Why does frozen milk sometimes thaw out into normal homogenised milk and sometimes into curds and whey?
Milk is a very complex liquid. It consists of many small droplets of fat, each surrounded by a membrane that helps to keep them suspended in a solution of proteins and sugars in water. The delicate balance that keeps the fat droplets suspended can be upset in many ways. The acidity of the milk increases as it gets older (and goes “sour”). This makes some of the milk proteins (casein) coagulate to form a solid mass and the milk curdles. If milk has started to sour a little, acidity can increase during freezing (as ice crystals grow, the amount of liquid water is reduced, so the concentration of acids increases). If the acidity gets high enough before the temperature drops too low, casein molecules can coagulate and the milk will curdle. Also, when you freeze milk, sharp ice crystals grow. These can puncture the membrane around the fat drops. So if, when you defrost the milk, you let the temperature get high enough to melt the fat (around 15 – 20°C) it can escape through the punctured me
Milk is a very complex liquid. It consists of many small droplets of fat, each surrounded by a membrane that helps to keep them suspended in a solution of proteins and sugars in water. The delicate balance that keeps the fat droplets suspended can be upset in many ways. The acidity of the milk increases as it gets older (and goes “sour”). This makes some of the milk proteins (casein) coagulate to form a solid mass and the milk curdles. If milk has started to sour a little, acidity can increase during freezing (as ice crystals grow, the amount of liquid water is reduced, so the concentration of acids increases). If the acidity gets high enough before the temperature drops too low, casein molecules can coagulate and the milk will curdle. Also, when you freeze milk, sharp ice crystals grow. These can puncture the membrane around the fat drops. So if, when you defrost the milk, you let the temperature get high enough to melt the fat (around 15 – 20°C) it can escape through the punctured me
