Lemon juice contains citric acid, adding the right amount of citric acid. Its main function is that citric acid in a certain PH range (PH2.8 ~ 3.3) is conducive to the gel of the jam. Because pectin in the freezing to form a network of organizations, the formation of this network of organizations need enough hydrogen ions, so moderate increase in acidity, you can improve the coagulation of pectin. the lower the PH value, the same amount of pectin can be formed in a harder gel. Adding citric acid can also adjust the sugar-acid ratio, improve the flavor of jam and adjust the ratio of converted sugar and granulated sugar to prevent granulated sugar crystals folded out.