Which candy-making technique is essential to control sugar crystallization?

The technique of beating vigorously is essential for controlling sugar crystallization in candy-making. When sugar syrup is heated and then cooled, it tends to form crystals, which can lead to an undesirable grainy texture in candies such as fudge or fondant. By incorporating air and agitating the mixture through vigorous beating, the formation of large sugar crystals is inhibited. This action helps to create a smoother texture and encourages the formation of smaller crystals instead, resulting in a creamy, well-textured candy.

In contrast, the other techniques listed do not primarily serve the purpose of controlling sugar crystallization. Rapid cooling can actually promote the formation of larger crystals if not managed correctly. Adding baking soda can affect the candy's flavor and color but does not address crystallization directly. Incorporating high fats can create a smoother texture as well, but it is not the primary technique used for controlling sugar crystallization. Thus, vigorous beating is the correct method to achieve the desired result in candy-making.