Why does milk sometimes separate in the fridge?

Milk separating in the fridge can be caused by several reasons:

1. Temperature Fluctuations: If your fridge isn't set to the right temperature or if the temperature fluctuates, milk can separate. Milk should be stored at around 1°C to 4°C to stay fresh and prevent separation [1].
2. Spoilage: When milk spoils, the bacteria in it break down the lactose (milk sugar) into lactic acid. This acid, in turn, interacts with the casein proteins in milk, causing them to curdle or separate [1].
3. Homogenization Process: Commercial milk is usually homogenized to break up the fat globules and distribute them evenly throughout the milk, preventing the cream from separating and rising to the top. If the homogenization process is not perfect, some cream may still rise to the top and cause the milk to appear separated [2].
4. Frozen Milk: If milk is frozen and then defrosted, the milk's texture can change and cause it to separate. This doesn't mean the milk has spoiled, but it might not be as pleasant to drink [3].

Learn more:

1. 5 Reasons Why Your Milk Might Be Spoiling Before the Sell-By Date | The Kitchn
2. Milk Separation: What's The Deal With Cream Top Yogurt? | The Kitchn
3. Does Milk Go Bad? | Does It Go Bad?