If you’ve been paying careful attention to my recipes, you may have noticed that in most my jam recipes I’ll indicate lemon juice amounts in the number of lemons, whereas when it comes to recipes that involve tomatoes or other moderately acidic ingredients, I specify bottled lemon juice.
The reason for this is that bottled lemon (or lime) juice has been uniformly acidified so that it has a consistent and dependable acid level. When you’re canning things like tomatoes (or watermelon jelly) and you need to reliably get those foods into a safe pH zone, that consistency is important.
I don’t make this recommendation because I prefer the flavor of the bottled stuff or because I’m lazy (although, some days I do appreciate the ease of just opening a bottle). It’s what the USDA recommends for canning and since I do my best to adhere to their rules, I follow suit.