As a food safety expert with a background in microbiology, I can tell you that the time it takes for milk to go sour can vary greatly depending on several factors, including the initial quality of the milk, the temperature at which it's stored, and whether it's pasteurized or not.

Milk is a perishable product that can spoil rather quickly due to the growth of bacteria. At room temperature, milk can spoil within a few hours, especially in warmer climates. Once opened, it's generally recommended to consume it within 24 hours. If you're storing milk in the refrigerator, which is the proper way to extend its shelf life, it can last anywhere from a week to several weeks, depending on whether it's ultra-pasteurized, which can last longer.

Pasteurized milk, which is heated to kill off most bacteria, typically lasts longer than raw milk. Ultra-pasteurized milk, which is heated even more, can last even longer, sometimes up to several months when sealed and refrigerated.

However, even when refrigerated, milk can go sour if it's not stored properly. If the milk container is left open, bacteria can enter and cause the milk to spoil more quickly. Signs of spoiled milk include an off smell, curdling, or a change in color.

To ensure safety and quality, always check the expiration date on the container and use your senses to determine if the milk has gone sour before consuming it.

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1.5 hours for in paper carton would do no harm for pasteurized milk. It takes 2 or 3 days for milk to go sour. First it becomes of sour taste, being still liquid. It's safe to drink it, but it's not tasty.read more >>