Unopened ultra-pasteurized milk, also known as shelf-stable milk, can be left out of the refrigerator for a limited time, but it's best to follow the manufacturer's guidelines for storage and handling. While this type of milk is processed to be more stable at room temperature, it is not entirely immune to spoilage.
Typically, unopened ultra-pasteurized milk can remain safe at room temperature (68°F to 72°F or 20°C to 22°C) for about 6 to 9 months, depending on the brand and packaging. However, this timeframe can vary, so it's crucial to check the product's packaging or the label for specific storage instructions.
Once the milk is opened, it should be treated like regular pasteurized milk and refrigerated promptly. Consume it within the time frame mentioned on the label, which is usually several days to a week.
Remember, when in doubt, it's always safer to refrigerate the milk, even if it's shelf-stable. Heat, light, and exposure to air can still impact the quality and safety of the product over time, so proper storage can help maintain its freshness and taste.