Our shelf-stable milk was left in a hot car of over 100 degrees for many days. it has not been opened. Would it still be good for use ? I have not opened any of the cartons and I will just throw them away if it may not be good to still drink.
As I understand it, as long as your milk is in the sealed, original packaging, it is still safe. In fact, the flavor might be ok, depending on the length of time at that storage temperature. These authors studied the effect of storage temperature on the quality of UHT milk.
For UHT milk stored at 30 and 37 °C a deviating taste was generally detected from storage weeks 24–32. For all storage temperatures, the deviating taste was described as sweet, cardboard, creamy or watery. Additionally, for UHT milk stored at 30 and 37 °C, the deviating taste was described as acidic or caramel.
Their highest test temperature ws 37C (just about 100F). From the paper, I would assume that higher temps speed the flavor change. However, they were also storing for a year, not "many days." In fact, they didn't notice a flavor change until week 24!
If it were me, I would chill one down, and give it a taste to see if I liked it.
I would throw it out. While UHT (Ultra-High-Temperature) Pasteurization is a great way to extend the shelf-life of milk and is packaged in a sterile environment in sterile packaging, it doesn't guarantee sterility, so it is not safe to consume the milk.
In addition, the high temperatures will most likely have altered the flavour and texture of the product, which might well make it unpalatable even if it were safe.