It is constantly recommended by many reputable canning sources that bottled lemon juice should be used with recipes opposed to fresh squeezed lemon juice. The reason for this is because commercial lemon juice is acidified per FDA regulations so that it has a consistent and dependable acid level. This is essential for the safety margin in canning low-acid foods and for making jams gel properly.
Acid strength is measured on the pH scale. The scale starts with the strongest acid at 1 and declines in strength as the number increases to 14, the strongest alkali. The neutral point is 7, neither acid nor alkaline. Fresh lemon juice is inconsistent with their pH levels due to where they were grown, the weather conditions during growth, the variety, the soil used, and the storage conditions. So make sure to use bottled lemon juice for canning instead of fresh squeezed lemon juice. Happy Canning!