Which factor does NOT affect the steaming of milk?

The correct answer is the customer's preference because it does not directly influence the physical process of steaming milk. Steaming milk involves creating a specific texture and temperature through the application of heat and pressure. This process is governed by the other factors listed: the quantity of milk, which affects how much steam is needed; the type of milk, as different milks have varying fat content and protein structures that impact how they froth; and the pressure of the steaming wand, which determines the effectiveness of the steaming process. Therefore, the customer's preference, while important for serving the right beverage, does not have a direct impact on the technical aspect of steaming the milk.