When you refrigerate an open carton of milk, it's normal to observe a reduction in the liquid level upon serving it due to a process called evaporation. Evaporation occurs when the liquid milk turns into vapor and escapes into the air. Several factors contribute to this phenomenon:
Surface Area: The larger the exposed surface area of the liquid, the more evaporation can occur. When you open the carton of milk and pour some out, you increase the surface area exposed to the air.
Temperature: Evaporation is influenced by temperature. The colder the environment, such as inside the refrigerator, the slower the evaporation process. However, even at refrigerator temperatures, some evaporation can still happen over time.
Humidity: The humidity of the refrigerator also plays a role in evaporation. If the refrigerator's humidity is low, the air can absorb more moisture, leading to increased evaporation.
Time: The longer the milk remains in the refrigerator, the more time it has to evaporate slowly.
While evaporation causes a slight reduction in the liquid level, it's generally not a significant concern as long as the milk is within its expiration date and hasn't gone bad. However, it's essential to keep the carton closed tightly after each use to minimize evaporation and prevent the milk from absorbing any unwanted odors or flavors from the fridge. If you notice any off smells or changes in texture, taste, or appearance, it's best to discard the milk, as it may have spoiled.