatoes floating to the top of the jar? or It looks like water got into my jars!

    Two reasons. One is that the tomatoes still have small amounts of air in them. The second is that there is actually an enzyme in tomatoes that activates during the canning process and causes a separation of the liquid and pulp. As your jars cool, give them a gentle shake and everything will normalize.


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