The acidic nature of the fruit inside also affects how good the jam will become (just like the pectin content). Adding natural acidic juices, such as lemon juice, also helps prevent jam from 'discoloring' and can improve the taste and color of the jam. You can use bottled or fresh lemon juice. Additionally, you can use other complementary juices for most fruit jams. Instead of using lemon juice, you can use fresh red grape juice in strawberry jam, or a type of apple acid in berry jam, etc.
