This comes from the Ball Blue Book - it is absolutely one of my favorites! The crystallized ginger makes it so special! The butter reduces the foam making skimming unnecessary but don't add more than 1/2 teaspoon as this may raise the ph level too much and make the recipe unsafe for canning. In canning recipes it is best to use bottled lemon juice which has a stable, known ph because fresh lemon juice can vary in acidity.