Many home preservers ask why tested and USDA approved canning recipes call for bottled lemon juice. This is especially true when it involves tomatoes and making jams. Why not fresh squeezed lemon juice?
A USDA RECOMMENDATION
It is a USDA recommendation that bottled lemon juice be used. And consistent with the recommendation, reputable canning sources will agree that the best source of lemon juice for canning is commercially bottled lemon juice, as opposed to the juice of a fresh lemon. The reason for the recommendation is that bottled lemon juice has been uniformly acidified or standardized per FDA regulations: “lemon juice prepared from concentrate must have a titratable acidity content of not less than 4.5 percent, by weight, calculated as anhydrous citrus acid.” With a guaranteed pH (5 percent2), there is a consistent and known acid level which is essential for the critical safety margin in canning low-acid foods and for making jams gel properly.
The amount of acid in canned food is critical to deter the growth of micro-organisms and insure that the food is safe. Foods with a pH less than or equal to 4.6 are labeled “high-acid” foods. Those with a pH greater than 4.6 are “low- acid.” This distinction is very important because only high-acid foods can be processed safely in a boiling water bath. Low-acid foods must always be processed in a pressure canner; if not, they can support the growth of the potentially harmful bacterium, Clostridium botulinum.
The pH of fresh lemon juice is inconsistent due to variety, maturity, weather conditions during growth, soil, fertilizer, rootstock, and storage conditions. There are even variations in acidity within a single variety. Lemons grown in hot climates tend to be less acidic than those grown in cooler climates. Lemon juice contains both ascorbic and citric acid; since ascorbic acid is destroyed by heat, only citric acid is measured. The average acid level of fresh lemon juice is about 5 percent, thus the “natural strength” labeling on the lemon juice bottle.
While acid consistency is the reason for using bottled lemon juice, bottled lemon juice is made from concentrate and preserved with sulfites. For people allergic to sulfites, bottled lemon juice may be a health hazard. If you or family members have a sulfite sensitivity or allergy, substitutes for bottled lemon juice include bottled lime juice (not Meyer or key lime) or frozen lemon juice (not lemonade) in equal amounts as bottled lemon juice or citric acid in appropriate ratios. Citric acid, sold as a white crystalline powder, is available where canning supplies are sold (note: citric acid is not ascorbic acid) and can be used to safely acidify foods if used correctly. Vinegar should not be used to replace bottled lemon juice unless a tested recipe allows it because white vinegar is weaker in acid strength. Equal amounts of bottled lemon juice can be used to replace white vinegar in recipes calling for vinegar, but not the reverse. When vinegar is an acceptable substitute, it will affect the flavor of the food. Never change the amount of acid, dilute with water, or substitute acid sources unless the recipe specifically allows you to do so. Aspirin should not be used as a substitute in canning. It cannot be relied on to lower pH or prevent spoilage .
ACIDIFYING TOMATOES FOR SAFE CANNING
When canning products with an unknown pH, they must be acidified to a pH of below 4.6 with lemon juice or citric acid. Tomatoes and figs are two examples where the pH values hover near or above 4.6. When acidified with lemon juice or citric acid, they may be processed as acid foods  making them safe for boiling water bath or atmospheric steam processing. Directions from the National Center for Home Food Preservation  for acidification of tomato products to insure safe acidity in whole, crushed, or juiced tomatoes state: Use 2 tablespoons of bottled lemon juice or 1/2 teaspoon of citric acid per quart of tomatoes. For pints, use 1 tablespoon bottled lemon juice or 1/4 teaspoon citric acid. Acid can be added directly to the jars before filling with product. Sugar may be used to offset the acid taste, if desired. Four tablespoons of a 5 percent acidity vinegar per quart may be used instead of lemon juice or citric acid. However, vinegar will likely cause undesirable flavor changes. Tomato salsas must also be acidified. To get an idea of how much difference bottled lemon juice makes, see Table 1 in the publication Why Add Lemon Juice to Tomatoes and Salsas Before Canning? by North Dakota State University.
pH MATTERS WITH JAMS
While many factors are involved in getting jams to “set” or gelatinize, pH plays a key role. When fruit is cut and heated with sugar, pectin strands are released from the fruit cells. The freed pectin strands repel each other because they carry a negative electric charge. Lemon juice lowers the pH of the jam mixture and neutralizes the negative charges on the strands of pectin allowing them to move together into a network to “set” the jam. The optimal pH for gelatinization is between 2.8 and 3.5. The best way to achieve this level of acidity is to use commercially bottled lemon juice. A second reason for using bottled lemon juice in jam recipes is to prevent the growth of bacteria and insure safe canning. With a lower pH, jams can be processed in a boiling water bath for a small amount of time dependent on altitude.
Whether using bottled lemon juice to acidify tomatoes or getting jam to “set,” bottled lemon juice has a ‘best used by’ date. Keeping the product in the fridge may extend its date but it is best to use a fresh bottle when canning or making jam to insure that the juice is at its best.
The verdict is in. The best way to insure a safe or desired pH for canning low-acid foods or jam gelatinization is to go with a commercially bottled lemon juice. Bottled juice is controlled and standardized with the acid content assured and is more reliable than fresh lemons. Fresh lemons, however, make excellent lemonade!
1 Code of Federal Regulations (Title 21, volume 2, revised April 1, 2010)
2 Green, Janet; Hertzberg, Ruth; Vaughan, Beatrice (June 2010). Putting Food By, Fifth Edition (p. 119). Penguin Books Ltd.