What is the purpose of adding lemon juice to a sugar syrup during cooking?

Adding lemon juice to a sugar syrup serves to prevent crystallization, a crucial aspect when making smooth and glossy syrups like those used in candy or dessert sauces. The acidity from the lemon juice interferes with the formation of sugar crystals during the cooking process. By introducing an acid, it helps to keep the sugar molecules in solution, ensuring that they remain dissolved rather than forming unwanted crystals, which can lead to a grainy texture. This technique is particularly important in confections where a smooth consistency is desired.

Using lemon juice also contributes some flavor to the syrup, but the primary function in this context is to control crystallization. It does not play a significant role in enhancing color or thickening the syrup, as those effects are achieved through cooking time and sugar concentration. The strategic inclusion of lemon juice is a fundamental technique in pastry making, helping to achieve the desired results in various applications.