Milk becomes like curd when it is not refrigerated due to the process of bacterial fermentation. Fresh milk contains lactose, a type of sugar, and beneficial bacteria, primarily lactic acid bacteria. When milk is left at room temperature, these bacteria start to consume the lactose and convert it into lactic acid through a process called fermentation.
Here's how the fermentation process works:
Lactic acid bacteria naturally present in milk or introduced through external sources (air, utensils, etc.) begin to multiply rapidly in the warm and nutrient-rich environment of milk.
As the bacteria grow, they metabolize the lactose present in milk, breaking it down into lactic acid.
The increasing amount of lactic acid lowers the pH level of the milk, making it more acidic.
The decrease in pH causes the milk proteins, primarily casein, to denature and coagulate, forming solid curds.
The remaining liquid is known as whey, which contains water, some dissolved proteins, and lactose.
The curdling or coagulation of milk due to bacterial fermentation is the basis for various dairy products like yogurt and cheese, which are intentionally made by introducing specific strains of lactic acid bacteria and controlling the fermentation process. When milk curdles unintentionally at room temperature, it is generally a sign that the milk has soured and is no longer suitable for drinking. Consuming soured milk can lead to digestive discomfort and is not advisable.
Refrigeration slows down the growth of bacteria and prevents the fermentation process, thus keeping milk fresh for a longer period and safe for consumption. If you want to prolong the shelf life of milk, it's essential to store it in the refrigerator at a temperature of around 40°F (4°C) or below.