Canned Tomatoes – Answers to Frequently Asked Questions

Tomatoes are the most popular food for canning at home.  They are versatile, abundant, and easy to can. There is no doubt that tomato canning season is here as the number of “tomato canning” questions rise—floating, separation, loss of liquid, air bubbles.  Do any of these problems affect the safety?   AnswerLine is here to answer these questions.  


Floating tomatoes (tomatoes or tomato pieces at the top of the jar, liquid at the bottom) may be a visual disappointment but does not affect the safety of the product. Floating is more likely to happen with whole or quartered tomatoes and with a raw pack.  The floating is caused by the natural air and water in the tomatoes which releases during processing. Raw food has a lot of air in it. The volume of food put in a jar before processing might actually only be ¾ food and ¼ air trapped inside the food. During processing, this air will escape from the food and rise to the top of the jar. Float can be minimized by choosing fresh, firm tomatoes, reducing the amount of liquid in the jar (replacing with product), packing the jar as firmly as possible without crushing the tomatoes, removing air bubbles, and using a regular mouth jar. Sometimes turning the jars upside down after they have cooled for 24 hours and letting them rest for a period of time will bring the liquid and solids back together.  Canning crushed tomatoes and/or using a hot pack often eliminates the problem. 


Sometimes crushed or puréed (juice) tomatoes will separate in the jar into tomato solids and liquid. Separation is another visually disappointing issue that does not affect safety. When tomatoes are cut or crushed before heating, exposure to the air activates a natural enzyme, Pectose (Pectinesterarse), found in high concentrations in tomatoes. The enzyme is activated when tomatoes are cut. This enzyme breaks down pectin which causes the liquids and solids to separate.  Heating tomatoes immediately after they are cut or crushed to 180F (82C) inactivates this enzyme.  This is the reason that many tested recipes direct one to cut small quantities of tomatoes and heat them in batches.  Gently shaking the jars after the product has cooled for 24 hrs may bring the solids and liquid back together.

Loss of Liquid

Loss of liquid does not cause food to spoil, though the food above the liquid may darken. If, however, the loss is excessive (for example, if at least half of the liquid is lost), refrigerate the jar(s) and use within 2 to 3 days.[1]  And jars with milder liquid loss should be used sooner rather than later so place them at the front of the shelf so they get used first.  Penn State Extension advises on the three likely causes of liquid loss or siphoning from the jar during processing of tomatoes or other fruits and vegetables—raw pack, rapid fluctuation of temperature in the canner, and removing the jars too quickly after processing. In addition to the causes noted, improper headspace and loose bands are other sources of liquid loss.  Food expands during processing and if a jar is overfilled there is insufficient room for the expansion.  When this happens, water will push out to make room for expansion. If canning ring bands are too loose, liquid will escape and may also cause seal failure. And like floating and separation, removing air bubbles from the jar prior to lidding helps to lessen liquid loss.

Air Bubbles

Removing of air also known as de-bubbling prior to processing is an important step in canning. Air trapped in jars can interfere with the jar’s ability to drive out the extra air in the top causing too much headspace, floating, loss of liquid, and a poor or no seal. Additionally, too much air space results in canned product above the canning liquid which can lead to discoloration and the development of off-flavors. Ball and Norpro make a bubble remover and headspace tool designed for air removal but a plastic or silicone knife or spatula handle will do the same; any tool used should be heat resistant to handle the heat of a hot pack.  Do not use anything metal to remove air as it may cause hairline cracks in the jar.  Simply run a bubble popper around the edges of the jar, gently shifting the food, so that trapped air is released as much as possible. After the air bubbles have been removed, more liquid may need to be added to the jar to ensure proper head space.

After processing, tiny air bubbles may be noticed in the product.  If these bubbles are inactive, they are benign or harmless.  If the bubbles are actively moving or fizzing up to the top of the jar when opened, the product may be fermenting or contaminated. Products with active air bubbles should not be used and properly discarded.

