Many home canners assume that white distilled vinegar is the most acidic ingredient in their arsenal.
Not so: lemon juice is actually even more acidic than white distilled vinegar.
pH of a few typical canning acids
Here’s a comparison in descending order of the pH of a few typical canning acids (remember, the lower the pH, the higher the acidity):  Source: FDA. Approximate pH of Foods and Food Products. April 2007. Accessed March 2015 at https://foodscience.caes.uga.edu/extension/documents/FDAapproximatepHoffoodslacf-phs.pdf.
- Apple cider vinegar: 3.10
- White distilled vinegar: 2.40 – 3.40
- Lemon Juice (fresh): 2.00 – 2.60
- Lime Juice (fresh): 2.00 – 2.35
What does lemon juice do in home canning recipes?
The Putting Food By authors say this about lemon juice,
Lemon juice contains both ascorbic and citric acids. Average acid-strength of fresh lemons is about 5 percent (also the labeled strength of reconstituted bottled lemon juice; some strains of California lemons are less strongly acidic, however). Being in solution naturally, it’s about one-sixth as effective volume for volume as ascorbic acid for preventing darkening. Even more of a flavor-masker than citric acid, it also adds a distinctive lemony taste to the food. It is used primarily to augment a food’s natural acidity.”  Hertzberg, Ruth; Greene, Janet; Vaughan, Beatrice (2010-05-25). Putting Food By: Fifth Edition (p. 36). Penguin Publishing Group. Kindle Edition.
Use bottled lemon juice
Beginning home canners are usually shocked to find that so many of the tested and approved home canning recipes call for bottled lemon juice.
The University of Georgia Extension Service explains why:
Bottled lemon juice is used to standardize acidity. Fresh lemon juice can vary in acidity and is not recommended.” E. M. D’Sa, E. L. Andress, J. A. Harrison and M. A. Harrison. 2006. Thermal Process Development to Ensure the Safety of a Home-Canned Lemon Curd Product. Athens, GA: The University of Georgia, Cooperative Extension Service. Accessed July 2016.
The Bernardin Guide (2013) says,
Lemon juice – bottled juice is preferred because the commercial product has a standardized pH (acidity) level.” Bernardin Guide to Home Preserving. Toronto, Canada: Bernardin Ltd. 2013. Page 48.
Fresh lemons can vary in acidity based on:
- variety (Meyer lemons are very low in acid);
- where in the world the lemons have been grown;
- the weather during their growing season;
- how long they are in storage along the way, in the store, and in your fridge;
- storage conditions, etc.
Bottled lemon juice, however, is governed by Food and Drug Administration rules. It must have “a titratable acidity content of not less than 4.5 percent, by weight, calculated as anhydrous citrus acid.”  Code of Federal Regulations (Title 21, volume 2, revised April 1, 2010), via Linda Ziedrich.
Given the variables that can affect juice from a fresh lemon, and given that the acidity of the bottled lemon juice is a known, guaranteed factor by law, it’s no wonder that the canning authorities often play it safe and decide to settle on bottled to provide the critical safety margin in a recipe. Asian pears, figs and tomatoes require an acid such as lemon juice as a critical element to make them safe for canning.
The authorities weren’t going to rely on the wildcard of a “fresh lemon” for safety. Nor could they possibly begin to even dream of having the resources to monitor on a regular basis the acidity of new cultivars of lemons coming on the market all the time, impact of varying countries of origin and weather conditions on acidity, etc. They need to make recommendations that can last several decades at least, because they never know when the next few shekels of funding to review the recommendations will come along. So, by calling for commercial bottled, they can ensure that everyone is getting the same safe acidity level.
Acidity conclusion disputed
For the record, respected home canning author Linda Ziedrich disputes the USDA’s view on the assured acidity of fresh lemon juice versus bottled. You may wish to review her analysis at : Real Lemon versus ReaLemon (link valid as of March 2015.)
Bottled lemon juice has an expiry date
Don’t forget, bottled lemon juice has a best-before date. Keeping the product in the fridge should extend its best-before date greatly.
What bottled lemon juice swaps can you do?
Safety-wise summary: You can swap bottled lemon and lime juice interchangeably, and you can always replace vinegar with bottled lemon or lime juice. But you cannot ever do the reverse and replace either of those two with white vinegar, because it’s weaker than those two. Note that while lemon and lime juice are quite acidic, sadly orange and tangerine juice (with a ph of 3.30 – 4.19) are far less so and so cannot be swapped in as safety equivalents.
Angela Fraser at North Carolina Extension says,
Bottled lemon juice tends to be more acidic than vinegar. It also has less effect on the overall flavor of the product in which it is used. Equal amounts of bottled lemon juice can be substituted for vinegar in recipes calling for vinegar. Vinegar, however, should not be used when a recipe calls for lemon juice.”  Fraser, Angela. North Carolina State University Cooperative Extension. Celebrate with Safe Salsa. FCSW-516. Accessed March 2015 at https://www.ces.ncsu.edu/depts/fcs/pdfs/FCS516WAccessibleApril09.pdf.
You can also swap in citric acid in appropriate ratios (for instance, when doing tomatoes, which allow for either bottled lemon juice or citric acid.)
Suzanne Driessen at the University of Minnesota says, “You can safely use bottled lime juice instead of bottled lemon juice.” Driessen, Suzanne. Canning Tomato and Tomato Products? Pay Attention to Directions. University of Minnesota Extension Service. Reviewed 2014. Accessed June 2016 at https://www.extension.umn.edu/food/food-safety/preserving/tomatoes-salsa/canning-tomato-and-tomato-products./
When you can use fresh lemon or lime juice
When a canning recipe calls for bottled lemon or lime juice, or just lemon or lime juice, use bottled, don’t substitute fresh.
If a tested recipe from a reputable source such as Ball or Bernardin labs test kitchens calls for fresh lemon or lime juice, go ahead and use that (though you may also use bottled if that is all you have to hand or if that is your preference.)
If a recipe calls for fresh lemon or lime juice, but you only have bottled you may use bottled.
Some canning authors say that whenever you are are sure that the safe acidity requirements are already met elsewhere in the recipe, you are free to use fresh lemon juice. For instance, home canning blogger Marissa McLaughlin says:
However, when it comes to recipes where the level of acidity isn’t crucial (for instance, when you’re adding lemon juice to a batch of jam to balance the sweetness), you can use fresh lemons.”  McLaughlin, Marissa. Canning in Vintage Jars. Blog posting: Some Canning Questions and Answers. December 2009. Accessed March 2015.
The issue with that advice, however, is that probably only .001% of home canners would have the knowledge of when a jam was acidic enough. Don’t forget: safety aside, high acidity in jams is also required (along with sugar) for traditional pectin to set to make the jam worth canning!
The Pacific Northwest Extension says that Meyer lemons or Key limes are less acidic:
Don’t use Meyer lemon juice or key lime juice, they are different, and weaker: “Key lime juice should not be used as lime juice.”  Salsa Recipes for Canning. Pacific Northwest Extension. PNW395. 2014. Page 4.
Ball’s All New Salsa recipes (2016) call for fresh lime juice. Healthy Canning confirmed in several telephone calls July 2016 to Ball that in their salsa recipes appearing in their 2016 All New Book of Canning calling for fresh lime juice, you can use either Key Limes (aka Mexican Limes) or the larger Persian Limes (aka Bearss, aka California) limes. So for those recipes, the tested lab results from Ball’s test kitchens overrule the Key Lime advice above from Pacific Northwest — for those particular lab-tested recipes only.
Bottled lemon juice and sulfites
Some people are concerned about sulfites in the bottled lemon juice.
The University of Minnesota Extension has this advice:
Bottled lemon and lime juice contain sulfites. If you or family members have a sulfite sensitivity or allergy, use citric acid or vinegar or substitute frozen lemon juice (not lemonade) that you find in the grocery store frozen section – use same amounts as bottled lemon juice.”  University of Minnesota Extension. Home Food Preservation Newsletter. August 2012. Accessed March 2015 at https://www.extension.umn.edu/food/food-safety/home-food-preservation-newsletter/docs/home-preservation-august-2012.pdf.
Or, again, you can also swap in citric acid in appropriate ratios.
History of the recommendation for bottled citrus juice
The recommendation for bottled lemon juice goes back to at least the 1980s. In June 1986, an extension agent from Maryland wrote:
Bottled, not fresh lemon juice should be used because the acid content of bottle lemon juice is standardized.” Jenkins, Kathryn. New guidebook available this fall giving up-to-date home canning tips. Frederick, Maryland: The Frederick News-Post. 19 June 1986. Page F-1.
The recommendation applied equally to lime juice.
In 2016, Ball created some controversy in the canning world with its All New canning book, when it called for fresh as opposed to bottled lime juice for its new salsa recipes in that book.
References [ + ]
|1.||↑||Source: FDA. Approximate pH of Foods and Food Products. April 2007. Accessed March 2015 at https://foodscience.caes.uga.edu/extension/documents/FDAapproximatepHoffoodslacf-phs.pdf.|
|2.||↑||Hertzberg, Ruth; Greene, Janet; Vaughan, Beatrice (2010-05-25). Putting Food By: Fifth Edition (p. 36). Penguin Publishing Group. Kindle Edition.|
|3.||↑||E. M. D’Sa, E. L. Andress, J. A. Harrison and M. A. Harrison. 2006. Thermal Process Development to Ensure the Safety of a Home-Canned Lemon Curd Product. Athens, GA: The University of Georgia, Cooperative Extension Service. Accessed July 2016.|
|4.||↑||Bernardin Guide to Home Preserving. Toronto, Canada: Bernardin Ltd. 2013. Page 48.|
|5.||↑||Code of Federal Regulations (Title 21, volume 2, revised April 1, 2010), via Linda Ziedrich.|
|6.||↑||Fraser, Angela. North Carolina State University Cooperative Extension. Celebrate with Safe Salsa. FCSW-516. Accessed March 2015 at https://www.ces.ncsu.edu/depts/fcs/pdfs/FCS516WAccessibleApril09.pdf.|
|7.||↑||Driessen, Suzanne. Canning Tomato and Tomato Products? Pay Attention to Directions. University of Minnesota Extension Service. Reviewed 2014. Accessed June 2016 at https://www.extension.umn.edu/food/food-safety/preserving/tomatoes-salsa/canning-tomato-and-tomato-products./|
|8.||↑||McLaughlin, Marissa. Canning in Vintage Jars. Blog posting: Some Canning Questions and Answers. December 2009. Accessed March 2015.|
|9.||↑||Salsa Recipes for Canning. Pacific Northwest Extension. PNW395. 2014. Page 4.|
|10.||↑||University of Minnesota Extension. Home Food Preservation Newsletter. August 2012. Accessed March 2015 at https://www.extension.umn.edu/food/food-safety/home-food-preservation-newsletter/docs/home-preservation-august-2012.pdf.|
|11.||↑||Jenkins, Kathryn. New guidebook available this fall giving up-to-date home canning tips. Frederick, Maryland: The Frederick News-Post. 19 June 1986. Page F-1.|