Despite one’s very best efforts to diminish floating, separation, loss of liquid, or air bubbles, it seems that there is one more non-scientific reason—the phase of the moon! Or that it just happens!  As long as the jar seals and there is at least half of the liquid in the jar, the tomatoes are safe inside the jar.

Marlene Geiger

I am a graduate of the University of Nebraska-Lincoln with a BS in Home Economics Education and Extension and from Colorado State University with a MS in Textiles and Clothing. I enjoy spending time with family and friends, gardening, quilting, cooking, sewing, and sharing knowledge and experience with others.

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12 thoughts on “Canned Tomatoes – Answers to Frequently Asked Questions

  1. I did not boil my tomato juice as long as instructions called for. All the jars sealed. As long as they stay sealed is it safe to use them? If not is there any thing I can do to save the juice? I canned them about a month ago, but left on vacation and just got home.

  2. Hi Mary, It is too late to rectify your mistake. The boiling time is probably not real critical to the process but acidification is. Did you acidify your tomato juice? Acidification is necessary to get the pH to 4.6 or lower to insure real safety.

  3. Do I have to save the lightly colored juice that separates from home canned tomatoes. I want to make spaghetti sauce and I have close to 2″ on top. It would save me a lot of boiling down if I just discarded all of it. I am wondering if I would discard a lot of flavor and nutrients by doing this?

  4. Hi Jim, you may drain the tomatoes if you desire; however, in doing so, you are draining off both nutrients and flavor. If you do drain, save the liquid and use it to flavor or thin sauces or add to soup broth. Thanks for contacting AnswerLine.

  5. Good evening. I am new to canning and just bought my first pressure canner. Many (official) canning recipes I see call for crushed tomatoes. Unfortunately, I have a condition called diverticulitis that forbids me from eating seeds. Is it safe for me to use the same amount of fresh tomatoes called for in the recipe, but to first strain them in a food mill instead of crushing them, and then proceed with the (pressure canning) recipe? Or should I add more acidification or more processing time to compensate for density? This is truly a question I’m going to face every time a recipe calls for chopped, stewed, or crushed tomatoes, so I want to make sure I do it right.

    I could also really could use your help in figuring out the canning lingo differences between tomato juice (like for the USDA beef in tomato juice recipe), tomato puree (not paste), tomato passata, and strained tomatoes. To my uneducated canning eyes, they are all one and the same.

    THANK YOU for what you do, and for your time. I just want to be canning safely.

    God bless,


  6. Hi Menolly, Removing the seeds from tomatoes does not affect the processing method or time. There are numerous ways to remove the seeds so which ever method you choose, it will not change the way you can them. Some people like to remove the seeds from the whole tomato and then can whole or halved. Others like to quarter, scrap out the seeds and can has crushed or diced. Still others, do as you are suggesting, use a food mill. Check the National Center for Home Food Preservation for canning directions of all tomato products. Diced tomatoes are processed like crushed tomatoes.Tomato juice and tomato puree are quite similar; however, tomato juice may have some added seasoning while puree does not. Tomato passata is the same as tomato puree; it is the Italian term and is also used through out Europe. Strained tomatoes usually refers to tomatoes which have been canned and upon opening, are run through a food mill or strainer to remove seeds. So, yes, they are as you say, nearly one in the same. As you move forward with your canning and have specific questions, please call us at AnswerLine, 800-262-3804. It is much easier to talk through questions than to try to explain everything via email as often additional information may be needed that is not included in the message.

  7. Thank you so very much for replying! I had lost the page, and that’s the only reason I took so long to thank you.

  8. I just made my first batch of tomato sauce today and when I went to fill the jars, I noticed bubbles on the top that kind of appeared like a little soapy. I was able to use my bubble remover and they went away but I am not sure if this is normal? It seems some people see bubbles after they water bathe them but this was when I put the sauce in.

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